Smoke

Investigation summary

What happened

On the morning of 21 July 2025, a Virgin Australia Airlines Boeing 737-800, registered VH-YFY, was being operated on a scheduled air transport passenger flight from Sydney, New South Wales, to Hobart, Tasmania. About 10 minutes prior to landing in Hobart, one cabin crew member was checking the cabin was secure for landing when they identified smoke and flames coming from the top of an overhead locker. When the overhead locker was opened, a passenger’s backpack was found to be on fire. The cabin crew doused the flames with a fire extinguisher, and with the assistance of some passengers, poured water on the bag until no smoke was emitted. The aircraft landed without further incident. 

After landing, aviation rescue firefighters retrieved a burnt power bank from inside the backpack. 

What the ATSB found

The ATSB found that the lithium-ion battery in a power bank experienced a thermal runaway, resulting in a fire in the overhead locker inside a passenger’s bag. Due to the timing of the fire, when the aircraft was already close to landing, the cabin crew had limited time to complete the lithium battery firefighting procedure. 

It was also identified that, while the cabin crew attempted to use the protective breathing equipment provided by the operator, difficulties during its fitment meant that they did not find it effective in this incident. 

What has been done as a result

Following this incident, Virgin Australia Airlines reviewed its policy regarding the carriage of power banks and spare batteries. As of 1 December 2025, guidance provided to passengers stated:

• Power banks, spare and loose batteries must be carried as carry-on baggage only and must be protected against damage.
• Each battery and power bank must be individually protected to prevent short circuiting by placing it in the original retail packaging, in a separate plastic bag, a separate protective pouch or insulating the terminals by applying tape over the exposed terminals.
• Only bring batteries and power banks that are clearly labelled and made by reputable manufacturers. Unlabelled, damaged, leaking, subject to product recall, and counterfeit batteries or power banks must not be brought on board the aircraft.
• Batteries and power banks must be stowed in the seat pocket, under the seat in front, or be kept on you/in your hands. Do not store them in the overhead lockers.
• Power banks must not be used to charge other devices on board the aircraft. Even when not in use, remove all cables/USB cables connected to power banks and batteries.
• Power banks and batteries must not be recharged using the aircraft’s power supply. 

Virgin Australia Airlines also stated that batteries that were damaged, swollen, leaking, recalled, showing signs of defects, or had been repaired or modified, could not be carried in either checked or carry-on baggage. 

Safety message

Passengers often travel with multiple devices that contain lithium batteries, including laptop computers, mobile phones, headphones, and power banks. To reduce the risk associated with lithium battery fires, passengers should ensure their devices are packed safely, easily accessible in the cabin and are not carried on board an aircraft if they show signs of damage or deterioration.

The thermal runaway of a lithium battery can be difficult to manage, particularly when the aircraft is airborne. In-flight fires pose a significant risk to the safety of an aircraft if not managed quickly and appropriately. An operator’s procedure to manage battery fires is designed to limit the risk and reduce the likelihood of re-ignition of the battery until the aircraft can land. However, it requires the batteries to be out of a bag and accessible to be easily completed.

Passengers are encouraged to review their airline’s website, and check the Civil Aviation Safety Authority ‘Pack Right’ website to confirm that equipment they are planning to take on board an aircraft is permitted and packed safely.

Summary video

The investigation

The occurrence

On the morning of 21 July 2025, a Virgin Australia Airlines Boeing 737-800 aircraft, registered VH-YFY, was being operated on a scheduled air transport passenger flight from Sydney, New South Wales, to Hobart, Tasmania. There were 2 flight crew, 4 cabin crew (comprised of a cabin manager under training (‘1L’), a cabin crew trainer (‘1R’), and 2 cabin crew (‘2L’ and ‘2R’) and 149 passengers on board. The first officer was pilot flying and the captain was the pilot monitoring.[1]

At about 0901 local time, as the aircraft descended through 10,000 ft with the seatbelt sign illuminated, the cabin crew began their final checks on the cabin prior to landing at Hobart. While near the front of the cabin, the 1R heard a sound that they described in their interview with the ATSB as a popping and hissing sound. On looking, they saw white smoke, then flames, emanating from the overhead locker above row 7DEF (Figure 1). They immediately instructed the passengers seated in both sides of rows 6,7 and 8 to move away from the area and into other seating in the aircraft. The 1R then retrieved a fire extinguisher, the portable breathing equipment (PBE), and water from the forward galley.

The 1L made a call from the front galley to the other cabin crew for assistance. The 2 rear cabin crew brought more water and another fire extinguisher forward. The 1R and 2R attempted to don the PBE around the same time, however, one was unable to stretch the neck ring sufficiently to don the PBE, and the other, after donning the PBE felt it restricted their ability to see and communicate effectively so decided to remove it. 

Figure 1: Aircraft seat plan showing the location of the overhead locker where flames and smoke were observed

Source: Virgin Australia Airlines, modified by the ATSB

When the overhead locker was opened, flames and smoke were observed emanating from a backpack. Although they could not see what was causing the fire, from their training, the cabin crew suspected it was from a portable electronic device overheating. The 1L discharged a fire extinguisher into the locker until the flames were extinguished. The 1L and 1R then poured water over the bag, with the assistance of the 2R and passengers. To reduce the risk of re‑ignition, a second fire extinguisher was discharged into the locker. The cabin crew instructed passengers to keep their heads down and cover their nose and mouth to avoid inhaling smoke. They also asked the passengers who was the owner of the bag but did not receive a response at the time.

While the other cabin crew managed the fire, the 2L called the flight deck. The captain recalled in interview that, prior to the call, they detected a smoky odour in the flight deck, which they thought was ozone. The 2L advised the captain there was a fire in an overhead locker, which was extinguished, but there was still smoke, and the 1L was dousing the bag with water. They also stated that there were still passengers standing. On ending the call, the captain asked the 2L to ensure everyone was seated for landing. 

Following receipt of this information, at 0905:17, just prior to descending through 5,100 ft, the captain made a ‘PAN PAN’[2] call to the Hobart approach air traffic control to advise of a possible fire in the cabin, and that they would require assistance on landing. The captain also asked to speak to the tower controller earlier to request clearances. The approach controller coordinated to get the tower controller on frequency. 

The captain decided to take control of the aircraft for landing due to the emergency. At 0906:00, the aircraft was cleared to land on runway 30 by the tower controller. At this time, the first officer also asked to cancel the approach they had been previously cleared for and to instead conduct a visual approach. This change was permitted. At 0906:28, air traffic control contacted the aviation rescue and firefighting service at Hobart Airport, which deployed in preparation for the aircraft landing.

The 2L then retrieved gloves and the portable electronic device fire containment bag and took it to the other cabin crew, intending for the burnt device to be placed in the bag, and the bag stored in the rear lavatory, as per procedure. The backpack was too large to fit in the fire containment bag and there was difficulty in locating the device inside the backpack. Given the short amount of time remaining before landing, the decision was made by the 1R to keep the device inside the backpack in the overhead locker above row 7. It was identified by the cabin crew that the rounded shape of the overhead locker retained the water that had been poured on the backpack, which kept the bag soaked, and would help reduce the risk of re‑ignition during landing. The cabin crew planned to keep the overhead locker open, with the 1R seated in seat 7C adjacent to the locker to monitor the device during landing, with water to use if necessary. The cabin crew also directed the passengers standing to be seated immediately, even if it required 4 passengers to be in a row.

As recalled by the captain in interview with the ATSB, the first officer contacted the 1L just prior to the aircraft turning onto the final approach. This was likely just prior to 0909, with the aircraft between 1,500 ft and 1,000 ft. The 1L confirmed the fire was out, and the passengers were seated, but that the cabin crew were still standing. They assured the first officer the cabin crew would be seated for landing. 

The 1L, 2L and 2R secured the final items for landing and seated themselves in their designated seats, except for 1L who sat in 1R’s seat to maintain better visibility of the cabin. On their way to their seat at the front, the 1L checked the overhead lockers around row 7 for heat or any developing hot spots. Once all the cabin crew were seated, the 1L signalled to the flight crew the cabin was secure. The 1L made a final announcement to passengers to ensure their seatbelts were fastened, to remain seated for landing and follow instructions of crew following the landing.

Flight data provided by Virgin Australia showed that the aircraft touched down at Hobart at 0910:29 (Figure 2). 

Figure 2: Flight path with key events

Source: Google Earth, annotated by the ATSB

The captain stopped the aircraft on the taxiway and exited the flight deck to speak to 1L and observed the cabin to determine whether an evacuation was required. As the fire appeared to be contained, the captain taxied the aircraft to the parking bay. At 0919, the aviation rescue and firefighting personnel boarded the aircraft and removed the backpack from the overhead locker. They confirmed the origin of the fire was a lithium power bank, stored in one of the backpack’s front pockets (Figure 3). 

Figure 3: Backpack containing the power bank 

Source: Airservices Australia

The passengers were cleared to disembark normally at 0927. After disembarkation, one cabin crew member (2L) was treated by paramedics. It was unknown if this was due to the effects of smoke as they had been unwell throughout the flight prior to the fire commencing. No other crew or passengers reported to the operator about seeking medical attention from the effects of the smoke. The fire caused minor damage to the overhead locker above row 7/8 DEF (see section Aircraft damage). 

Context

Cabin crew information

The flight comprised of 4 cabin crew, 2 located at the front set of aircraft doors (called ‘1L’ and ‘1R’) and 2 at the rear of the aircraft (called ‘2L’ and ‘2R’). Each cabin crew member had designated roles on the flight, based on which door they were operating. On this flight, the 1L position was filled by a trainee cabin manager who had previous experience in this role working for other airlines, but was completing their first flight as a cabin manager under training for this operator. The 1L was supervised by a cabin crew trainer in the 1R position.

Each of the cabin crew had between 1 and 17 years of experience as cabin crew. They had all completed their annual emergency procedures training between November 2024 and June 2025. This training included both theoretical information and a practical review of the lithium battery firefighting procedure. All cabin crew reported having completed simulated scenarios where the fire occurred either during cruise or on the ground, where there was plenty of time to complete the firefighting procedure, and store the damaged battery appropriately. None of the cabin crew had completed a simulated scenario involving a time‑pressured situation.  

Aircraft cabin information

VH-YFY was a Boeing 737-800 aircraft, with a single aisle in the cabin and seating for 182 passengers. In economy, where this fire occurred, there were 3 seats on either side of the aisle. 

The cabin on this aircraft was fitted with the Boeing 737 sky interior design. In this design, the overhead lockers lowered from the ceiling, creating a contained basket for the bags to sit in. In comparison, the doors in other locker designs would open upwards. 

Lithium batteries

Overview

There are 2 primary types of lithium batteries – lithium metal and lithium-ion. Lithium metal batteries cannot be recharged and are designed to be disposed of once their initial charge has been used, whereas lithium-ion batteries are rechargeable. Compared to lithium metal, lithium-ion batteries store a high amount of energy and are commonly found in many portable electronic devices (PEDs) such as smartphones, tablets, cameras, laptops, and power banks. 

Guidance on the safe carriage of lithium-ion batteries on board aircraft

Lithium batteries are classified by the United Nations as dangerous goods. As such, the International Civil Aviation Organization’s Technical Instructions for the Safe Transport of Dangerous Goods by Air (2025-2026 Edition) stated that lithium batteries, including power banks must be carried as carry-on baggage only.

The Civil Aviation Safety Authority stated that spare batteries and power banks should be packed in carry-on baggage only, so that trained aircrew can manage any issues quickly and safely. Their Pack right website provided safety tips when travelling with lithium batteries:

These simple steps help keep you and your fellow passengers safe:

• choose reputable suppliers when buying devices and spare batteries

• follow airline and manufacturer rules for carrying and charging lithium batteries

• keep spare batteries with you in the cabin and protect them from damage

• stop using or charging batteries that show signs of damage, overheating, or swelling

To prevent short circuits, protect spare battery terminals by:

• keeping them in original packaging

• covering terminals with tape

• placing each battery in a separate plastic bag or case.

At the time of the incident, the operator’s policy permitted power banks to be carried in the cabin only, but they could be stored inside a bag and there were no restrictions on their use on board. Advice on the carriage of power banks and other electronic devices was provided to passengers on the operator’s website and during check-in. 

Thermal runaway

Thermal runaway is a rapid and uncontrolled increase in temperature and occurs when the internal cell(s) of a lithium battery become damaged for reasons including:

• internal short circuits
• breakage from dropping or crushing
• exposure to excessive heat
• failure of the battery cell due to manufacturing defects.

The operator’s Aircrew Emergency Manual stated that crew should be alert not just for signs of smoke or fire in the cabin but also to the smell of overheating electronic devices. 

The smell of overheating may be the first sign of an impending lithium battery/PED fire. PEDs approaching a thermal runaway start initially displaying hissing, crackling sounds, as well as bubbling or blistering casings.

Once a lithium battery cell thermal runaway starts, it quickly leads to the failure of adjacent cells in a chain reaction that can produce fire, which is especially difficult to extinguish. A fire caused by lithium batteries can produce a fire burning with a temperature as high as 1,000^°C, explosions releasing toxic gases and flammable electrolytes as well as shrapnel from damaged PED casings. The manual also stated that in some cases, the explosive force of the venting gasses could be significant enough to cause spikes in cabin pressure.

Managing lithium battery fires

The operator’s Aircrew Emergency Manual stated a general risk about onboard fires was that:

Any fire, no matter how small, may rapidly become out of control if not combatted quickly. Research has shown that if left uncontained, a smoke-filled cabin can be consumed by fire in as little as 6-10 minutes. The first priority shall always be to put the fire out.

In order to manage the risk, the manual outlined 3 important principles of firefighting:

• Immediately locate the source of fire, smoke or fumes. Specific to lithium battery fires, all smoke or fire events occurring in baggage within an overhead locker should be assumed to be a PED/lithium battery fire until the source is positively confirmed.
• Aggressively attack and extinguish the fire using all available resources, which can include able bodied persons.
• Communicate to other crew and the flight crew, as soon as possible, specifying the location and source of fire.

Furthermore, the manual also guided crew that: 

It is important to protect yourself from the effects of smoke and fumes while attempting to flight a fire. In some circumstances it may be safe and thus more important to attack a fire first than fitting of PBE [portable breathing equipment]. PBE should be worn by at least one person when a team is formed to flight a fire and anytime by the primary firefighter when they are in smoke, a confined space of affected by fumes.

In regard to managing a lithium battery fire, the manual stated:

The primary method for stopping a lithium battery from thermal runaway or overheating is to cool it down by pouring water or other non-flammable liquid on the battery or device. This should be done once any flames have been extinguished and continued until the device is cooled and there is no evidence of smoke, heat, crackling or hissing sounds usually associated with an overheating lithium battery. This could take as long as 10–15 minutes. 

Another recommendation in the emergency procedures manual was a suggestion not to open any baggage if there was smoke or flames emanating from it, unless it was required to get a fire extinguisher and liquid onto an identified battery. Once the device was cooled, it could then be placed in a fire containment bag or other suitable container.

Aircraft firefighting equipment

Overview

The operator fitted the Boeing 737 aircraft with firefighting equipment. In addition to this designated equipment, the cabin crew were taught to use other equipment available on board as required, such as drinks used in the cabin service as well as wet blankets and pillows to smother a fire.

Fire extinguishers

The aircraft had 4 bromochlorodifluoromethane (BCF)/halon fire extinguishers. These fire extinguishers are designed to be used on all types of fires by discharging a colourless, odourless, non-corrosive, liquified gas. This gas is not cooling, meaning that further steps are required in fighting a lithium battery fire.

Fire protection gloves

Fire protection gloves were designed to protect the wearer’s hands when fighting fires, including lithium battery incidents. The emergency procedures also suggested that crew should use gloves whenever fighting fires, including using oven gloves if the fire protection gloves were not available.

The L2 retrieved the fire protection gloves from the galley with the intention to move the power bank once the fire was suppressed, but the gloves were not used during the firefighting process.

Portable breathing equipment

Portable breathing equipment (PBE), commonly referred to as a smoke hood, was provided for each cabin crew member on board. The PBE supplied was designed to protect the wearer from fumes and smoke by forming a secure seal using an elastic neoprene neck ring (Figure 4). An oxygen generator on the nape provided oxygen to the wearer. 

Figure 4: Exemplar PBE, showing how to open the neck ring (left) and when donned (right)

Source: Virgin Australia Airlines, modified by the ATSB

Operator procedures recommend PBE should be used by at least one person involved in firefighting, or when one person was fighting a fire in an enclosed environment. Although, consideration was given that the first person to a fire might need to immediately attend to the fire while others gathered and donned the PBE. 

The 2 cabin crew who attempted to use the PBE stated that the equipment provided in training was much easier to don as the neck ring was stretched from repeated usage.

In October 2025, the Airbus Safety First magazine contained an article about PBE, reviewing a case study where 7 cabin crew had difficulties using the PBE provided. The analysis indicated that, similar to this incident, despite having regular training in the use of PBE, the cabin crew found it difficult to use the PBE in a real emergency. The article recommended that ‘dummy’ PBE used in training may not represent the equipment found on board.

Fire containment bags

There were 2 sizes of fire containment bags carried on the operator’s Boeing 737 aircraft. The smaller sized bag, designed to fit an iPad, was carried in the flight deck in the event of a thermal runaway involving one of the flight crew’s electronic flight bags.

A larger bag, designed to fit a device the size of a laptop, was carried in the aircraft cabin (Figure 5). The purpose of this bag was to place the device in after it was cooled post‑fire, so that it could be stored in a water filled container in a secure place.

Figure 5: Fire containment bag

Source: Virgin Australia Airlines

Damage to the power bank

The power bank was inspected by the aviation rescue firefighters after being removed from the aircraft. 

The power bank had a rated output of 37 watt hours and had both USB‑A and USB‑C charging ports. It was not charging a device when the fire started, but there was a cable plugged into the USB‑A charging port. When orientated with the manufacturer’s label facing up, most of the damage was observed along the top and right side of the power bank. The damaged right end contained the USB-C charging port. External observations suggest that the power bank contained 2 internal cells, of which only 1 was affected by the thermal runaway by the time the fire was extinguished (Figure 6). The power bank was not protected from short circuits as it did not appear to have the terminals covered and was not separated from other items in the bag by being placed in a protective pouch or the original packaging. The backpack was reported by the operator to be substantially damaged by the fire.

The owner of the power bank advised the operator that it was purchased in 2024 and:

• it had no pre-existing damage
• was fully charged the day prior to the incident, and there were no previous issues during charging or use
• it was not dropped, exposed to moisture or heat prior to the incident. 

Figure 6: Power bank showing damage from the back (left) and side view (right)

Source: Airservices Australia (left) and Virgin Australia Airlines (right)

Aircraft damage

An inspection of the aircraft found fire damage in the panels above and behind the overhead locker above row 7 and 8 DEF. The passenger service unit containing the reading lights, call bell and information signals under this locker also sustained fire and water damage. The overhead locker needed to be replaced post-fire (Figure 7). 

Figure 7: Overhead locker above row 7DEF where the smoke and flames were observed (left) and area behind the overhead locker showing fire damage (right)

Source: Airservices Australia (left) and Virgin Australia Airlines (right)

Related occurrences

Australian data

This incident was the first reported power bank-related in-flight fire either in Australia or on an Australian‑registered aircraft. A review of the ATSB’s occurrence database identified 3 previous incidents where smoke was reported emanating from a power bank in the aircraft’s cabin, but no fire occurred:

• OA2025-01608: On 20 January 2025, a Boeing 737 aircraft was being operated on a flight from Brisbane, Queensland, to Melbourne, Victoria. During cruise, a power bank overheated in the cabin, and smoke was observed emanating from the attached charging cable. The power bank and cable were placed in a container and stowed in the rear lavatory for the remainder of the flight.
• OA2019-04325: On 28 May 2019, an Airbus A330 aircraft was being operated on a flight from Melbourne, Victoria, to Hong Kong. During cruise, smoke was detected emanating from a passenger's power bank.
• OA2019-02629: On 15 April 2019, an Airbus A330 aircraft was being operated on a flight from Hong Kong to Melbourne, Victoria. During passenger disembarkation, smoke was observed emanating from a passenger's power bank. The crew doused the power bank in water.

ATSB records showed that, in the past 10 years, there were 4 in-flight fires resulting from mobile phones, 3 of which occurred after the mobile phone was crushed in the seat mechanism, damaging the lithium battery contained inside. The reason for the fourth fire was unknown.

International incidents

While this was the first reported incident in Australia, there have been a number of significant fires resulting from power banks, including: 

• In January 2025, an Air Busan A321 aircraft was preparing for flight at Busan, South Korea. Prior to taxi, a power bank fire started in an overhead locker. The fire spread, resulting in an evacuation. The aircraft was destroyed.
• In March 2025, a Hong Kong Airlines A320 aircraft diverted after a power bank experienced a thermal runaway. The fire was extinguished.
• In October 2025, an Air China A321 aircraft had to divert after a battery caught fire in an overhead locker. 

The United States Federal Aviation Administration recorded 8 in‑flight fire incidents involving power banks occurring in the past 10 years. 

Safety analysis

Power bank thermal runaway

For unknown reasons, one of the cells in a lithium-ion power bank stored in an overhead locker failed during the descent into Hobart. In this case, there was no reported pre‑existing damage, or any other identified problems with this power bank prior to flight. However, the power bank was stored with a cable in it and the ports uncovered, both factors which can increase the risk of a fault.

Inspection of the power bank post-flight, combined with the cabin crew reports of sounds they were trained to expect in the event of a lithium battery fire, suggested that the fire was characteristic of a thermal runaway. As the temperature of the power bank continued to increase, smoke, followed by flames resulted.

Completion of firefighting procedures

As described in the operator’s procedures, fires on board aircraft can spread quickly. As the aircraft was already on descent when the smoke was initially observed, there was limited time for the cabin crew to manage the power bank fire by completing all the procedures they were trained to do in response to an in-flight fire. In addition, they had the responsibility to ensure the cabin was secure for landing. 

In less than 8 minutes, the cabin crew worked together to identify a fire, gather the required equipment, and aggressively fight the fire to a point where the fire appeared suppressed. In addition, they communicated the problem with the flight crew and managed moving passengers to alternative seating. 

While the cabin crew had received emergency procedures training, they had never trained for a lithium battery fire in a compressed time. They completed as many of the procedures as they were able to in the available time, but by the time the fire was considered controlled, there was only around 90 seconds for the cabin crew to clear up the cabin and be seated for landing. Besides the logistics and risk that would result from handling the power bank to remove it from the backpack and place it in a fire containment bag, there was no time available. 

The cabin crew identified an alternate solution to moving the burnt power bank. While the overhead locker held the water poured on the backpack, there was no assurance that the power bank was going to remain fully submerged for landing, in accordance with the operator’s procedures, or isolated from other lithium battery devices. Although there was no consequence as a result of the power bank remaining in the overhead locker, there was an increased risk of cabin occupant injury and aircraft damage if the power bank re‑ignited due to further exposure to fire and smoke. 

Protective breathing equipment

Protective breathing equipment (PBE) was available for cabin crew to use if deemed necessary in preparation to fight a fire, and all crew were trained in their use. Two of the cabin crew attempted to use the PBE, but did not find it effective due to fitment and communication/visibility issues. As the cabin crew were unable to use the PBE, they had no protection from the smoke and were placed at an increased risk of smoke inhalation. While these cabin crew did not experience any residual effects from the smoke, any protective equipment provided should be efficient to don and wear continuously while managing an emergency situation.

Findings

From the evidence available, the following findings are made with respect to the in-flight fire involving Boeing 737, VH-YFY, 56 km north‑north‑east of Hobart Airport, Tasmania, on 21 July 2025. 

Contributing factors

• During the descent, a passenger's lithium-ion power bank, located in the overhead locker, overheated due to thermal runaway and began to emit flames and smoke.

Other factors that increased risk

• Due to the timing of the fire starting on descent, the cabin crew had limited time to complete the procedure for managing a lithium battery fire.
• The cabin crew attempted to use the protective breathing equipment provided by the operator but did not find it effective when managing the lithium battery fire.

Safety actions

Safety action by Virgin Australia Airlines

Virgin Australia Airlines advised it has reviewed its policy regarding the carriage of power banks in the cabin. As of 1 December 2025, guidance provided to passengers stated:

• Power banks, spare and loose batteries must be carried as carry-on baggage only and must be protected against damage.
• Each battery and power bank must be individually protected to prevent short circuiting by placing it in the original retail packaging, in a separate plastic bag, a separate protective pouch or insulating the terminals by applying tape over the exposed terminals.
• Only bring batteries and power banks that are clearly labelled and made by reputable manufacturers. Unlabelled, damaged, leaking, subject to product recall, and counterfeit batteries or power banks must not be brought on board the aircraft.
• Batteries and power banks must be stowed in the seat pocket, under the seat in front, or be kept on you/in your hands. Do not store them in the overhead lockers.
• Power banks must not be used to charge other devices on board the aircraft. Even when not in use, remove all cables/USB cables connected to power banks and batteries.
• Power banks and batteries must not be recharged using the aircraft’s power supply. 

Virgin Australia Airlines also stated that batteries that are damaged, swollen, leaking, recalled, showing signs of defects, or have been repaired or modified, cannot be carried in either checked or carry-on baggage. 

It has updated the cabin crew’s pre-flight announcement and website to inform passengers of the revised policy.

Virgin Australian Airlines has also acknowledged that, while it made changes to its policy, there are challenges in monitoring passengers’ compliance with these measures. It also stated that greater awareness about the risks of travelling with lithium batteries should be delivered by all airlines and airports.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the cabin crew
• the captain
• Virgin Australia Airlines
• Airservices Australia
• Civil Aviation Safety Authority.

References

Airbus. (2025). Focus on protective breathing equipment. https://safetyfirst.airbus.com/focus-on-protective-breathing-equipment/   

Civil Aviation Safety Authority. (n.d.). Lithium batteries. Retrieved 25 July 2025, from https://www.casa.gov.au/packright/lithium-batteries

International Civil Aviation Organization (2025). Technical instructions for the safe transport of dangerous good by air 2025-2026. https://www.icao.int/Dangerous-Goods/Technical-Instructions 

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• the cabin crew
• the flight crew
• Virgin Australia Airlines
• Airservices Australia
• Civil Aviation Safety Authority.

Submissions were received from:

• Virgin Australia Airlines
• 2 cabin crew members
• Civil Aviation Safety Authority.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. About ATSB reports ATSB investigation reports are organised with regard to international standards or instruments, as applicable, and with ATSB procedures and guidelines. Reports must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2025   Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Investigation summary

What happened

On 11 June 2025, a Robinson R44 Raven I helicopter, registered VH-OOE, was being operated on a personal transport flight from Daly Waters Aerodrome to Wally’s Airstrip, Northern Territory, with a pilot and one passenger on board. As the helicopter neared the destination, the pilot felt the onset of severe airframe vibration. The pilot elected to conduct a precautionary landing in an area of open farmland, resulting in a hard landing. The pilot and passenger were uninjured, and the helicopter sustained minor damage.

What the ATSB found

The helicopter’s engine was found to have suffered a mechanical failure due to in-service loosening of the nuts on the connecting rod bolts, leading to separation of one of the connecting rods from the crankshaft. The reason the nuts became loose was not determined.

While there was no indication of influence on this occurrence, independent inspection of the connecting rod attaching hardware performed during the overhaul of the engine did not involve a physical torque check of the connecting rod bolts. While the inspection was not a regulatory requirement, this was a missed opportunity to verify the installation torque.

During the most recent periodic inspection the helicopter maintenance provider did not refit the spark plugs using new gaskets, as required by the spark plug manufacturer. It was also found that the Civil Aviation Safety Authority guidance on spark plug gasket fitment was inconsistent in this respect. 

What has been done as a result

The Civil Aviation Safety Authority acknowledged the inconsistent information contained within the 2 airworthiness bulletins. CASA advised that Airworthiness Bulletin AWB 20‑001 is scheduled for cancellation and Airworthiness Bulletin AWB 85-023 is to be amended to reflect current recommendations. 

The helicopter maintenance provider advised the ATSB it now installs new gaskets when refitting spark plugs.

Safety message

This incident highlights the importance of managing inflight anomalies through a comprehensive understanding of aircraft systems and the application of emergency procedures. The pilot’s timely actions following the onset of the vibrations ensured a safe outcome for the occupants and resulted in minimal damage to the helicopter.

The incident also emphasises the importance of adhering to manufacturer requirements when installing aircraft components, as well as the additional assurance provided by a thorough independent inspection of completed work.

The investigation

The occurrence

On 11 June 2025, a Robinson R44 Raven I helicopter, registered VH-OOE, was being operated on a personal transport flight with a pilot and one passenger on board. The flight was conducted under the visual flight rules,[1] and the planned route was from Daly Waters Aerodrome to Wally’s Airstrip, Northern Territory (NT) (Figure 1).

On the morning of the flight, the pilot completed their pre-flight inspection and refuelled the helicopter. Shortly after starting the engine, the pilot recalled sensing an unusual sound and vibration through the helicopter, but it resolved when the engine speed was increased. The pilot completed their pre-take-off checks, and the helicopter departed Daly Waters Aerodrome at about 0900 local time. The pilot did not recount any issues with the helicopter’s performance during the take-off, climb or initial cruise.  

Figure 1: VH-OOE flight track

Source: Google Earth, annotated by the ATSB

At about 1015, when the helicopter was about 46 km to the south‑east of Wally’s Airstrip, the pilot contacted Tindal Airport air traffic control (ATC). Several exchanges with Tindal Airport ATC took place, during which the pilot was instructed to follow a railway line and maintain an altitude not above 1,500 ft above mean sea level. The pilot complied with these instructions and continued towards their destination. At about 1020, when the helicopter was at an altitude of about 1,100 ft, the pilot felt the onset of severe airframe vibration. They recalled initially thinking the helicopter tail may had been struck but later discounted that possibility when they identified they still had directional control. The pilot was unable to diagnose the cause of the vibration and decided to undertake a precautionary landing. 

At 1020:42, and an altitude of about 1,100 ft, the pilot alerted Tindal Airport ATC that they had a ‘problem’ (Figure 2). The pilot selected a paddock for the landing that had recently been harvested of its crop and commenced a right turn towards the landing location at 1020:49. At 1020:52, they communicated that operations were not normal, and at 1021:00 they advised Tindal Airport ATC that they would be landing immediately. The pilot recalled noting the engine gauges and the rotor and engine speed indications at that time were normal.  

Figure 2: VH-OOE flight path from the onset of vibrations until landing

Source: Google Earth, annotated by the ATSB

At 1021:05, and an altitude of 700 ft, the pilot made a transmission to Tindal Airport ATC during which a low speed warning horn could be heard in the background (see Low rotor speed). The pilot did not recall hearing the horn. At about 150 ft above ground level, the pilot recalled noting a low oil pressure light on the helicopter’s caution warning panel (see Oil warning caution light). They continued the approach and, as the helicopter slowed for landing, they observed smoke blowing forward from the rear and recalled having concerns about a fire. 

The helicopter landed heavily in the paddock. The pilot recalled that the landing was probably completed ‘quicker’ and with a lower tail position than normal, due to their concerns about a fire. Once the helicopter had landed, the pilot instructed the passenger to exit and run forward. They then shut down the helicopter’s engine, and at 1021:41 advised Tindal Airport ATC that they had landed and were safe. The pilot then exited the helicopter. Both occupants were uninjured, and the helicopter sustained minor damage.  

Context

Pilot information

The pilot held a valid Commercial Pilot Licence (Helicopter) with single engine and low‑level ratings. The licence was issued on 6 June 2025 following the successful completion of a commercial pilot licence flight test in May 2025. The pilot had held a Private Pilot Licence (Helicopter) since October 2023. They also held a current class 1 aviation medical certificate valid to 6 August 2025. At the time of the incident, they had a total flying time of 194 hours of which 118 hours were on the Robinson R44.

Helicopter information

General information

The Robinson R44 Raven I is a 4-place helicopter with a 2-bladed main rotor system and a conventional 2-bladed tail rotor. VH-OOE was manufactured in the United States in 2008 and first registered in Australia in July 2008. At the time of the incident, the helicopter had accumulated 1,995 hours total time in service. 

It was powered by a Lycoming O-540-F1B5, 6-cylinder, horizontally opposed piston engine that is naturally aspirated and rated at 235 horsepower. The overhauled engine was installed in September 2022 and had operated for 291 hours at the time of the incident, with a total time of about 1,614.6 hours. The last periodic inspection was undertaken on 6 May 2025, and the helicopter had flown about 25 hours since that inspection. 

Airworthiness and maintenance history

Recent maintenance

The last periodic inspection was undertaken by Platinum Helicopters on 6 May 2025. During the inspection, the Champion REM38E spark plugs fitted to the engine were removed, inspected and then refitted by the maintenance engineer. The maintenance engineer recalled that it was not their practice to fit new spark plug washers (gaskets) when refitting the spark plugs, instead electing to use annealed[2] gaskets (see Spark plug maintenance). 

Engine overhaul

In September 2022, VH-OOE underwent a 12 year/2,200 hour inspection. During the inspection, the engine was removed and an overhauled engine was fitted to the helicopter. This engine had been salvaged from a Robinson R44, and was overhauled by South West Aviation, a CASA‑approved maintenance organisation. 

During the overhaul of the engine, additional components were used to replace some aspects, including:

• 6 new cylinder kits
• new connecting rod hardware (bolts and nuts) with parts manufacturer approval[3]
• a crankshaft that had been salvaged from a different Robinson R44.

Records show all salvaged components were inspected and tested to assess serviceability prior to fitment. Once the engine overhaul had been completed, it underwent ground runs and checks prior to being installed in VH-OOE.

Records show that independent inspections were undertaken during the engine overhaul of the engine fitted to VH-OEE. The purpose of an independent inspection is to verify that a maintenance task has been completed correctly. The inspection is undertaken by an appropriately authorised person who did not undertake the original activity. While there was no regulatory requirement for the independent inspection of maintenance work carried out on engine systems, South West Aviation had included these inspections as part of the organisation’s worksheets for engine overhaul. 

The worksheets for the engine overhaul stated that an independent inspection of the engine sub-assembly was completed during the engine rebuild. Figure 3 shows the sub‑assembly of the crankshaft and the connecting rods, which were secured to the crankshaft by 2 connecting rod bolts and nuts. The crankshaft has 2 dynamic counterweight assemblies fitted, which assist in removing torsional vibration during engine operation.

Figure 3: O-540 crankshaft and connecting rod sub-assembly 

Source: Lycoming O-540-F1B5 Illustrated Parts Catalogue, annotated by the ATSB

During interview, when asked about a torque check of the connecting rod nuts, the engineer who conducted the independent inspection stated they would check the torque was set correctly on the tooling that had been used, but it was not their normal procedure to physically check the torque on each nut. South West Aviation did not have a documented procedure that detailed how the independent inspection of the connecting rod hardware should be conducted.

Helicopter systems and procedures

Vibration

The Robinson R44 pilot operating handbook (POH) contained advice for the management of vibration, and stated:

A change in the sound or vibration of the helicopter may indicate an impending failure of a critical component. If unusual sound or vibration begins in flight, make a safe landing and have the aircraft thoroughly inspected before flight is resumed. 

Low rotor speed

The helicopter was fitted with a low rotor speed horn. The activation of the horn indicated that rotor speed may be below safe limits (97%). Power available from the engine is directly proportional to rotor speed. With less power the helicopter will start to sink. If the collective is raised to stop it from descending, the rotor speed will reduce even further causing the helicopter to sink faster. To restore rotor speed, the Robinson R44 POH stated that a pilot should lower the collective, roll throttle on and, in forward flight, apply aft cyclic.

Oil warning caution light

The helicopter was fitted with an oil warning caution light. The illumination of the light indicated a loss of engine power or oil pressure. The Robinson R44 POH stated the actions to take in response should be to check the engine tachometer for power loss and the oil pressure gauge. If oil pressure loss was confirmed, the POH stated the pilot should land immediately. Continued operation without oil pressure causes serious engine damage and engine failure can occur.

Spark plug maintenance

The Champion Aviation Service Manual,[4] which included recommended service, handling and reconditioning practices for Champion spark plugs stated:

Always install both new and reconditioned Champion aviation spark plugs with a new copper gasket. 

Additionally, Champion Aviation Technical Bulletin 95-11[5] stated:

Gaskets that have become too hard with normal usage won’t “hold torque” correctly, and spark plugs can come loose with disastrous results. An annealed gasket will not meet new specifications.

The maintenance engineer stated they carried out the periodic inspection in accordance with the Lycoming O-540 Operator’s Manual.[6] However, this manual, which covered both the O-540 and IO-540 engines, contained no information regarding spark plug gasket fitment. The guidelines for the installation of spark plugs were contained in Lycoming service instruction 1042 Approved Spark Plugs, which stated:

Always install a spark plug with a new gasket.    

The Civil Aviation Safety Authority (CASA) had produced 2 advisory airworthiness bulletins (AWBs) that included information on spark plug fitment. However, the advice within these 2 documents was not consistent. 

AWB 20-001 Spark Plug Care, issued in September 2001, stated:

Most modern spark plugs have a solid copper gasket that requires annealing prior to spark plug installation to ensure a tight, gas sealed fit. The maintainer should check that the spark plug has only one washer, is of correct dimensions and is annealed. If the engine is equipped with a thermocouple probe in the form of a spark plug gasket, a normal gasket is not required.

Whereas AWB 85-023 Piston Engine Spark Plug Cracking, issued in June 2021, stated:

Always install a new spark plug gasket when servicing spark plugs or installing new spark plugs. Failure to install a new spark plug gasket may result in incomplete sealing of the combustion chamber, loss of heat transfer with spark plug overheating leading to possible pre-ignition.

Meteorological information

The weather at the time of the incident, recorded at Tindal Airport around 13 km to the north of the landing site, captured a wind of between 9–13 kt from the east, clear skies and a temperature of 23°C. 

Recorded information

The helicopter was not fitted with a flight data recorder or a cockpit voice recorder, nor was it required to be. During the incident flight, data was being transmitted by the helicopter’s transponder. This data, recorded by ground-based receivers, captured the aircraft’s position, altitude, and groundspeed during the final 25 minutes of the flight. All radio communications made and received by Tindal Airport ATC throughout the flight were recorded.

Helicopter damage

The ATSB did not attend the landing site. A post-incident inspection of the helicopter was completed by a maintenance organisation located at Wally’s Airstrip, NT. This inspection identified:

• damage to the engine with scattered material within the cowling
• damaged and displaced drive belts
• impact damage to the engine oil cooler caused by engine material
• engine oil on external areas of the engine and airframe
• the skid landing gear was spread outwards (Figure 4).

The engine and a selection of components were removed for a detailed examination by the ATSB.

Figure 4: VH-OOE shortly after landing showing oil leak and smoke haze

Source: Supplied, annotated by the ATSB

Engine examination 

The engine was disassembled and examined at a CASA‑approved engine overhaul facility under the supervision of the ATSB. The examination found that the number 4 connecting rod had separated from the crankshaft journal, resulting in mechanical damage to the internal engine components and fracture of the adjacent crankcase. Both connecting rod bolts had been fractured, with one connecting rod nut missing and the other unwound (see Component examination). There were also witness marks from impact between the number 4 piston crown and cylinder head.

Prior to removal of the remaining connecting rods, the nuts were checked for torque. The check found that the number 3 cylinder connecting rod nuts were at 20 ft/lb, while numbers 1, 2, 5 and 6 connecting rod nuts were at the correct torque of 40 ft/lb. 

The number 4 cylinder spark plugs were found loosened, but the spark plug leads were attached tightly. Subsequent testing of the spark plugs found both were serviceable. Figure 5 depicts the engine prior to disassembly.

Figure 5: Engine assembly showing damage 

Source: ATSB 

Component examination 

Several components were retained from the engine disassembly and were examined at the ATSB’s technical facilities in Canberra, Australian Capital Territory. 

Extensive deformation and fracture of the number 4 connecting rod (Figure 6) and deformation of the crankshaft journal, was consistent with initial separation of the connecting rod, followed by repeated impacts to the connecting rod by the still-rotating crankshaft. 

Figure 6: Number 4 cylinder connecting rod and piston

Source: ATSB 

This resulted in significant damage to the adjacent cylinder wall, piston skirt, camshaft and the hole in the crankcase. The fractured connecting rod showed no evidence of fatigue cracking or other defect.

The number 4 connecting rod bearings were deformed due to contact with the moving internal engine components but were found to be the correct parts and did not exhibit any abnormal signs of wear. Bearings from some of the other connecting rods displayed minor surface wear, which was attributed to low engine oil volume during the final part of the flight. 

There were visibly fewer combustion deposits on the number 4 piston crown, compared to the remaining pistons. However, a considerable amount of sand-like contamination was recovered from the number 4 cylinder during engine disassembly, which was found to be chemically similar to the piston deposits. There was no evidence of destructive combustion issues such as pre-ignition or significant detonation. 

The connecting rod was secured to the crankshaft by 2 connecting rod bolts (Figure 3). Both number 4 cylinder connecting rod bolts were fractured in approximately the same location (Figure 7). The fracture surface features of both bolts and deformation of the adjacent shank were consistent with overstress failures. 

Figure 7: Cylinder number 4 connecting rod bolts

Source: ATSB

One of the cylinder 4 connecting rod bolts had no nut and heavily damaged threads. The nut was not located. The other connecting rod bolt had a partially unwound nut retained on the threads (Figure 8).[7] The exposed threads were damaged. The nut could not be further unwound by hand, likely due to impact damage. The bolts and nut material was in accordance with their specification. The extent of deformation precluded a detailed inspection of the threads; however, the threads were not stripped and the remnants of a compound consistent with thread lubricant was identified. 

Figure 8: Number 4 cylinder connecting rod bolt showing position of retained nut

Source: ATSB

The examination also identified evidence of abnormal fretting[8] wear in the number 4 cylinder connecting rod bolt holes. A comparison between the number 4 cylinder and number 6 cylinder connecting rod bolt holes is depicted in Figure 9.  

Figure 9: Number 4 cylinder connecting rod end cap bolt hole fretting wear and exemplar

Source: ATSB

The abnormal fretting wear indicated relative movement (micro-slip) between the bolted surfaces during operation, which would occur if the bolt tension was insufficient to restrain movement under normal operational loads. The missing nut from one of the bolts, and the other nut retained in an improper position on the fractured bolt, was also an indicator that the nuts had loosened in-service.

Possible mechanisms that could result in the in-service loosening of the nuts included:

• Abnormal loading or vibration from engine overspeed or abnormal combustion that could lead to bolt stretch and nut loosening.
• Variations in thread condition or installing the threads dry versus lubricated could produce a lower bolt stress than desired.
• Unintended deformation due to improper or defective parts leading to reduced bolt stress over time.
• Microscopic surface deformation and fretting at contact interfaces could reduce clamping force by a small margin over time, which could then make the nut susceptible to further loosening during service.
• Inadequate torque applied to the nuts during installation that could lead to relative movement between the clamped surfaces and nut loosening during normal operation.

Related occurrences

In 2007, the ATSB published a research and analysis report (B20070191) into aircraft reciprocating (piston) engine failures. The report examined 20 high-power[9] piston engine structural failure occurrences in Australia, between 2000 and 2005. The report focused on failures of the combustion chamber, connecting rods and crankshaft assemblies. It included several engine failure investigations, including investigation 200105866 (below).  

ATSB investigation 200105866

On 14 December 2001, a Piper PA31-350 aircraft, registered VH-JCH, was in cruise flight at 8,000 ft when the flight crew noticed that the propellers went out of synchronisation. Adjustments were made to correct the problem but were unsuccessful. Following right engine speed fluctuations, the crew shut the engine down, feathered the propeller and conducted a single‑engine landing. 

During the subsequent disassembly of the engine, the crankshaft was noted to have fractured at the number 6 connecting rod journal, and the number 6 connecting rod big end had separated from the crankshaft and impacted the camshaft. The separation of the number 6 big end permitted the piston to strike the top of the combustion chamber with sufficient force to deform the top of the piston. 

The number 6 connecting rod disconnection from the crankshaft was due to the loosening of the nuts on the connecting rod bolts, and eventual loss of one nut. Evidence of nut loosening, leading to fretting wear damage, was observed on the bolt threads and the connecting rod cap bolt hole locations. The reason for the loosening of the number 6 connecting rod nuts could not be determined. 

The damage of these components was almost identical to the damage noted in the engine from VH-OOE.

Safety analysis

The ATSB examination of the engine components determined that the engine failure resulted from mechanical damage caused by the separation of the number 4 cylinder connecting rod from the crankshaft. 

The initiating factor of the separation was almost certainly the in-service loosening of the connecting rod nuts of the number 4 cylinder. This was evidenced by the fretting wear in the connecting rod bolt holes, which was illustrative of engine operation after a loss of bolt tension, allowing relative movement between the bolts and holes. The absence of one of the associated nuts, and the opposite one mostly unwound was also evidence of the nuts loosening prior to the engine failure. There was also an absence of fatigue cracking of the number 4 bolts or connecting rod that might otherwise account for the component fractures and separation of the connecting rod. 

Of the possible mechanisms identified that could have led to the connecting rod nuts loosening:

• Abnormal loading or vibration: there was no evidence of engine overspeed, or of piston melting or structural damage consistent with severe abnormal combustion. There was no evidence that the spark plugs, found loose during the disassembly, had any negative impact on the engine performance.
• Variation in thread condition and lubrication: this could not be fully assessed due to thread damage and the absence of one of the nuts.
• Improper or defective parts: the connecting rod bolts and nut material was correct; however, a full assessment of their original condition was not possible.
• Embedding (microscopic deformation): it is possible that the initial bolt tension reduced by a small margin due to microscopic deformation of the clamping or thread surfaces, which would then make the nut more susceptible to further loosening during service.
• Inadequate installation torque: like the above, it was possible that the nuts were slightly under-torqued during installation and progressively loosened during the subsequent 291 hours of operation. The number 3 connecting rod nuts being found at the incorrect torque value during the engine disassembly further supports this scenario.

Given most of the possibilities above could not be definitively ruled out, the reason for the nuts loosening was ultimately not determined. 

Despite this, it was identified that during the overhaul of the engine fitted to VH-OOE, the independent inspection of the engine sub-assembly did not involve a torque check of the connecting rod nuts. While there was no evidence of influence on this occurrence and while the inspection was not a regulatory requirement, the ATSB considered it a missed opportunity to positively verify the installation torque.

Additionally, during the engine examination, both spark plugs in the number 4 cylinder were found to be loose. The reason for the loose spark plugs was not determined and, as above, there was no evidence identified to indicate influence on the engine failure. However, it was identified that during the most recent periodic inspection, the helicopter maintenance provider did not refit the spark plugs using new gaskets as required by the engine and spark plug manufacturer. 

On the same subject, the Civil Aviation Safety Authority guidance on spark plug gasket fitment was inconsistent. Airworthiness Bulletin AWB 20-001 stated that annealed gaskets could be used, whereas Airworthiness Bulletin AWB 85-023 stated new gaskets must be used in all circumstances. 

The unusual sound and vibration noted by the pilot during engine start was possibly a precursor to the eventual failure inflight, however the vibration disappeared when engine speed was increased. In response to the onset of severe vibration inflight, the pilot assessed the controllability of the helicopter and noted there were no abnormal engine indications at that time. In accordance with the Robinson R44 POH, the pilot conducted a precautionary landing in a suitable location. They also communicated the issue to Tindal Airport ATC, which increased the likelihood of a timely emergency response had one been necessary.

During the late stages of the approach, the low rotor speed warning horn and low oil pressure caution light activated. Both indicated a reduction in power, almost certainly due to the mechanical failure, resulting in less power than normal to arrest the rate of descent in the final stages of landing. This, in combination with the pilot’s concern about a possible fire and recollection of landing ‘quicker’ than normal, likely resulted in the helicopter landing heavily which spread the landing gear skids.

Findings

From the evidence available, the following findings are made with respect to the engine failure and forced landing involving Robinson R44, VH-OOE, 13 km south of Tindal Airport, Northern Territory, on 11 June 2025.

Contributing factors

• In-service loosening of the connecting rod nuts resulted in the eventual separation of the connecting rod from the crankshaft and the mechanical failure of the engine. The reason for the nuts loosening was not determined.
• During the engine overhaul, the torque on the connecting rod nuts was not physically checked as part of the independent inspection of the engine assembly. This was a missed opportunity to verify that the installation of the connecting rod nuts had been completed correctly.

Other findings that increased risk

• During the most recent periodic inspection, the helicopter maintenance provider did not refit the spark plugs using new gaskets as required by the spark plug manufacturer. This increased the risk of loosened spark plugs, insufficient heat transfer and pre-ignition.
• The Civil Aviation Safety Authority guidance on spark plug gasket fitment was inconsistent. Airworthiness Bulletin AWB 20-001 stated that annealed gaskets could be used, whereas Airworthiness Bulletin AWB 85-023 stated new gaskets must be used in all circumstances. The inconsistency in this guidance could have led to incorrect procedures being performed which were not in accordance with spark plug maintenance requirements.

Safety actions

Safety action by Civil Aviation Safety Authority

The Civil Aviation Safety Authority acknowledged the inconsistency between Airworthiness Bulletin AWB 20-001 (that stated that annealed gaskets could be used) and Airworthiness Bulletin AWB 85-023 (that stated new gaskets must be used in all circumstances) and advised the ATSB that AWB 20-001 will be cancelled and AWB 85‑023 will be amended to reflect current recommendations.

Safety action by Platinum Helicopters

Platinum Helicopters advised the ATSB that new spark plug gaskets are now fitted each time spark plugs are reinstalled. 

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot of VH-OOE
• the engine overhaul organisation
• the maintenance provider for VH-OOE
• Civil Aviation Safety Authority
• the aircraft manufacturer
• the engine manufacturer
• the PMA parts manufacturer
• Airservices Australia
• the Bureau of Meteorology.

References

Australian Government (1988), Civil Aviation Regulations 1988 (Commonwealth), reg 42G. AustLII. https://classic.austlii.edu.au/au/legis/cth/consol_reg/car1988263/s42g.html 

Australian Government (2021), Aircraft Reciprocating-Engine Failure: An Analysis of Failure in a Complex Engineered System, Australian Transport Safety Bureau, Canberra, ACT. /publications/2007/b20070191

Civil Aviation Safety Authority (2025). Airworthiness Bulletin 20-001. Retrieved from https://www.casa.gov.au/aircraft/airworthiness/airworthiness-bulletins/spark-plug-care

Civil Aviation Safety Authority (2025). Airworthiness Bulletin 85-023. Retrieved from https://www.casa.gov.au/aircraft/airworthiness/airworthiness-bulletins/piston-engine-spark-plug-insulator-cracking 

Lycoming Engines Operator’s Manual 4^th edition 2006, O-540, IO-540 Series

Lycoming Engines Overhaul Manual, Direct drive engines 1974

Lycoming Engines Parts Catalogue 2009, O-540-F1B5

Robinson Helicopter Company 2024, R44 Pilot’s Operating Handbook, section 10, p.10-2

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• the pilot of VH-OOE
• Civil Aviation Safety Authority
• the maintenance provider
• the engine overhaul organisation
• Textron Lycoming
• Robinson Helicopters
• National Transportation Safety Board (NTSB).

Submissions were received from:

• the pilot of VH-OOE
• the maintenance provider
• the engine overhaul organisation
• Textron Lycoming
• Robinson Helicopters.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. About ATSB reports ATSB investigation reports are organised with regard to international standards or instruments, as applicable, and with ATSB procedures and guidelines. Reports must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2025  Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Executive summary

What happened

On the morning of 23 April 2023, a Saab 340A, registered VH-KDK and owned by Pel-Air Aviation, was being operated by Regional Express Airlines (Rex) for a non-revenue flight from Wagga Wagga, New South Wales, to Charleville, Queensland. While in cruise at 22,000 ft and passing to the east of Cobar, New South Wales, the flight crew received a cargo smoke indication on the central warning panel. As a precaution, the crew fitted their oxygen masks and smoke goggles. Shortly after, the cockpit filled with smoke.

The captain commenced a diversion to Cobar while the first officer made a PAN-PAN[1] call to Melbourne Centre. Thick smoke then filled the flight deck preventing the crew from effectively seeing external visual references or the aircraft’s flight instruments. While completing the emergency checklists, the crew received further warnings for avionics smoke, followed by a cabin depressurisation, and then a right engine fire detection fail indication. The crew landed at Cobar and elected to stop on the runway and evacuate the aircraft.

Shortly after landing, Fire and Rescue New South Wales personnel arrived from the Cobar station and located a heat source at the air cycle machine and in the associated wiring. After gaining access to the cabin underfloor area, the source of the heat was doused with water. An internal inspection of the aircraft found fire damage in the area around the right recirculating fan. The aircraft was substantially damaged, and the crew were not injured.

What the ATSB found

The ATSB found that a likely failure of the right recirculating fan electronic box sub-assembly resulted in an in-flight fire under the cabin floor. The fire filled the cabin with smoke, which then entered the flight deck due to a smoke barrier curtain not being fitted in place and the flight deck door being open. 

When the crew fitted their oxygen masks, it was found that the first officer’s mask microphone was not working correctly, which delayed emergency checklists being actioned. The fire also caused substantial structural damage and led to a breach of the fuselage, resulting in a depressurisation of the aircraft. 

It was also found that the Rex flight crew had not been trained or had knowledge of the differences in the cargo‑configured Saab 340 aircraft, leading to them having no familiarity with specific systems fitted. This prevented them from completing some of the required steps in the emergency checklists. 

The flight crew did not receive training on the cargo‑configured aircraft differences prior to conducting freight operations. Further, the operator’s flight crew operating manuals did not reflect the differences in the cargo‑configured aircraft interior checklists, which may have alerted the flight crew to these differences during pre-flight preparation. Additionally, the manufacturer did not have any specific pre-flight check for correct fitment of the smoke barrier curtain for cargo‑configured aircraft preparation. 

What has been done as a result

On 13 May 2023, Rex issued an Operations Notice to all pilots, highlighting the guidance on the cross-valve handle as outlined in the Saab Aircraft Operating Manual (AOM). Furthermore, this guidance has been incorporated into the Rex Flight Crew Operating Manual (FCOM). 

On 13 November 2024, Rex amended the internal inspection checklist that is contained in their Saab 340 FCOM. The amendment now requires flight crews verify the position of the cross‑valve handle during the pre-flight checks.

Rex have also updated the training information delivered in their ground school to include the cross-valve system for the cargo‑configured Saab 340 aircraft into the training syllabus.

Rex indicated they are implementing a fleetwide inspection of the recirculating fan assemblies at the next aircraft heavy maintenance cycle, with a focus on the electronic sub-assembly module.

Pel-Air have included a revision to their flight crew operating manual with a caution that the smoke barrier curtain must be installed whenever combustible material is carried in the cargo compartment. Due to contract completion, Pel-Air have ceased conducting freight operations using the Saab 340 aircraft and have since sold the aircraft. 

Saab has revised their preparatory and walk-around pre-flight checklists to include the fitting of the smoke barrier curtain when carrying cargo in the cargo‑configured aircraft. 

Safety message

It is essential for operators to ensure that flight crew are conversant with differences in aircraft configurations when required to conduct operations on aircraft they may be unfamiliar with. It is important that information is readily available and accessible and be delivered in a manner to inform flight crews on the operational requirements of the aircraft. 

Operator flight crew operating manuals need to be relevant for the aircraft configuration being utilised. Further, manufacturer checklists for pre-flight inspections are required to cover the modifications fitted, so that this is available to operators to enable them to write the appropriate documentation for their flight crews.

The occurrence

History of the flight

On the morning of 23 April 2023, a Pel-Air Aviation cargo‑configured Saab 340A, registered VH‑KDK and operated by Regional Express Airlines (Rex), was being prepared for the first stage of a non-revenue freight flight from Wagga Wagga, New South Wales, to Charleville, Queensland. The purpose of the flight was to pre‑position a Rex Saab 340 engine to Cairns, Queensland, utilising a Rex crew, consisting of a captain and first officer (FO), and operating under the Part 91 flight rules. 

The crew had flown the aircraft from Cairns the previous day, and were tasked to fly the return leg, including a refuelling stop at Charleville. The flight crew arrived at the aircraft at about 0830 local time and conducted their pre-flight checks. The captain performed the interior checks while the exterior walk around checks were conducted by the FO. The aircraft departed Wagga Wagga at about 0949, tracking for Charleville, and cruising at FL 220.[1] The captain was the pilot flying (PF), and the FO was pilot monitoring (PM).[2]

In-flight fire and diversion

At about 1052, the crew was alerted by a cargo smoke detection warning[3] on the central warning panel. They began to manage the warning by identifying and cancelling the indication. The crew then donned their oxygen masks and smoke goggles. However, once fitted, the crew had difficulty communicating due to the FO’s mask microphone not functioning correctly and being very faint. A review of the cockpit voice recorder (CVR) recording showed it took another 57 seconds for the crew to establish effective communications with each other before they could commence the emergency checks.

At 1055 the crew received a right (engine) fire detection fail caution light, followed shortly after by an air conditioner caution light. This was due to the detection of a right distribution duct over temperature which automatically closed the right engine low pressure bleed valve. The crew initially detected no smoke or fumes, however within about 60 seconds, thick black smoke began filling the cockpit.

During the process of establishing the nature of the warnings, the flight crew discussed which airport would best suit their needs for the emergency, and opted to divert to Cobar, New South Wales (Figure 1). The captain had flown to this airport on several previous occasions and was familiar with it, and from their position, they could conduct a direct approach with minimal manoeuvring.

Figure 1: VH-KDK flight overview

Source: Google Earth and Flightradar24, annotated by the ATSB

At 1056, the FO gave a PAN PAN[4] radio call and advised air traffic control (ATC) that they were diverting to Cobar. ATC arranged for emergency services to meet the aircraft upon arrival into Cobar. Due to the radio coverage in the Cobar area, ATC utilised an overflying aircraft to relay communications as the diversion progressed.

While descending through FL 160, the crew received a cabin pressure failure warning, indicating that the aircraft had lost pressurisation. This occurred 4 minutes after the initial warning of the cabin smoke. The cabin depressurisation led to the crew increasing their rate of descent to get below FL 100. 

At approximately 1058, they commenced the ‘cargo compartment smoke’ emergency checklist. The CVR indicated the crew completed the first checklist item, which called for the left bleed valve to be closed. The checklist then called for closing the cross-valve handle, however the crew was unable to locate it, with the CVR recording indicating the crew tried to find it for 61 seconds before resuming the checklist flow without actioning the cross-valve handle closure. The crew completed the cargo compartment smoke checklist as visibility on the flight deck reduced to less than 10 cm. The captain recalled sliding their seat forward to enable them to see the instrument panel through the thick smoke.

Five minutes after the PAN PAN call, the crew commenced the checklist for ‘avionic or electrical smoke or fire’.[5] The crew then conducted the ‘smoke removal’ checklist to clear the smoke from the flight deck. 

One requirement of the smoke removal checklist called for aircraft speed reduction to below 160 kt, and to open a crew hatch to aid in smoke removal. Due to the unknown severity of the on‑board fire, the crew decided to maintain their speed and not complete all the checklist items as required, enabling them to expedite their landing.

As the crew continued their approach, the volume of smoke in the flight deck began to dissipate. This allowed them to navigate using external visual cues and conduct a visual approach for the landing. Because of the unknown source of the fire, upon arrival in Cobar, the crew elected to stop the aircraft on the runway and evacuate the aircraft. 

As part of the emergency evacuation procedure, the crew activated the fire extinguisher system[6] on both engines after shutting down, and then exited the aircraft. The incident had taken 22 minutes from the first smoke warning to landing.

After evacuation, the crew noted smoke to be coming from the vicinity of the right air cycle machine at the right wing root. Shortly after landing, Fire and Rescue New South Wales personnel arrived from the Cobar station and assessed the aircraft. After gaining access to the cabin underfloor area, an electrical harness was initially found melted and smoking. Consequently, the area was doused with water. 

Further inspection of the aircraft found significant fire damage concentrated in the area around the right recirculating fan. The right recirculating fan was also significantly fire damaged. An assessment of the right engine found no evidence of a fire. The aircraft underfloor structure was substantially damaged, and the crew were not injured.

Context

Aircraft information

The Saab 340A[7] is a twin-engine turboprop aircraft designed and initially produced by Saab and Fairchild Aircraft. It is designed to seat 30‍–‍36 passengers in standard configuration and is powered by 2 General Electric CT7-5A2 turboprop engines.

VH-KDK was manufactured in Sweden in 1984 and first registered in Australia in February 1985. It was operated in passenger configuration by Regional Express Airlines (Rex), before being modified to cargo configuration in 2009. VH-KDK was then owned and operated by Pel‑Air Aviation.[8] 

At the time of the accident,[9] VH-KDK had accrued a total time in service of 48,130.6 hours and 60,046 landings and had flown 13.5 hours since the last maintenance. 

Flight crew information

Captain

The captain held an air transport pilot licence (ATPL) (Aeroplane), and a valid Class 1 aviation medical certificate. They reported a total flying time of 7,579 hours with about 5,070 of those being on the Saab 340. They had flown for the operator for about 10 years, and previously had flown for another regional airline and had operated into Cobar airport on numerous occasions. 

The captain had previously flown the Saab 340A in a passenger configuration but had not flown either the Saab 340A or B variant in a cargo configuration.

First officer

The first officer (FO) held an ATPL (Aeroplane), and a valid Class 1 aviation medical certificate with a restriction for vision correction. They had reported a total flying time of about 18,297 hours, having flown about 1,480.9 in the Saab 340B. The FO had not previously flown any Saab 340 variants in a cargo configuration.

Meteorological conditions

Graphical area forecasts provided by the Bureau of Meteorology (BoM) stated that generally good weather conditions, with little cloud, and visibility greater than 10 km existed during the flight. The terminal area forecast for Cobar indicated light winds from the east at about 10 kt with the flight crew reporting CAVOK[10] conditions existing for the time of the diversion.

Cargo configuration

VH-KDK cargo conversion

The cargo conversion modification was carried out in accordance with Saab service bulletin (SB) 340-25-280. As an overview, the SB involved removal of the passenger fit‑out and replacing it with an interior cargo liner, blanked over windows, additional cargo barrier nets, and a floor roller system (Figure 2). In conjunction with this SB, other SBs were incorporated on the cargo version, which modified the air conditioning system. This was done by removing the left recirculating fan and introducing a cross-valve handle. 

A removable smoke barrier curtain was added at the forward section of the cargo compartment. The fire extinguishing system for the passenger cargo area at the rear of the cabin (zones C1 and C2) was also removed, and additional smoke detectors were added to the cabin.

Figure 2: Cargo configuration modification

Source: Saab, annotated by the ATSB

Smoke curtain

A removable smoke barrier curtain was designed to be installed between compartment A and the front-left fuselage (Figure 2). The purpose of the curtain was to provide containment of smoke and fire within the cargo compartment in the event of an on-board fire and prevent smoke ingress to the flight deck (Figure 3). The aircraft carried a placard at the top of the entrance stairs which stated: 

Smoke barrier must be installed for all cargo operation flights.

The smoke barrier was constructed of a fibreglass impregnated vinyl which was secured in place by a Velcro perimeter and metal press studs. The aircraft owner stated that the standard procedure was that it would be left attached by the left side attachment points and secured in place by the freight handlers after loading of cargo. It was then to be checked by the FO prior to flight. 

During interview, the flight crew stated that they were not aware of the use of the smoke barrier, and that it had been located after the incident in compartment A of the cargo area, and that it was not in place on the left of the cabin. The ATSB did not determine why the engineers who loaded the engine into VH-KDK had not secured the smoke curtain.

Figure 3: Smoke barrier curtain location in exemplar aircraft

Source: Saab, annotated by the ATSB

Air conditioning system 

In passenger configuration, the Saab 340 air conditioning system is comprised of a left and right air conditioning pack (ACP) which is supplied by bleed air from its respective engine. Each ACP is mounted externally under a fairing on the lower fuselage near the rear of each wing. The system has 2 recirculating fans under the adjacent cabin floor, ducting for the cabin and flight deck conditioned air supply and return, temperature sensors and controls, and cabin and flight deck air outlets. 

The left and right ACPs supply conditioned air to the cabin, and a portion of the right ACP conditioned air is supplied to the flight deck. The left recirculating fan returns the air to the left ACP from the aircraft cabin, while the right fan extracted air from the flight deck. The avionics fan draws air for cooling the avionics from the cabin conditioned air supply. The air is then expelled under floor. 

As part of the modification from passenger to cargo configuration, the left recirculating fan and ducting were removed. This resulted in limited extraction and recirculation of any contaminated air from the cabin interior, while the right recirculating fan would extract and recirculate air solely from the flight deck. 

Cross-valve handle

In the cargo configuration, a cross-valve and handle were added to the air conditioning system ducting between the left and right ACP (Figure 4), with the manual operating handle being located at floor level, next to the FO seat. Closing of the left bleed valve and cross-valve in accordance with the emergency checklist would isolate the supply of air to the cabin in the event of cargo smoke or fire, and the right ACP would supply only to the flight deck. As part of the emergency checklist for ‘cargo compartment smoke’, the left ACP would also be isolated from supplying the cabin. 

Figure 4: Schematic of modified air conditioning system

Source: Saab, annotated by the ATSB

Recirculating fan

The right recirculating fan was a centrifugal impeller type fan and was driven by an AC motor. The fan had an in‑built inverter, supplied by 28-volt DC. The majority of the electronics for the fan unit were contained in the box sub-assembly. This included the inverter, an electromagnetic interference (EMI) filter unit and the electronic card sub-assembly. The box sub-assembly controlled the fan operation, including the thermal control. Each electric motor was equipped with:

• a thermal switch[11] located in the cooler which guarded against an abnormal temperature increase. This switch would cut off the power if the motor temperature exceeded 110°C +/− 5° (230°F +/− 41°). The fan would start again when the temperature decreased to 65°C +/− 5° (149°F +/− 41°)
• a speed sensor which guarded against an abnormal decrease of nominal speed. If the speed fell below 80% of nominal speed for more than 17 seconds, the fan would be stopped.

In 1987, Saab released a service bulletin which gave operators the option to install an upgraded recirculating fan. The original fan was a brush type motor which required regular maintenance, including brush replacement when they had worn from use. Saab had received reports of smoke and burning smells which were attributed to brushes that were incorrectly installed. The brushless fans were introduced to help eliminate this issue and also required less maintenance. 

The fan installed in VH-KDK was the new brushless fan, manufactured in 1990 and fitted in April 1996. The fan history prior to installation was unknown by the operator. The fan accrued 27,585 hours while fitted to VH-KDK. 

Recirculating fan examination

Fire damage was found in the area around the right recirculating fan and on the fan itself (Figure 5). There were no other components in the vicinity of the fan with significant fire damage. As such, it is likely the right recirculating fan was the source of the fire.

Figure 5: Damaged recirculating fan

Source: ATSB 

An examination of the recirculating fan was conducted at the ATSB’s technical facilities in Canberra. The examination found that the fire damage was most significant at the external box sub‑assembly which housed the EMI filter, the resistor support plates and electronic card sub‑assembly. The aluminium cover of the box sub-assembly had melted, the electrical wiring was damaged, and some terminals had disconnected as a result of the fire damage. There was heavy soot and melting of solder in the cooler assembly.

The electronic card sub-assembly was made up 3 circuit boards, mounted to the resistor support plate. The function of the circuit boards was for motor speed detection, timer control, and a logic card. The damage exhibited on the circuit boards showed significant burning, consistent with the other components within the aluminium cover.  

The electromagnetic interference (EMI) filter sub-assembly had considerable damage to the filter itself and to the mounting plate with signs of melting and heat tinting of the steel plate structure, indicating a significant heat source. The heat tint was indicative of temperatures of approximately 310° to 330°C. 

Of note, the fire did not appear to be associated with the motor and there was no indication of damage to the internal components of the fan and was able to rotate freely. There was carbon and soot observed on the external surface on the motor. The crew stated that there had been no circuit breakers tripped that would be associated with the failure of the electronic card sub‑assembly.

The ATSB examination of the recirculating fan could not determine the cause of the failure of the electronic components attached to the assembly. 

Pressurisation system

The Saab 340 cabin is pressurised by the 2 air conditioning packs. The pressurisation system uses bleed air drawn from each engine and was either automatically controlled by a pressurisation controller, or manually controlled by a control valve operated by the flight crew from the flight deck. 

Pressure is able to be regulated by the opening and closing of 2 outflow valves, located in the empennage. The primary outflow valve is electro-pneumatically operated by the pressurisation controller, while the secondary outflow valve is pneumatically controlled from the cockpit and used as a manual standby system.

When emergency pressure relief is required, the primary outflow valve is able to be opened with the emergency pressure dump switch. When the crew of VH-KDK experienced the smoke in the cabin and flight deck, dumping cabin pressure as stated in the ‘smoke removal’ emergency checklist was the method used to assist in rapid removal of the smoke.

Aircraft depressurisation

The severity of the fire resulted in significant damage to the surrounding underfloor furnishings, ducting and airframe structure. The damage was then sufficient to rupture the skin (Figure 6) and caused a subsequent depressurisation of the aircraft. 

Figure 6: Underfloor fire damage showing fuselage hole

Source: Operator, annotated by the ATSB 

Checklists

The pre-flight procedures for the Saab 340A aircraft, including cargo configuration aircraft, were covered by the Aircraft Operations Manual (AOM) normal procedures, produced by the aircraft manufacturer. The AOM pre-flight normal checklist contained a check of the cross-valve handle position prior to flight but did not include a specific check for correct fitment of the smoke barrier curtain. 

A flight crew operating manual (FCOM) was carried on board VH-KDK which was developed by Pel-Air, based on the aircraft manufacturer’s AOM. Unlike the aircraft manufacturer's AOM, the pre-flight checks in the operator's FCOM did not contain any reference to the cross-valve handle. Consistent with the aircraft manufacturer’s AOM, the operator’s FCOM also did not include a specific pre-flight check for correct fitment of the smoke barrier curtain. The weight and balance chapter of the FCOM did show a diagram of the smoke curtain fitted but did not include a requirement to ensure the smoke curtain was in place.

The manufacturer had an airplane flight manual supplement in place, implemented as part of the cargo configuration SB, which included fitting the smoke barrier as a limitation (Figure 7 left). There were no identified interior checks in this supplement, which included the checking of the cross-valve handle prior to flight.

The aircraft manufacturer did have emergency checklists specific to cargo‑configured aircraft. These were compiled into the quick reference handbook (QRH) which was available to flight crew in the aircraft (Figure 7 right). 

Figure 7: Flight manual supplement (left) highlighting smoke curtain use and QRH checklist (right) highlighting cargo cross-valve handle and cockpit door 

Source: Saab, annotated by the ATSB

Flight deck door

The Pel-Air FCOM stated in the Operating Limitations section that during flight, the flight deck door must be kept closed and locked at all times. It is further listed in the engine start checklist that all doors are closed before engine start. 

Evidence from the accident flight, however, shows that before the fire, the crew were operating with the cockpit door open. During interview, the flight crew indicated they closed the door, whilst performing the cargo compartment smoke checklist. This was further supported by the CVR review, where the crew were heard to state the door was to be closed as per the checklist steps, which was followed by the sound of the door shutting. 

Flight crew training

Rex conducted a ground school, including simulator training, for their new Saab 340 flight crew. Under a commercial agreement, Pel-Air flight crews were also trained by Rex. The ground school covered the 340 variants of A, B and B WT. The aim of the ground school was to provide pilots from both operators with the necessary knowledge to gain a Saab 340 type rating. The type rating covered all Saab 340 aircraft and did not distinguish between any variant or configuration of the aircraft.

Following the Rex ground school, Pel-Air then conducted further training through its line training program for its flight crews allocated to freight operations. Delivery of this training was in a practical environment, with pilots learning the systems and differences of the cargo‑configured Saab 340. This included the use of the smoke barrier and the operation of the cross-valve handle. 

The ATSB asked what the process was for Rex pilots to receive this training and knowledge. Pel‑Air advised that a pilot employed by Rex would only receive this if they were to transition to freight operations with Pel-Air in a permanent role. 

Crew familiarity of cargo aircraft

The flight crew operating VH-KDK were both Rex pilots, who were normally rostered for passenger operations. The night before the flight to Wagga Wagga, they were rostered to fly the cargo‑configured 340A. The captain recalled asking the scheduler if they needed a briefing for flying the cargo 340A, and was told that they did not, but it could be arranged if needed. 

The captain decided to speak to a Rex colleague who had flown the cargo‑configured 340A several times previously. They were told there were no differences other than the removal of the seats and a freight interior being fitted and that there were no special procedures to be aware of. 

Both Rex and Pel-Air advised that their respective operations could roster a crew to fly either operator’s aircraft if it was available. Crewing arrangements were such that there was never any mixing of crew, that is that the flight crew would consist of either 2 Rex or 2 Pel-Air flight crew on any flight.

Oxygen mask

The flight crew fitted their oxygen masks and smoke goggles shortly after receiving the cargo smoke warning and smelling the smoke. The FO conducted the functional tests after fitment and found their microphone was barely readable by the captain. The cockpit voice recorder (CVR) indicated that, although muffled, the speech from the FO was recorded and they were also able to be heard by air traffic control (ATC) throughout the emergency.  

The pre-flight checks relating to the crew oxygen system were conducted as part of the interior checklist. The FCOM detailed the checks as: 

RIGHT OXYGEN MASK .................................................................................CHECKED

Check flight crew oxygen mask and microphone in accordance with the following: 

•  …, 

•  set audio panel BOOM - MASK switch to MASK, 

•  increase INT/SPKR volume and knock on mask, 

•  speaker noise indicates proper microphone function, 

•  set BOOM - MASK switch back to the BOOM position, ….

The check on the left oxygen mask was to be performed in the same manner. Although not detected on the CVR, the captain advised the masks were both tested prior to departure. 

Aircraft damage

The damage caused by the underfloor fire was substantial. The fire had damaged underfloor air conditioning ducting and electrical wiring (Figure 8).

Figure 8: Underfloor fire damage

Source: Operator, annotated by the ATSB

Structural components in the surrounding area had been distorted by the extreme heat, including the floor panels, which had collapsed when fire crews entered the aircraft and inadvertently walked over the affected area. The seat track support structure had distorted, and the fuselage was weakened by the fire which breached the outer skin, preventing the aircraft from remaining pressurised (Figure 9).

Figure 9: Fuselage skin breach

Source: Operator, annotated by the ATSB

The fuselage in the immediate area above and below the cabin floor was buckled and delaminated. The heat from the fire most likely travelled between the interior panels and freight lining, leading to the damage observed (Figure 10). Following an engineering inspection of the fire damage, the aircraft was withdrawn from service and not repaired.

Figure 10: Right side delamination on fuselage

Source: Operator, annotated by the ATSB

Recorded data

The ATSB was supplied raw data by the operator from the flight data recorder (FDR). This data was analysed and was found to include the previous 4 flights. 

The recorded data from the occurrence flight showed that at about 59 minutes after becoming airborne at Wagga Wagga, the FDR stopped recording. The recording stopped at about the same time as the initial smoke indication. This was most likely due to fire damage to the electrical wiring which controlled the FDR. 

Flight track information was also obtained from FlightRadar24, which showed the entire flight, including the diversion and landing at Cobar (Figure 11).

Figure 11: VH-KDK flight track and diversion

 Source: Google Earth and Flightradar24, annotated by the ATSB based on CVR recordings

The CVR was removed and sent to the ATSB technical facilities in Canberra. The CVR data was downloaded, with the recovery of 4 channels of audio data of about 120 minutes duration which included the in-flight fire event. Reviewing the recorded CVR data also revealed that coincidentally, just prior to the indication of the cargo smoke caution, the flight crew had been discussing alternate airports in the area, and which one they would select if they had a need to divert. 

The recording contained information from the end of the previous flight, and from the day of incident. It included the:

• flight crew’s initial reaction to the caution warning for the smoke in the cabin
• conduct of the emergency checklists
• additional warnings as they occurred
• crew intercom and communications with Melbourne Centre
• landing at Cobar and subsequent exiting of the aircraft.

The recording also revealed that while conducting the emergency checklist for ‘cargo compartment smoke’ the crew closed the cockpit door as a loud bang was heard, indicating it was open during the flight.  

Safety analysis

Introduction

On 23 April 2023, the Regional Express (Rex) Airlines flight crew operating a Pel-Air Aviation Saab 340A, registered VH-KDK were conducting an internal revenue cargo flight from Wagga Wagga, New South Wales, to Charleville, Queensland. About 1 hour into the flight, the crew experienced an in-flight fire and diverted to Cobar, New South Wales. After experiencing thick smoke on the flight deck and then a cabin depressurisation, the crew performed a safe landing at Cobar. The aircraft was substantially damaged, and the flight crew were not injured. 

This analysis will explore:

• origin of the in-flight fire
• aircraft preparation for the flight
• oxygen mask use and technical problem of the microphone
• cabin depressurisation
• flight crew knowledge of aircraft systems
• flight crew operating with the cockpit door open
• Pel-Air and Rex flight crew operating manual information deficiencies
• training of Rex flight crews
• Saab pre-flight inspection checklists
• Rex crew familiarity of cargo aircraft. 

Origin of the in-flight fire

The source of the in-flight fire was traced to the right recirculating fan assembly. Although the fan was not damaged internally, the fire damage was most significant at the box sub-assembly, which was mounted external to the fan and housed the electrical control circuit boards. It is likely that an electrical component or components within the box sub-assembly failed, resulting in the underfloor fire. The fire damaged underfloor insulation and plastic air conditioning ducting components, which led to thick smoke filling the cabin and cockpit and aircraft structural damage. 

The avionics warning received by the crew during the diversion was most likely associated with the avionics cooling air that was being drawn from the now smoke-filled cabin. This was also stated in the ‘cargo compartment smoke’ checklist. 

The ATSB examination of the recirculating fan could not determine a cause for the failure in the electronic control cards which led to the fire. 

When the crew received the air conditioning system right duct over temperature caution light, it was most likely due to the distribution duct over temperature being affected by the fire and the melting which occurred as a result. When the over temperature was sensed, the right bleed valve closed automatically as a function of system logic for over temperature protection.    

Smoke barrier curtain

Cargo operations can have a greater fire risk than passenger operations due to the carriage of cargo that could be the source of a fire and the lack of cabin crew available to fight a fire. As such, additional protection was available to minimise flight crew exposure to cabin smoke in the form of additional smoke detectors and a smoke curtain. 

However, the smoke curtain was not installed into position by anyone involved in the flight preparation. The flight crew, who normally operated the same aircraft type but in a passenger configuration, did not notice there was a placard at the aircraft entrance stating the smoke curtain was to be fitted for all cargo flights. The flight crew remained unaware of the smoke barrier curtain and its use for cargo operations. Further, the engine being transported was positioned in the cargo area of VH-KDK, on both occasions, by Rex engineers. As the curtain was usually installed by freight handlers during normal cargo operations, it is possible the Rex engineers were also unaware of its requirement to be fitted.  

In this accident, the source of the fire was an aircraft component rather than the cargo being carried. If a similar fire occurs in a passenger‑configured Saab 340 aircraft, then the smoke curtain would not be in place. However, the smoke curtain was available and was required for use for this flight, so its non-use increased risk for this event.

The flight deck door was not closed during flight as prescribed in the operator’s FCOM operating limitations and checklists. Having the door closed would have likely prevented the smoke being able to flow into the flight deck. 

The result of not having the smoke barrier fitted and the flight deck door closed as part of the aircraft pre-flight preparation was that smoke from the fire was not contained to the cabin area and was able to move forwards toward the flight deck. 

Oxygen mask fault

The flight crew fitted their oxygen masks and smoke goggles when alerted of the presence of smoke by the central warning panel. This decision may have prevented the crew from being overcome by smoke and fumes in the cockpit in the next several minutes. However, once fitted, the crew had difficulty communicating with each other, as a result of the mask microphone being very faint and difficult for the captain to hear the first officer (FO). This appeared to be an internal fault only, as the cockpit voice recording (CVR) showed that the FO was able to be adequately heard by ATC. 

This breakdown of communication delayed the crew by 57 seconds, in which emergency checks were not initiated due to the breakdown of communication. It created confusion and distraction between the crew while trying to execute the emergency checklist. 

A review of the CVR captured prior to flight could not positively determine if the pre-flight check action in relation to the oxygen mask was performed. This is an important check of the emergency communication system whilst on oxygen and was designated as a mandatory check item for a daily inspection as required by the Rex and Pel-Air FCOM.

Cross-valve handle

While the flight crew were conducting the emergency checklist items for cargo compartment smoke, they were unable to locate the cross-valve handle. This was due to the combination of the thick smoke obscuring their vision and their lack of knowledge of the differences in the cargo‑configured aircraft. 

Had the location and function of the cross-valve handle been known by the flight crew, the time taken to identify it during completion of the emergency checklist would have been minimised, which would have limited the delay of smoke removal from the flight deck. 

In this case, the subsequent depressurisation resulted in the smoke dissipating even in the absence of the cross-valve.     

Cabin depressurisation

The weakening of the fuselage structure due to the underfloor fire resulted in a breach of the fuselage skin, which led to a subsequent depressurisation of the aircraft during the descent. Although adding to another caution alert indication for the flight crew and subsequent checklist to be conducted, it also benefited in the removal of smoke from the cabin and flight deck. 

At the time of the depressurisation, VH-KDK was at FL 160 and descending. The crew, when alerted to the depressurisation, increased the rate of descent to below 10,000 ft. The cabin depressurisation occurred 4 minutes after the initial smoke warning occurred.

Due to the size of the hole created, the smoke removal most likely occurred at a greater rate than using the aircraft pressurisation outflow valves alone. The resulting fortuitous reduction in the amount of smoke in the flight deck improved visibility and allowed the crew to carry out a safe landing into Cobar.

Crew familiarity of cargo‑configured aircraft

The flight crew had not flown or had any prior training on the cargo‑configured Saab 340 and were not familiar with the differences of the passenger configuration. The captain chose to liaise with a colleague to gain information on the cargo‑configured aircraft instead of accepting the company offer for a briefing. 

This non-formal approach to understanding the differences between the 2 aircraft types ultimately did not pass on the required operational differences and potential safety aspects of the change of aircraft configuration.

Rex and Pel-Air manuals

Both operators (Rex and Pel-Air) manuals, which were designed to provide essential information to flight crews, did not include the required information to enable the pre-flight checks to be conducted adequately. While the weight and balance chapter of the FCOM showed the smoke barrier curtain location, there was no information on its importance to cargo operations, and as the crew had not been informed of any differences, they would not have been expecting that this section contained this information. The smoke barrier curtain installation information that was contained in the Saab service bulletin and flight manual supplement was not included in the pre‑flight checklists. As a result, the flight crew did not have awareness of its use. 

The operators' manuals also did not have a check to verify the position of the cross-valve handle. As discussed above, when the checklist called for the crew to use this handle when the aircraft was already filling with smoke, the crew could not locate it.

Flight crew training

The Rex ground school provided type rating training on the Saab 340 series aircraft to both Rex and Pel-Air pilots. This training was based on the passenger‑configured aircraft. Pel-Air pilots undertook further training which gave them the knowledge and skills for the cargo‑configured aircraft. 

In scheduling their flight crews to operate the cargo‑configured Saab 340, Rex did not have a process to ensure that the additional training or knowledge sharing for their crews in the differences applicable to aircraft operated by Pel-Air was delivered.

Saab pre-flight checklists

As discussed above, both operators’ manuals had no inclusion of a pre-flight interior check for the smoke barrier curtain or the cross-valve handle. Likewise, there was no pre-flight interior check for these items in the manufacturer’s documentation. Saab confirmed that there were no checks in the pre-flight checklist for the crew to specifically verify that the smoke barrier curtain was correctly fitted.

The result of the manufacturer’s pre-flight and interior checklists not detailing information for the smoke curtain was that the operator did not detail these in their own FCOM. This information was not available for the flight crew who, even without prior knowledge of the cargo‑configured variant, would have been alerted to these changes while conducting these pre-flight checks in accordance with the FCOM. 

Findings

From the evidence available, the following findings are made with respect to the in-flight fire and cabin smoke involving a Saab 340A, VH‑KDK, 114 km east-north-east of Cobar, New South Wales, on 23 April 2023. 

Contributing factors

• It was likely that an electrical component of a control circuit board on the recirculating fan failed, resulting in an in‑flight fire under the cabin floor.
• The smoke curtain was not fitted as required for the cargo configuration, and the flight deck door was open, which allowed smoke from the in‑flight fire to enter the flight deck.
• The underfloor fire caused weakening of the fuselage structure, which led to a subsequent depressurisation of the aircraft during the descent. However, the depressurisation aided in the removal of enough smoke from the flight deck on approach to allow an unhindered visual approach at Cobar.
• Crew were not familiar with the cargo configuration and were unaware of the smoke curtain requirements and location of the cross-valve handle.
• The Pel-Air and Rex Saab 340 flight crew operating manuals did not include reference to the location and operation of the cross-valve handle or the operation and use of the smoke curtain. (Safety issue)
• Rex did not ensure its flight crews received training in the differences between passenger and freight‑configured Saab 340 aircraft, prior to being scheduled to fly freight operations. (Safety issue)
• Saab did not include the smoke curtain fitment in pre-flight documentation for the cargo‑configured Saab 340 aircraft to inform flight crew of this difference from the passenger-configured version. (Safety issue)

Other factors that increased risk

• When the flight crew donned their oxygen masks, the first officer's oxygen mask microphone did not function correctly. This led to difficulty in communication between the flight crew and a delay in responding to the emergency.
• Due to the combination of the smoke density and lack of prior knowledge, the flight crew were unable to locate the cross-valve handle during the emergency, therefore delaying the removal of smoke from the flight deck.

Safety issues and actions

Saab documentation for cargo configured aircraft

Safety issue number: AO-2023-020-SI-01

Safety issue description: Saab did not include the smoke curtain fitment in pre-flight documentation for the cargo‑configured Saab 340 aircraft to inform flight crew of this difference from the passenger‑configured version.

Operator documentation and crew familiarity

Safety issue number: AO-2023-020-SI-02

Safety issue description: The Pel-Air and Rex Saab 340 flight crew operating manuals did not include reference to the location and operation of the cross-valve handle or smoke curtain.

No formal company training

Safety issue number: AO-2023-020-SI-03

Safety issue description: Rex did not ensure its flight crews received training in the differences between passenger and freight‑configured Saab 340 aircraft, prior to being scheduled to fly freight operations.

Safety action not associated with an identified safety issue

Additional safety action by Regional Express

Rex advised that it is implementing a fleet‑wide inspection of the flight deck and passenger compartment recirculation fans at the next aircraft heavy maintenance visit. The inspection will be focused on the electronic sub-assembly module of the recirculation fan due to this component being identified to have the most significant fire damage on the fan assembly removed from VH‑KDK.

Glossary

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the crew of VH-KDK
• Regional Express Airlines
• the chief pilot of Pel-Air Aviation
• the manager training and checking and head of operations for Regional Express Airlines
• Civil Aviation Safety Authority
• Bureau of Meteorology
• Fire and Rescue New South Wales
• Saab
• Airservices Australia
• the cockpit voice recorder and flight data recorder
• recorded data from Flightradar24. 

References

Australian Government 2023, Part 91 (General Operating and Flight Rules) Manual of Standards 2020, Civil Aviation Safety Authority, Canberra, ACT, viewed 30 April 2024, <Federal Register of Legislation - Part 91 (General Operating and Flight Rules) Manual of Standards 2020> 

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• Civil Aviation Safety Authority
• Airservices Australia
• Crew of VH-KDK
• Regional Express Aviation
• Pel-Air Aviation
• Swedish Accident Investigation Authority (SHK)
• Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA).

Submissions were received from: 

• the captain of the crew
• Regional Express Aviation
• Pel-Air Aviation
• Swedish Accident Investigation Authority (SHK)
• Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA).

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2024 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

On 16 March 2020 a Bombardier DHC-8-402 registered VH-QOE, departed Jacksons International Airport, Port Moresby, Papua New Guinea. Shortly after take-off, the flight crew detected fumes in the cockpit. Passing FL 100 on climb, the fumes became stronger and the cabin crew also reported detecting a fume smell in the cabin.

The flight crew donned oxygen masks and levelled off at FL 180. The first officer then made a PAN call and requested a return to Port Moresby. After switching off the number one bleed air, as per the quick reference handbook for smoke and fumes, the flight crew observed smoke emanating from the air vents. The crew switched off the number 2 bleed air and depressurised the cabin once the aircraft had descended below FL 100, and the smoke dissipated.

The Papua New Guinea Accident Investigation Commission (AIC) investigated the incident and requested assistance from the Australian Transport Safety Bureau (ATSB).

To facilitate this work the ATSB initiated an external investigation under the provisions of the Transport Safety Investigation Act 2003. 

The ATSB has now concluded its involvement in the investigation. 

The AIC is responsible for and will administer the release of the final investigation report into this incident.

Any enquiries relating to the investigation should be directed to the Papua New Guinea Accident Investigation Commission.

What happened

On 14 October 2017, a Boeing 777-300 aircraft, registered A6-ETR and operated by Etihad Airways, was on a scheduled passenger service from Abu Dhabi, United Arab Emirates (UAE) to Sydney, New South Wales. An augmented flight crew, consisting of two pilots in each crew (crew A and crew B) conducted the flight.^[1]

At about 0407 Central Daylight-saving Time,^[2] while in the cruise and with flight crew B flying the aircraft, the flight crew noticed a burning smell coming from an air vent. In an attempt to establish the source of the smell, they requested that cabin crewmembers check the forward galley. The cabin crew confirmed that the forward galley was clear of any burning smells or smoke. The flight crew then requested two other cabin crewmembers enter the flight deck, who confirmed the burning smell. Around this time, the aural fire bell activated, a master warning light illuminated and a warning message ‘FIRE CARGO FWD’ was displayed on the engine-indicating and crew‑alerting system.

In response, the flight crew actioned the non-normal checklist, which included arming the forward cargo fire switches located in the flight compartment overhead panel. This action resulted in numerous mechanical and electrical actions, including de-energising the recirculation fan^[3] and closing the air vents in the forward cargo compartment. The flight crew then selected the cargo fire discharge switch, which discharged the two fire extinguisher bottles located in the forward cargo compartment.^[4] The flight crew declared a MAYDAY^[5] to air traffic control and advised of their intention to divert to Adelaide Airport, South Australia, as it was the nearest suitable airport for the aircraft type.

Flight crew A had just completed their scheduled rest period and entered the flight deck where they were briefed by flight crew B of the situation. Flight crew A assumed control of the aircraft as they were the designated crew for landing. Flight crew B remained on the flight deck to provide assistance. A rapid descent to flight level (FL)^[6] 125 was conducted and the aircraft was diverted to Adelaide.

During the remainder of the flight, the cabin crew, operator and passengers were informed of the situation and the diversion. The flight crew also advised air traffic control that, if smoke or fire from the forward cargo compartment was confirmed by emergency services upon landing, they would evacuate the aircraft on the runway.

At 0455, the aircraft landed uneventfully. The emergency services advised the flight crew that they did not observe any smoke or fire emanating from the aircraft. The aircraft was taxied from the runway to taxiway ‘F6’, where the emergency services inspected the aircraft externally with a thermal imaging camera. They confirmed that there were no identified hot spots indicating an on‑going fire in the forward cargo compartment. Based on this information, as a precaution, the crew decided to conduct a rapid deplane of the passengers through passenger door 5L using mobile boarding stairs. All passengers and crew disembarked in a controlled manner and were transported to the passenger terminal. Nil injuries were reported during the disembarkation.

Initial engineering inspection

Once the forward cargo compartment was emptied of cargo, maintenance engineers inspected the cargo hold for evidence of fire. A small quantity of soot was identified in the cargo ceiling area, between the fiberglass ceiling panel and fiberglass joint sealing tape about aircraft body station (BS) 508 (Figure 1).

Figure 1: Illustration showing aircraft structure in the forward cargo hold in relation to heat damage from electrical arcing about BS 508

Source: The Boeing Company, modified by the ATSB

The ceiling panels were removed where soot was identified in the area between the lower side of the cabin floor and the upper side of the cargo-ceiling panel (Figure 2). Inspection of that area found heat damage and chafed 115-volt electrical wire in wiring loom P/N W5279-3002R-12 (W5279), which supplied power to the right lower recirculation fan. The chafing enabled the wire core to come in contact with a cargo ceiling panel retainer screw where the short circuited wire tracked through the polyetheretherketone resin (PEEK) stand-off brackets and carbon fibre floor beam.

Figure 2: Damaged floor beams, webs and wiring covered in soot with ceiling panel opened

In consultation with Boeing and operator’s aviation regulator, the United Arab Emirates General Aviation Authority, the operator temporarily repaired the wiring and the damage to the floor beams were evaluated. The operator conducted a non-revenue flight (nil passengers) where they flew the aircraft back to the UAE for the purpose of further inspections and permanent repairs.

Detailed engineering inspection

A detailed inspection between the forward cargo ceiling and passenger floor was conducted at the operator’s maintenance facility in the UAE. Wire bundle W5279, located at about BS 508 was found to have been incorrectly routed. Consequently, the wires had come into contact with screws and nutplates used to close out the cargo-ceiling panel to the ceiling standoff clips.

Over a prolonged period of time, the 115V recirculation fan wire located within that bundle chafed through the insulation coating, allowing the wire to short circuit. The electrical wiring and fourteen of the cargo ceiling panel standoff clips manufactured from PEEK were heat damaged. Sections of the carbon fibre beam web and beam flange at BS 508 between the left buttock^[7] lines 40 to 60 were also found to be heat damaged and delaminated between 6 and 7 percent in three locations where the current tracked (Figure 3).

Figure 3: Boeing 777 aircraft showing the approximate location of the heat damaged ceiling panel, soot and heat damaged wiring loom

Source: The Boeing Company, modified by the ATSB

Boeing determined that the wiring loom W5279 was likely to have been incorrectly positioned during the aircraft build in 2013. Boeing reported that this was the fifth reported incident involving wire chafing and arcing in the cargo area of a Boeing 777 aircraft. However, this was the first event that triggered the cargo fire warning system and that had been detected in flight. In all of these cases, the wiring loom had been installed incorrectly during manufacture, allowing screws to chafe wires and short circuit.

Recirculation fan wiring protection system

The 115V recirculation fan wiring system is protected by an electrical load control unit (ELCU) that is located in the aircraft’s main equipment centre. The ELCU is designed to protect the electrical circuit from over-current or differential loads by automatically opening (‘tripping’) to remove power. In this incident, despite the damage sustained to recirculation fan wiring it was reported that the ELCU did not open.

The circuit was tested during the repair of the wiring loom where it was identified that the ELCU functioned as designed. Boeing surmised that in this case, it was possible that the chafed wire may have been intermittently shorting to earth through the PEEK stand-off brackets. It is likely that the insulation properties of the PEEK prevented sufficient current draw to trip the ELCU.

Boeing also surmised that it was likely that the crew’s action of arming the forward cargo fire switches de-energised the chafed wire within loom W5279, thereby preventing further current flow and short circuit.

Cargo compartment fire protection

Materials used in the construction of passenger compartment interiors and in the space between the cabin floor and cargo ceiling are required by the United States Federal Aviation Administration to be self-extinguishing (i.e. stop burning after the heat source has been removed) or better. For example, electrical wire and cable insulation must be self-extinguishing. Cargo liners form part of the passive fire protection feature. In addition, the primary purpose of a cargo liner is to prevent a fire, originating in a cargo compartment, from spreading to other parts of the aircraft and to seal the compartment to help contain the suppression agent in that area.

In Class C cargo compartments, which include the lower cargo compartments of all passenger aircraft, the sidewall and ceiling liner panel installations are fire tested to determine flame penetration resistance. All other materials must be self-extinguishing.

Safety analysis

The flight crew identified a burning smell in the flight deck and completed the appropriate actions to manage the situation. By arming the forward cargo fire suppression system, electrical power was removed from the recirculation fans, which prevented further arcing and damage to the structural carbon fibre beam, support brackets and wiring. Even though there was a significant amount of soot and electrical arcing, de-energising the electrical circuit manually before sufficient current went to ground negated the electrical load control unit from tripping. It was likely that, once the electrical current was deactivated by arming the forward cargo fire switches, the smoke had also stopped. Discharging the fire bottles in the forward cargo space, even though procedurally correct, had nil effect on this occasion as the source of the electrical arcing was in the sealed zone between the cargo ceiling panel and the passenger floor compartment, not in in the cargo compartment.

A post-incident inspection of the aircraft found an electrical wiring harness (W5279) was in an incorrect location. Consequently, one of the forward cargo ceiling liner retainer screws chafed on the wires, which resulted in the electrical current from the chafed wire dispersing through the passenger floor carbon fibre beam about body station 508. That electrical current generated significant heat where 14 of the cargo ceiling polyetheretherketone resin standoff brackets were heat damaged and several areas of the structural carbon fibre beam were chafed and delaminated. The smoke generated from the arcing was of a magnitude that it migrated through the forward cargo ceiling liner into the forward cargo compartment and activated the forward cargo fire detection system.

This incident was the fifth reported case where damage to the wire bundles in the forward (and aft) cargo compartment of a Boeing 777 aircraft has occurred from chafing on a ceiling liner screw and/or nutplate. This was the first event that triggered the cargo fire warning and the only event to have been detected in air. A subsequent investigation conducted by Boeing found that the wire bundle W5279 had been incorrectly routed, likely during aircraft manufacture, and had not been installed as per the design drawings. Slight variations of the wire bundle position allowed it to run directly above the screw and nutplate, which chafed the wire bundle over time.

Findings

These findings should not be read as apportioning blame or liability to any particular organisation or individual.

• During cruise, a burning smell was detected in the flight deck and the forward cargo compartment fire warning activated. The flight crew armed and set the forward cargo fire suppression system and diverted the aircraft to the nearest airport for a safe landing.
• A wiring loom situated above the forward cargo compartment about body station 508 was incorrectly routed, likely during manufacture of the aircraft. Over several years, wires in that loom chafed against the support structure and short circuited. Electrical arcing created smoke that activated the forward cargo smoke detector.

Safety actions

Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may proactively initiate safety action in order to reduce their safety risk. The ATSB was advised of the following proactive safety action in response to this occurrence.

Aircraft manufacturer

As a result of this occurrence, Boeing advised the ATSB that they have taken the following safety actions:

Fleet communication

Boeing issued a ‘Fleet Communication’, 777-FTD-24-18001 to all Boeing 777 aircraft operators informing them of this issue:

Boeing has received reports of five separate in-service events where a ceiling liner screw in the forward cargo compartment was found in contact with a wire bundle, resulting in a short to ground that damaged cargo ceiling standoffs, the wire bundle, and the floor beam in some cases.

Wire installation inspections

Boeing recommend interim action:

Operators can choose to inspect the wire bundle runs in the forward cargo compartment and locate instances where the ceiling liner screws and nutplates are in contact or do not have 0.13 Inch clearance with the wire bundle. Wire harnesses should have a minimum separation distance of 0.13 inch to sharp edges of structure and equipment per SWPM (D6-54446), Sec 20-10-11 Page 39, table 21 ‘Minimum Clearance’. If a riding condition is found, Boeing can provide technical assistance if required to provide corrective action. The wire routing installation drawings can be reviewed to determine the correct routing of wire bundles in the cargo compartment.

In addition, Boeing has issued Service Bulletin 777-24-0157, which would require operators to inspect for and correct similar conditions that led to this occurrence. Service Bulletin 777-24-0157 relate to all Boeing 777-200,777-200LR, 777-300ER aircraft line numbers 1-1527 inclusive.

Boeing Engineering performed an investigation of all cargo ceiling wire bundle installation engineering drawings. Boeing will add additional spacing as a precaution when wire bundles are in close proximity to ceiling liner screws.

…inspect and made changes to wire bundles near ceiling liner nutplate locations, in the forward and aft cargo compartments. If this service bulletin is not done, wire chafing can result in a short circuit and a system failure.

There have been five reports of wire chafing on ceiling liner screws or nutplates. Several ceiling liner support standoffs were damaged by heat which was caused by the grounding path. The floor beam was also damaged. Wire bundles near heat damaged ceiling liner support standoffs have also been damaged. The wire bundles that do not have the correct clearance from the ceiling liner screws, can result in chafing causing exposed conductors and shorting.

Boeing has also taken action in their production line by inspecting aircraft from line number 1529 for correct installation. Boeing are also considering installation and design changes to new production aircraft to alter the position of the effected wiring loom to prevent recurrence.

Safety message

Despite complex systems of design and manufacturing, training, and quality control, errors do occur during manufacturing that may not be apparent for some time. In this case, the aircraft was manufactured 4 years prior to the incident.

While this was a serious incident, the severity of the damage sustained was minimised through regulatory design requirements, material composition, system protections and crew actions. In response to this, and four other incidents, the aircraft manufacturer utilised their system of communication to alert all operators of the issue and took actions in an effort to prevent reoccurrence. Regardless of this, operators and maintenance providers are another line of defence for detecting errors. Due diligence during scheduled aircraft maintenance and defect rectification will assist with ensuring that aircraft systems meet the design intent and function accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through: identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information  Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2018 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence. Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work. The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

__________

1. A crew that comprises more than the minimum number required to operate the aircraft and in which each crew member can leave their assigned position and be replaced by another appropriately qualified crew member for the purpose of inflight rest.
2. Central Daylight-saving Time (CDT): Coordinated Universal Time (UTC) + 10.5 hours.
3. Recirculation fans move air throughout the cabin.
4. After a time delay, the remaining three extinguisher bottles discharge at a reduced flow rate into the selected compartment. If the aircraft lands before all the bottles discharge, one of the remaining bottles discharges into the selected compartment at a reduced rate on touchdown.
5. MAYDAY: an internationally recognised radio call announcing a distress condition where an aircraft or its occupants are being threatened by serious and/or imminent danger and the flight crew require immediate assistance.
6. Flight level: at altitudes above 10,000 ft in Australia, an aircraft’s height above mean sea level is referred to as a flight level (FL). FL 125 equates to 12,500 ft.
7. Buttock (butt) line is an axis of measurement. Buttock line zero is the theoretical longitudinal line down the centre of a fuselage. In this instance, left buttock line 40 is 40 inches to the left of the centre-line.

What happened

On 23 June 2017, at about 1549 Central Standard Time,^[1] a QantasLink Bombardier DHC-8-315 aircraft, registered VH-TQH, was being operated on a scheduled passenger service from Port Lincoln to Adelaide, South Australia. There were two flight crew, two cabin crew and 46 passengers on board. The captain was designated as the pilot monitoring and the first officer (FO) was the pilot flying.^[2]

When on final approach to runway 23 at Adelaide, at about 300 ft, the captain noticed fumes in the cockpit and mentioned this to the FO, who did not notice the smell. Shortly after, at about 200 ft, both crew detected the fumes, which smelt like electrical/chemical burning. They looked down at the centre console and noticed light grey smoke coming from the switch on the aileron/rudder trim control panel. The captain instructed the FO to focus on the landing and that they would manage the problem when they were on the ground. The captain notified air traffic control of smoke in the cockpit and requested emergency services. The controller considered this a ‘PAN PAN’^[3] call.

After landing, the aircraft was stopped on the taxiway and the captain called for the Smoke checklist from the Quick Reference Handbook to be completed. This involved donning oxygen masks, switching the microphone to mask, and turning the recirculation fans off to prevent the smoke being circulated within the aircraft. The smoke had dissipated from the cockpit, but fumes were still present.

Air traffic control called back to ask if the flight crew could continue taxiing. The FO responded they were using oxygen masks and would be shutting down on the taxiway. The captain delivered a public address to the passengers advising there was an issue and to await further instructions. The captain then made a call to the cabin crew to inform them of the smoke, that they were using oxygen, and were planning to do a precautionary disembarkation. The cabin crew member indicated that passengers seated in rows 4 and 5 could also smell the fumes, but there was no smoke.

The FO retrieved the On ground non-normal checklist. The captain completed the checklist and made the ‘precautionary disembarkation’ public address to the passengers. The FO exited and directed the passengers outside to an area away from the aircraft. The cabin crew cleared the cabin to ensure all the passengers had disembarked and the captain switched off all power before they exited as per the Precautionary disembarkation and Evacuation checklists.

The captain spoke to the airport fire personnel and provided them a description of the issue and where it occurred. The fire personnel assessed the situation and determined there was no fire risk.

The captain briefed the passengers on the incident, describing what happened and why they disembarked. There were no reported injuries or ill effects from the smoke and fumes, and the aircraft was not damaged.

Aileron/rudder trim control panel inspection

Following the incident, engineers removed the aileron/rudder trim control panel for inspection. That inspection identified visible damage underneath the rudder trim switch. Specifically, the rudder trim potentiometer^[4] was blackened and burnt (Figure 1). The trim control panel was subsequently replaced.

Figure 1: Burnt potentiometer in the aileron/rudder trim control panel (circled in red)

Source: Operator, modified by the ATSB

Additional comments

The following additional comments were made by the captain and operator:

• When the smoke and fumes were detected, the aircraft was about 1 minute from landing. Consequently, the captain elected to continue the landing, rather than action the appropriate checklist. The captain reported that, if the checklist was commenced during the approach, the aircraft’s controls would have been handed between the crew while donning masks. The captain considered this to be dangerous while hand flying the aircraft and when close to landing.
• While the crew were disembarking, the aviation rescue and firefighting personnel attempted to enter the aircraft, but unintentionally blocked the exit. This resulted in a minor delay for crew exiting.
• The aviation rescue and firefighting personnel did not allow the cabin crew to remove the first aid kit from the aircraft, which contradicted the operator’s emergency procedures.
• The operator reported that communications throughout the incident were well managed and the crew were commended for their response to the incident.

Airservices Australia comments

Airservices advised that the airport fire personnel were present at the exit only after all the passengers had disembarked and then attempted to enter the aircraft to talk to the crew and assess the internal conditions. The request not to remove the first aid kit was made in consideration that fire personnel carry first aid kits at all time. In addition, it was standard practice when securing a site not to remove anything from the aircraft until an internal assessment had been made. Airservices were not aware that it was company policy to remove the first aid kit.

Previous occurrences

A search of the ATSB’s database found the following occurrences involving smoke or fumes in DHC-8 aircraft originating from in the cockpit:

• On 29 July 2013, the crew of a Bombardier DHC-8-315 were en route from Sydney to Wagga Wagga, New South Wales (ATSB investigation AO-2013-120) when they noticed a blank area in the centre of the flight management system screen. About 10 minutes later, the screen went completely blank and thick, light-grey smoke was observed coming from the unit. Examination of the unit found that two capacitors failed, resulting in the smoke and failure of the unit.
• On 8 June 2014, the crew of a Bombardier DHC-8-202 were on take-off at Cairns, Queensland (ATSB occurrence 201405530). During the take-off, the FO’s electronic displays failed and fumes were detected in the cockpit. The take-off was rejected and the aircraft returned to the bay. An engineering inspection revealed water contamination to the No.2 symbol generator.
• On 10 November 2016, the crew of a Bombardier DHC-8-315 were on approach to Adelaide, South Australia (ATSB investigation AO-2016-151). At about 9,000 ft, the FO noticed the captain’s electronic attitude director indicator screen had gone blank and the crew conducted the display failure checklist. After the crew were cleared to descend, they noticed an electrical smell, which was suspected to originate from the failed screen. The crew actioned the fuselage fire or smoke checklist and made a ‘PAN PAN’ call to air traffic control. After landing, a precautionary disembarkation on the taxiway was conducted. An engineering inspection found the fumes were caused by damage to a circuit card assembly due to a blown resistor on the video driver.

Findings

These findings should not be read as apportioning blame or liability to any particular organisation or individual.

• The rudder trim potentiometer burnt in-flight, resulting in fumes and smoke in the cockpit during a critical phase of flight.
• Shortly after landing, the crew conducted a precautionary disembarkation on the taxiway, which reduced the risk of fumes exposure to the aircraft’s occupants.

Safety message

This incident highlights the effective flight crew management of an in‑flight issue during a critical phase of flight. The ATSB has published a research report, An analysis of fumes and smoke events in Australia from 2008 to 2012, which found that, from a flight safety perspective, the majority of fumes/smoke events were minor in consequence and the most common source was aircraft systems issues. The research also identified that fumes and smoke events were generally appropriately managed by flight and cabin crew due to effectiveness of crew training and operational procedures, such as using checklists.

Aviation Short Investigations Bulletin - Issue 62

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through: identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information  Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2017 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence. Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work. The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

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1. Central Standard Time (CST): Coordinated Universal Time (UTC) + 9.5 hours.
2. Pilot Flying (PF) and Pilot Monitoring (PM) are procedurally assigned roles with specifically assigned duties at specific stages of a flight. The PF does most of the flying, except in defined circumstances; such as planning for descent, approach and landing. The PM carries out support duties and monitors the PF’s actions and aircraft flight path.
3. PAN PAN: an internationally recognised radio call announcing an urgency condition which concerns the safety of an aircraft or its occupants but where the flight crew does not require immediate assistance.
4. An instrument for measuring electromotive force or difference in potential between two points in a circuit; the measurement is made without drawing electric current.

What happened

On 22 February 2017, at 1433 Eastern Daylight-saving Time (EDT), a Virgin Australia ATR 72-212A aircraft, registered VH-VPJ, departed Port Macquarie Airport, New South Wales (NSW) to operate scheduled flight VA1188 to Sydney, NSW. There were four crew and 23 passengers on board.

At 1453:35, during cruise at Flight Level (FL) 180,^[1] the Centralized Crew Alerting System (CCAS) alerted the flight crew to a failure of the number one static inverter (Figure 1).^[2] The CCAS then displayed multiple messages indicating a loss of power to systems associated with the number one static inverter. The aircraft electrical system power transfer function automatically transferred these systems to the number two static inverter and the CCAS warnings extinguished.

At 1453:43, the cockpit master warning activated and the CCAS displayed an electrical smoke warning. The flight crew immediately donned their oxygen masks and enacted the smoke checklist memory items. As the flight crew fitted the oxygen masks, they detected a strong electrical type burning odour and observed faint wispy smoke within the cockpit. After conducting the memory items, the flight crew then completed the electrical smoke checklist. The checklist included selecting the avionics vent exhaust mode to overboard. After completing this selection, the flight crew reported the smoke quickly dissipated. Flight data shows the electrical smoke warning extinguished at 1454:56.

After completing the electrical smoke checklist, the captain identified Williamtown Airport about 65 km (35 NM) south-east of the aircraft and elected to divert the flight to Williamtown.

At 1455, the captain contacted air traffic control (ATC) and declared a MAYDAY.^[3] The captain advised that they intended to divert to Williamtown Airport. ATC cleared the flight to descend and track directly to Williamtown.

After contacting ATC, the captain requested that the senior cabin crew (SCC) report to the aircraft interphone using the cabin public announcement system. The SCC heard the announcement, but due to muffling caused by the captain’s oxygen mask, they did not understand the request. The second cabin crewmember heard the announcement more clearly and communicated the request to the SCC. The SCC contacted the flight deck using the aircraft interphone. The captain advised them of the emergency and that the flight was diverting to Williamtown. The SCC advised the other cabin crewmember of the diversion and commenced securing the cabin.

After securing the cabin, the SCC returned to their seat and contacted the flight deck. The captain provided them with a full briefing, advising the nature of the emergency and to expect a precautionary disembarkation^[4] after landing. Recognising the high workload of the flight crew, the SCC advised the captain that they would conduct the passenger briefing tasks on behalf of the flight crew. The captain instructed the SCC to begin the precautionary evacuation once the seat belt sign extinguished after landing.

As the aircraft descended through 10,000 ft, the flight crew removed their oxygen masks. The captain found the remaining odour very strong and elected to refit the oxygen mask. The captain identified that the aircraft was too high to commence an approach to Williamtown and conducted a descending orbit to lose height prior to commencing a visual approach for runway 12. While approaching runway 12, the captain found the oxygen mask blurred their vision. The captain briefly handed control of the aircraft to the first officer and removed the oxygen mask.

At 1512, the aircraft landed on runway 12. After landing, ATC instructed the flight crew to taxi the aircraft to Bay 11. Once the aircraft stopped and the flight crew shut the engines down and extinguished the seat belt sign, the SCC initiated the precautionary disembarkation. The SCC used the cabin public address system to direct passengers to disembark the aircraft using the cabin door. Emergency services personnel met the disembarking passengers and guided them clear of the aircraft to a safe area.

After shutting down the engines, the flight crew noticed the smell intensifying. The captain elected to immediately vacate the flight deck. The flight crew followed the last passenger and the cabin crew in vacating the aircraft through the cabin door.

The aircraft was not damaged, and no persons were injured during the incident.

Figure 1: Number one static inverter

Source: Operator

Captain comments

The captain provided the following comments:

• Time was lost due to difficulties with the first officer refitting their headset after donning the oxygen mask. The oxygen mask also created difficulties in communication between the flight crew and cabin crew. Managing these communication difficulties added to the flight crew workload during the emergency.
• While the company did not operate the ATR 72 to Williamtown and the captain had not previously operated there, the captain commented that the best place for an aircraft with smoke in the cockpit is on the ground. The long runway, available emergency services and clear weather between their position and the airport enabled the captain to quickly elect to divert to Williamtown.

Senior cabin crew member comments

The senior cabin crew (SCC) provided the following comments:

• They had not expected and were not prepared for the communications difficulties caused by the flight crew’s use of oxygen masks. Their voices were heavily distorted which led to difficulty in understanding information. After the initial briefing from the captain, the SCC did not realise there was a smoke issue and believed the aircraft was experiencing an unspecified ‘leak’. After the initial briefing, they began to prepare the cabin for a possible depressurisation.
• Due to the communications difficulties caused by the flight crew oxygen masks, the SCC did not realise that they were being requested to contact the flight crew and did not immediately respond.
• The aircraft interphone does not allow the flight deck to address all cabin crew at the same time. Therefore, the SCC was required to relay information to the other cabin crewmember. This made it difficult for the other cabin crewmembers to be fully aware of the progress of the incident and increased the SCC’s workload.
• Cabin preparation procedures for the precautionary disembarkation require that the SCC use designated Cabin Preparation cards. These cards provide guidance for full and reduced cabin preparation procedures and associated passenger briefings. The cards were located under the SCC’s seat and were inaccessible while seated. As the SCC was unable to leave their seat during the period between receiving the full briefing from the captain and landing, they were unable to access these cards.

Engineering examination

The manufacturer of the static inverter conducted an engineering investigation of the failed static inverter. The investigation found that the failure of the number one static inverter and associated smoke and odour was caused by a failure of a C60x series capacitor within the number one static inverter.

The aircraft manufacturer also noted that the operator experienced two previous static inverter failures in November and December 2016. These failures were also caused by failure of a C60x series capacitor.

Vendor Service Bulletin

On 22 June 2016, the manufacturer of the static inverter released vendor service bulletin

This service bulletin identified an issue with capacitor C311 which led to instances of reduced reliability and premature failure, sometimes with associated smoke emission. As part of this service bulletin, the C311 capacitor is replaced with a modified capacitor of increased reliability.

The service bulletin recommended that the modification be incorporated at the next shop visit for the static inverter units. After completion of the service bulletin modifications the static inverters are designated as ‘Amendment E’ status.

From October 2016, a retrofit campaign was undertaken by the static inverter manufacturer to refit all in-service static inverters to ‘Amendment E’ standard.

In December 2016, the aircraft manufacturer advised operators of the vendor service bulletin. The bulletin was classified as a minor change and did not imply safety concerns.

Continued static inverter issues

Following reports of failures of ‘Amendment E’ static inverters, the aircraft manufacturer identified an issue with additional capacitors within the static inverter. These capacitors are of the C60x (C601 through C605) series. Failures of these capacitors also led to failure of the static inverter unit and associated smoke emission.

Static inverter failure

In the event of failure of a static inverter, the power transfer function automatically transfers power of the associated electrical systems to the second static inverter.

The operator’s Minimum Equipment List allows dispatch of an aircraft with an unserviceable static inverter for a period of up to two days.

Safety analysis

A C60x series capacitor within the number one static inverter failed in a manner consistent with other C60x series capacitor failures. Failure of the capacitor resulted in failure of the static inverter and smoke being emitted into the cockpit.

Difficulties in communication with the flight deck led the SCC to initially believe the flight crew were managing an unspecified ‘leak’. Therefore, the SCC began preparing for a possible depressurisation. However, as the required actions were similar to those required for the smoke event in progress, the misunderstanding did not impact on the management of the cabin during the incident.

The Cabin Preparation cards were inaccessible during the period that procedures directed the SCC to use them. However, as the SCC was able to complete the required actions without reference to the cards this did not impact on their ability to prepare the cabin for landing and the precautionary disembarkation.

Findings

These findings should not be read as apportioning blame or liability to any particular organisation or individual.

• A C60x series capacitor within the number one static inverter failed leading to failure of the static inverter and associated smoke.
• Difficulties in communication caused by oxygen mask use led to misunderstandings between the flight crew and cabin crew and increased flight crew workload.
• The Cabin Preparation cards were inaccessible to a seated cabin crewmember.

Safety action

Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may proactively initiate safety action in order to reduce their safety risk. The ATSB has been advised of the following proactive safety action in response to this occurrence.

Operator

As a result of this occurrence, the aircraft operator has advised the ATSB that they are taking the following safety actions:

• Verifying the integrity of the Power Transfer function across the ATR fleet. The operator advised that after completing this campaign no adverse findings were reported.
• The operator has initiated a retrofit campaign to route all in-service static inverters to the vendor to have the modification to both the C311A and the C60x capacitors completed (‘Amendment E’ and ‘Amendment F’ (see Aircraft manufacturer) standard).
• The operator has initiated a fleet wide inspection and operational test of the oxygen mask integrated microphone.
• The operator has undertaken a risk assessment for single and dual static inverter failure.

Aircraft manufacturer

As a result of this occurrence, the aircraft manufacturer has advised the ATSB that they are taking the following safety actions:

Short term actions:

• Replacement of C311 capacitor resulting in service units being modified to Amendment E standard. New production units have been modified and the retrofit process is on-going.

• Change in the manufacturing process with ‘burn in’ test on C60x capacitors resulting in units being modified to ‘Amendment F’ standard. New production units have been modified. The retrofit process is being prepared.

  • Mid-term actions:
  • A design change is under development including the replacement of the C60x capacitors. The redesigned unit will be identified with a new part number. The design change certification expected to be complete before the end of quarter 2, 2017.

  Safety message

  This incident underlines the value of effective training and procedures. Despite the communications difficulties and the inaccessible cabin preparation cards, the cabin crew were able to effectively prepare the cabin during the diversion and manage the subsequent precautionary disembarkation. This enabled all aircraft occupants to disembark the aircraft quickly and without injury.

  Aviation Short Investigations Bulletin Issue 61

  Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through: identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information  Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2017 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence. Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work. The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

   

  1. Flight level: at altitudes above 10,000 ft in Australia, an aircraft’s height above mean sea level is referred to as a flight level (FL). FL 180 equates to 18,000 ft.
  2. Static inverter: a component of the aircraft electrical system which changes direct electric current to alternating current.
  3. MAYDAY: an internationally recognised radio call announcing a distress condition where an aircraft or its occupants are being threatened by serious and/or imminent danger and the flight crew require immediate assistance.
  4. Precautionary disembarkation: a disembarkation of the aircraft using the normal aircraft exits in as timely a manner as possible. Passengers are briefed to clear exit paths, leave belongings on the aircraft and that the precautionary disembarkation may become an evacuation at any time. Passengers in emergency exit rows are briefed on their actions during the precautionary disembarkation and possible evacuation.
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