What happened

On 25 October 2016, at about 0640 Central Daylight-saving Time (CDT), an Embraer-120 ER aircraft, registered VH-YEI (YEI), departed from Adelaide Airport, South Australia (SA), for a chartered passenger flight to Challenger, SA with 4 crew and 29 passengers on board.

During the initial climb, the pilot attempted to fully retract the flaps (flaps 0) from their take-off position (flaps 15). The crew received a flap disagreement warning from the outboard pair of flaps. The other two pairs of flaps (inboard and nacelle) retracted without issue. A flap disagreement fault is triggered when one flap within a pair is unable to move to its selected position. To prevent wing asymmetry and control issues, the pairing flap will also not move. The crew reported no noticeable aircraft handling or control issues as a result.

The crew contacted air traffic control (ATC) to notify them of the fault and requested a climb to 4,000 ft on their current heading. After levelling off and reducing power to a cruise setting, the crew consulted the quick reference handbook, which advised cycling the flaps to their original position and back again. The fault cleared when the flap control was lowered to 15 degrees but occurred again upon retraction.

The captain and first officer discussed the situation and agreed to return to Adelaide. The crew then informed ATC. The first officer continued flying the aircraft, burning off fuel to reduce landing weight. The captain conducted the landing calculations and determined that they had sufficient runway to land with flaps 15, in case they were unable to deploy further.

During approach, the flaps fully extended (flaps 45) and the aircraft landed without further incident. The flaps were retracted after landing, and all three pairs moved to flaps 0 without the fault reoccurring. The crew visually inspected the flap and immediately identified a damaged bracket and fibreglass shroud.

There were no injuries as a result of the occurrence and the aircraft sustained no damage beyond the fractured bracket and shroud.

Component Failure

The failed bracket secured a roller to the underside of the wing. This roller supported the inboard side of the left-hand outboard flap shroud and was used to guide the flap shroud as it was deployed and retracted. Figure 1 shows failed the bracket, still fixed to the aircraft after it was identified by the flight crew.

The support of the roller pictured was not required for the flaps to actuate. This was apparent after landing, when the flaps were successfully retracted, despite the bracket fracture. It is likely that a portion of the bracket or its fibreglass housing obstructed the flap, preventing it from fully retracting during flight.

Figure 1: Bracket on the aircraft after it had failed. Most of the bracket is obscured by the fibreglass housing. Its outline is shown by the dotted red line.

Source: Flight crew

Figure 2 shows the failed component after it was removed from the aircraft. Figure 3 shows the fracture surface on the smaller of the two fragments. Two distinct regions were visible on the fracture surface. The upper region was tarnished and had likely been exposed to the atmosphere for a longer period of time. It was probable that this was the result of a pre-existing crack, and the stress in the remaining section resulted in failure of the component during normal operation.

Figure 2: The failed bracket after it had been removed from the aircraft.

Source: ATSB

Figure 3: Fracture surface illustrating the pre-existing crack and the overstress region, which fractured when the bracket finally failed.

Source: ATSB

Aircraft Maintenance

YEI was manufactured in 1992 and, at the time of the incident, had accumulated 41,961 hours total time in service. The aircraft was being maintained by a Civil Aviation Safety Regulations (CASR) approved maintenance organisation and the most recent comprehensive inspection was approximately 600 service hours prior, in November 2014. No problems with the outboard flap shroud or bracket were identified at that time.

Similarly, no problems were identified during the more recent line checks and daily inspections. However, the maintainer believes that the location of the bracket and the positioning of the shroud would make it difficult to see any cracks during this type of inspection.

Similar Occurrences

The CASA Service Difficulty Report (SDR) database shows two other reports of unserviceable flaps on EMB-120s. Both involved the left-hand outboard flap, however the fault in both cases was unrelated to the inboard bracket or flap track. Likewise, no reports of issues with this part could be found in the US Federal Aviation Administration (FAA) SDR database.

Safety analysis

A pre-existing crack on a partially concealed bracket propagated to the point where it failed in overstress during normal operation. The origin and age of the crack could not be determined, so it is not known whether there was an opportunity for it to have been detected during a base check conducted in 2014.

As a result of the bracket failure, the left-hand outboard flap could not be fully retracted during climb and a flap disagreement warning occurred. It was unclear exactly how the failed bracket prevented the flap from retracting, but it may have been caused by a bracket fragment physically obstructing the flap.

While there were no adverse control or handling issues, the flight crew returned the aircraft to Adelaide Airport. The flaps extended for landing without any further problems.

All risk controls during this incident worked well. The aircraft’s flap monitoring system identified the misalignment of the outboard flaps and prevented an asymmetry occurring. The crew were then adequately warned of the fault and advised ATC of the situation. The crew followed the quick reference handbook in an attempt to rectify the problem, but when the fault could not be cleared, returned the aircraft to Adelaide Airport. A contingency plan was formulated in the event the flaps could not be extended, but ultimately it was not necessary.

Findings

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

• Pre-existing cracking and subsequent fracture of a bracket supporting the left, outboard flap shroud prevented the flap from being fully retracted during climb and resulted in a flap disagreement warning.

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.

The Operator

Following the identification of the failed bracket, the operator inspected similar parts on YEI as well as the rest of their Embraer-120 fleet. No further damage or signs of cracking were identified.

Safety message

This incident highlights the importance of comprehensive maintenance inspections in maintaining aircraft airworthiness. However, when technical failures occur in spite of rigorous maintenance procedures, it is important to have adequate risk controls in place, as well as trained crew capable of adjusting plans to account for unforeseen circumstances. In such an event, a positive outcome can be achieved by maintaining composure, planning ahead, and communicating with other crewmembers, ATC.

Part of Aviation Short Investigations Bulletin - Issue 60

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.

What happened

On 26 October 2016, a Beechcraft B200 aircraft, registered VH-XGV (XGV), taxied at Brisbane Airport for a private ferry flight to Archerfield Airport, Queensland. The pilot was the only occupant of the aircraft.

The pilot taxied the aircraft to holding point A7 (Figure 1).

According to recorded air traffic control communications, at about 2010 Eastern Standard Time (EST), the aerodrome controller (ADC) asked the pilot of XGV whether they were ‘ready to go’ and the pilot responded advising they were ready. The ADC issued an instruction to the pilot to conduct a right turn onto a heading of 090° after take-off and cleared the pilot to line up on runway 01 and wait due to wake turbulence from an aircraft that had just taken off. The pilot read back the line-up instruction, but as they omitted to read back the assigned heading, the ADC repeated the turn and heading information and then also advised the pilot of an amended Departures frequency (from the one issued in their initial airways clearance).^[1]

After the pilot received the line-up instruction, they started to taxi the aircraft onto the runway, and commenced the line-up checks. The pilot read back the frequency initially, but subsequently asked the controller to repeat it. The pilot lined the aircraft up on what they thought was the centreline of runway 01, and the aircraft remained stationary on the runway for about 30 seconds before the controller cleared the pilot for take-off.

At about 2011, the aircraft commenced the take-off run. During the take-off, the pilot realised that the aircraft was not on the centreline and steered the aircraft right towards the centreline prior to the aircraft becoming airborne. A subsequent runway inspection found damage to a runway edge light and the aircraft sustained minor damage to the nose landing gear.

Figure 1: VH-XGV commencing take-off on runway 01

Source: Airservices Australia

Pilot comments

The pilot provided the following comments:

It seemed very dark on the taxiway and runway 01. They had difficulty detecting the taxiway centreline markings.

The aircraft had its LED taxi light on, but it did not seem effective, as it did not appear to illuminate the area.

• The pilot advised that the taxiway and runway lights were difficult to see and was not sure if there were runway centreline lights and whether they were activated.

Once they had changed to Tower frequency, they felt rushed by air traffic control to line up on the runway.

The pilot was not expecting the change in Departures frequency, which was given to them with the departure instructions as they were in the process of entering the runway and completing their line-up checks. This distracted them from ensuring the aircraft was lined up correctly on the centreline.

• The pilot thought something was not right but did not think the aircraft hit anything and realigned the aircraft on the runway centreline. Only after airport ground personnel found the parts of damaged lighting was it evident the aircraft took off on the runway edge.

Airport lighting

At Brisbane Airport, the taxiway has green centreline lights and lead-off lights from the runway, but no lead-on lights from any intersection departure points. The lead-off lights are one-way lights and would not be visible to the pilot entering the runway. Runway 01 has white runway centreline lights, runway threshold lights, and both high and medium intensity runway lights along the runway edge.

Air traffic control selects the lights on with one switch, which turns on both edge and centreline lights on the runway. There were no reports of any technical problems with the lighting that night.

Aircraft lighting

The aircraft had two LED landing lights below a single LED taxi light, all of which are attached to the nose landing gear leg of the aircraft. The pilot confirmed that the taxi light was switched on during taxi and take-off. Switching on landing lights is part of the line-up checklist, but the pilot was uncertain whether it was switched on during take-off.

Previous incidents

A search of the ATSB database found similar misaligned take-off occurrences at night during the absence of a particular type of runway lighting and perceived pressure on the pilot to take-off:

• Operational event, Brisbane Airport, Queensland, 25 November 2007 (ATSB investigation AO-2007-064^[2]): The pilot of a Gulfstream Aerospace Corporation G-IV was operating a charter flight from Brisbane to Sydney, New South Wales (NSW). The pilot-in-command commenced take-off on taxiway A, which was adjacent to runway 01. The aerodrome controller instructed the pilot to cancel the take-off clearance. It was found that the electronic flight bag (EFB) was not functional, and the pilot-in-command relied on memory of the aerodrome from landing earlier that morning to take-off. On the entrance to runway 01 (at the A7 intersection), there were no runway threshold markings and lights to indicate the beginning of the runway.
• The crew of a SAAB 340B was preparing to take-off from Sydney Airport to Lismore, New South Wales. The aircraft was lined up for take-off on runway 25 on the left runway edge runway lights. During take-off run, the captain thought something was wrong and realised that the aircraft was incorrectly lined up on the runway edge lights and re-aligned with the centreline. The crew were completing the line-up checklist at the time. Furthermore, runway 25 does not have centreline lighting.
• Collision on ground, Townsville Aerodrome, Queensland, 11 February 2009 (ATSB investigation AO-2009-007^[3]). During take-off, the pilot-in-command realised that the Bombardier DHC-8 was aligned with the left runway edge of runway 01. The aircraft was manoeuvred to the centre of the runway, and the take-off was rejected. It was found that the aircraft’s left mainwheel had damaged a runway edge light. Factors that may have led to the misaligned take-off related to the adverse weather. These included the weather being a distraction to the flight crew to monitor the aircraft’s path onto the runway, pressure to depart, and reliance on aerodrome lighting. Furthermore, the centre taxiway lighting stopped prior to the runway threshold making it difficult for crew to identify the runway edge.

Safety analysis

Animation of the recorded radar data from Brisbane Airport showed the aircraft commencing take-off on the runway edge, then moving across to the centreline.

Brisbane Airport taxiway and runway lights were operational on the night of the incident. However, lead-in lights were not installed to assist pilots lining up on the runway centreline. In addition, the aircraft’s taxi light used during the taxi, and possibly the take-off, supplied only very limited visibility to the pilot.

The pilot stated they felt rushed when the ADC gave them clearance to line up, and they were still completing the line-up checks when the controller issued the change in frequency. This combination of time pressure and distraction affected the pilot’s ability to detect the aircraft was not on the runway centreline. Airservices commented that as there was a 30 second delay on the runway for wake turbulence and no other aircraft on final approach, there was no actual urgency for the pilot. Despite this, the pilot had a perception of time pressure.

Findings

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

• The pilot lined the aircraft up and commenced take-off on the runway edge lighting from an intersection (A7) departure.
• The misalignment of the aircraft was influenced by the pilot rushing due to perceived pressure to commence the take-off when the line-up clearance was given, the issuing of a frequency change while the pilot was completing the line-up checks as well as lining up the aircraft on the runway, the lack of taxiway lead-on lights to the runway, and limited brightness of the aircraft taxi light.

Safety message

This incident highlights the effect time pressure and distraction can have on flight safety, particularly during critical stages such as while completing checklists. An ATSB research report Dangerous distraction: An examination of accidents and incidents involving pilot distraction in Australia between 1997 and 2004 provides an overview of occurrences resulting from pilot distraction. One strategy outlined is that if completion of checklists are disrupted, go back and start the checklist again (if possible) to reduce the potential for error.

In addition, ATSB research report Factors influencing misaligned take-off occurrences at night outlines conditions including intersection departure, air traffic control clearance/s issued during runway entry, and divided attention of flight crew. The ATSB has also developed a Pilot Information Card featured within the report to help flight crew identify factors that could increase the risk of a misaligned take-off.

Aviation Short Investigations Bulletin - Issue 57

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. An airways clearance authorises a flight to operate in controlled airspace along a designated track or route at a specified level to a specified point or flight planned destination (AIP ENR 1.1-2 para 3.6).
2. /publications/investigation_reports/2007/aair/ao-2007-064/
3. /publications/investigation_reports/2009/aair/ao-2009-007/

What happened

On 19 October 2016, at about 1037 Western Standard Time (WST), a vehicle was travelling along the northern perimeter road at Perth Airport, Western Australia. The vehicle had a driver and three passengers who were going to conduct a customs inspection on an aircraft, which was scheduled to depart Australia.

The driver had been advised that the aircraft would be parked on the international apron in preparation for the inspection. When the vehicle arrived at the international apron, the aircraft was not there, so the driver decided to show the other passengers some different areas of the airport while they waited for the aircraft to arrive.

As they drove towards the domestic apron, an area unfamiliar to the driver, the driver observed the aircraft that they were to meet, stationary in a position different to what was expected. They thought the aircraft was on the domestic apron, and instead of continuing along the northern perimeter road to the domestic apron, the driver turned left (Figure 1 item 1) and crossed the red and white zipper markings^[1] that denote the road is crossing taxiway R (Figure 2). About another 50 m past taxiway R, was a left turn (item 2) for vehicles to access the domestic apron.

Figure 1: Airport map showing the path of the vehicle

Source: Google earth, modified by the ATSB

Figure 2: Photo showing stop sign and red and white zipper markings that denote a taxiway like that for taxiway R

Source: Airport operator

Rather than being parked as assumed by the driver, the aircraft was actually being towed in company with two safety vehicles and was stationary at taxiway D holding point, waiting for clearance to cross-runway 21. The vehicle then turned left onto taxiway W (item 3) and then right onto the 700 lane (item 4). At about 1038, the surface movement controller (SMC) cleared the aircraft that was being towed (item 5), to cross runway 21. At about the same time, a senior airport operations officer (SAOO), who was in a vehicle and parked near the 700 lane (item 6), noticed the vehicle traveling along the 700 lane but was not able to identify it. The vehicle turned right onto taxiway B (item 7) and then left onto taxiway H3 (item 8).

At about 1040, the tower air traffic controller cleared a Fokker Aircraft F28 MK 100 aircraft (Fokker 100) that was on final approach, to land on runway 21.

At about the same time, the SAOO contacted the SMC to determine the identity of the vehicle that was just turning onto taxiway D (item 9). The SMC advised that they assumed that the vehicle was associated with works being conducted at the intersection of taxiways A and D. The vehicle was not fitted with a transponder. A transponder was not required for operating in the non-manoeuvring area of the airport, but the vehicle was displayed intermittently on the surface movement control system.^[2]

A work safety officer (WSO), who was supervising a work site at the intersection of taxiways A and D heard this exchange and observed the vehicle in the rear-view mirror (item 10). They turned their vehicle around, and followed the unidentified vehicle that was traveling along the paved edge of taxiway D, towards the runway 21 holding point. As the vehicle should not have been there and did not appear to be stopping, the WSO used their vehicle’s siren, horn and loudspeaker to alert the driver, and informed them that they were about to enter the runway and to stop immediately.

The vehicle passed over the holding point for runway 21 before it stopped. The vehicle stopped about two to three car lengths past the holding point (item 11). The WSO stopped about 5 m before the holding point and over the loudspeaker instructed the driver to go no further and to turn around. The WSO also advised the SMC that they were talking with the driver. The vehicle had stopped about 20 m before the white gable markers that denote the runway strip. The vehicle did a hard right turn and crossed back over the holding point at about 10:41:05 (Figure 3). About 3 seconds later, the Fokker 100 aircraft that had just landed on runway 21, passed the vehicle position. At this time, the estimated distance between the vehicle and the Fokker 100 was about 100 m. The crew of the Fokker 100 did not notice the vehicle.

Figure 3: Location of Fokker 100 aircraft landing on runway 21 and vehicle as it crossed back over the taxiway delta holding point for runway 21

Source: Airport operator, modified by ATSB

Vehicle driver comment

The vehicle driver indicated that the three passengers were new to the airside area of Perth Airport and they had used the delay to show the passengers the charter area that was on the same side of the airport as the domestic terminal. As they were traveling back to the international terminal, the driver saw, in the distance, the aircraft that they were to meet. The driver indicated that they were not familiar with the domestic area of the airport where the incident occurred, and they were focused on the aircraft that they needed to meet to complete their job, which also had time constraints.

The driver reported that as they approached the runway, they had stopped to allow the landing aircraft to pass before they saw the flashing lights of the WSO vehicle.

The driver reported that during the training to obtain the authority to drive airside (ADA), they would take every opportunity to gain more experience but would generally go to the same places.

The vehicle was not fitted with a VHF radio to communicate on or hear any of the air traffic control frequencies, nor was it required to be, as it was only authorised to travel on the perimeter road and apron areas.

The driver advised that there was no airport map or airside driving manual located in the vehicle.

Work safety officer comment

The WSO advised that the vehicle did not seem to be slowing down as they crossed the holding point and only seemed to start to slow when the WSO commenced calling on the loudspeaker.

The driver seemed confused and was focused on getting to the aircraft to complete their inspection, rather than where they were on the airport.

Airport operator comment

The airport operator conducted an investigation into the incident and determined that:

• The vehicle was not fitted with a transponder, as the vehicle was not permitted to access the airport manoeuvring area.
• The vehicle driver was issued with a category 2 authority to drive airside (ADA) in April 2016, which included approval to drive on all aprons and roadways but was not permitted on any taxiways or runways.
• The actions of the vehicle driver in leaving the marked perimeter roadway, traversing several taxiways and then crossing a marked runway holding point, were the actions of an individual confused with their location on the airport.
• The lost procedures for Perth Airport are promulgated in the airport driver-training programme and the Airside drivers pocketbook.

Safety analysis

The driver observed the aircraft they were scheduled to inspect and thought they were stationary on the domestic apron. They did not realise that the aircraft was being towed and was stopped at the holding point for runway 21. The driver entered the taxiways without a clearance and without having authorisation to do so. They became confused and crossed the holding point for runway 21. The WSO used their vehicle’s siren, horn, and loudspeaker to alert the driver, and informed them that they were about to enter the runway and to stop immediately.

The driver was focused on completing a time critical activity in an area of the airport that they were not familiar with. These probably combined to affect the driver’s ability to recognise that the aircraft they were to meet was being towed and stationary on a taxiway and not parked on the apron. The driver also did not identify that they had turned off the perimeter road and traversed several taxiways before crossing a holding point and entering an active runway.

Findings

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

• The driver incorrectly assumed that the aircraft was parked on the domestic apron and in an effort to save time, entered several taxiways without the required approval and authorisation.
• The driver was confused with their location, as they were unfamiliar with that area of the airport, and continued traveling down taxiway D and crossed the holding point for runway 21 before the WSO stopped the vehicle.

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.

Airport operator

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

• A review of the zipper markings will be conducted in areas where taxiways cross roadways to provide an enhanced level of identification to airside drivers.
• The incident and lessons regarding airfield familiarity, lost procedures and general situational awareness will be communicated in an incident alert.
• Explore the option to implement an authority to drive airside (ADA) ’zoning system’, to include a review of all organisations and their requirement to access different areas of the airport.
• Review the system of ADA categories to determine if there is scope to implement a system that controls the access of particular organisations to prevent them from entering areas that they do not have a requirement to regularly access.

Safety message

The International Civil Aviation Organisation (ICAO) has identified runway safety as one of its priorities and has been working with countries and aviation organisations globally to reduce runway safety accidents. ICAO has developed a runway safety website, which offers a range of information and products to assist the aviation community to improve runway safety.

In addition, ICAO has published a

and this is available from the ICAO website. The manual includes information on the prevention of runway incursions. The manual discuses that deficiencies in design, training, technology, procedures, regulations and human performance can result in a system break down and safety being compromised. It is important in a complex and dynamic airport environment that all people working in that environment remain vigilant, maintain open communications, and use the systems in place to minimise the risk of a system break down.

Additional information on runway safety is also available from the Airservices Australia webpage Runway safety.

In addition, Airservices Australia has published a guide for airside drivers, The Airside Drivers Guide to Runway Safety, which focuses on four aspects of operating safely on an aerodrome:

• planning your aerodrome operation
• aerodrome procedures
• communications
• aerodrome markings, signs and lights.

Aviation Short Investigations Bulletin - Issue 58

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. Unless directly involved in the servicing of an aircraft on a bay, vehicles must use the marked roadways for traversing the apron. Where white edge lines of the road turn into red and white chequered markings (referred to as zippers), denotes an active taxiway. The driver must stay clear of this part of the roadway when aircraft are using the taxiway. At taxiway R there were stop signs where vehicles must stop and check for aircraft before proceeding across the taxiway following the road.
2. The Advance Surface Movement Guidance and Control System (A-SMGCS) uses data sent from aircraft and ground vehicles fitted with a transponder. This data is displayed pictorially on a screen in front of the surface movement controller in the tower, showing the position of aircraft and ground vehicles on a map of the airport. Those vehicles that are not fitted with a transponder are detected with the surface movement radar and their position is shown on the same screen. The A-SMGCS did not generate an alert, as the vehicle did not enter the runway strip.

What happened

On 24 October 2016, the operator (pilot) of a Lockheed Martin Stalker eXtended Endurance (XE) unmanned aerial system (UAS), configured in vertical take-off and landing (VTOL) mode,^[1] prepared to conduct a demonstration flight at Mount Disappointment, Victoria.

Figure 1: VTOL Stalker XE

Source: Lockheed Martin Corporation. All rights reserved. Copyright 2017.

The previous day, the operator had loaded the latest autopilot software onto the aircraft’s main autopilot. One aim of the flight was to demonstrate the use of a particular radio frequency for the command and control communication link. The ground radio equipment incorporated a narrow beam width directional antenna.

The operator programmed a simple mission: launch segment to a first waypoint and one primary waypoint, which was a coordinate centred on the launch location, then a landing pattern and an alternate landing pattern.

The crew then conducted a physical inspection of the aircraft and ensured all was mechanically correct. They also completed the pre-flight checks. The operator then commanded the aircraft to launch. All four vertical-lift rotors were energised, and the aircraft lifted off the ground. At about 20 ft above ground level (AGL), the aircraft paused in accordance with normal procedures, to conduct an airborne check.

The aircraft then climbed to about 300 ft AGL, which was the programmed transition altitude, but did not transition to forward flight. As the aircraft climbed above the ground antenna, it flew into a null in the antenna pattern above the antenna where communication between the ground control station (GCS) and the aircraft was interrupted.

The aircraft hovered and, after about five minutes, the operator commanded ‘abort’ from the GCS. At that time, the operator observed that communication with the aircraft had been lost and repositioned the antenna to point directly at the aircraft. After observing continued lost link indications for about 30 seconds, the operator enabled the alternate radio datalink, restoring communication with the aircraft. However, the previously selected ‘abort’ command was unavailable for re-issue after the initial selection.

About 11 minutes after launch, the aircraft’s power failed, the vertical propellers stopped, and the aircraft pitched about 95 degrees nose down, descended vertically and collided with the ground.

The aircraft sustained substantial damage.

Vertical take-off and landing (VTOL) configuration

In the VTOL configuration, four vertical lift motors are fitted in addition to a conventional tractor motor/propeller. The aircraft takes off vertically and then transitions to horizontal flight at a set altitude. Following the period of horizontal (fixed-wing) flight, the aircraft transitions back to vertical flight for landing.

Transition altitude

The typical time needed to climb to departure altitude and transition to forward flight is less than 1 minute.

The transition altitude is specified by the operator and is usually 150-200 ft higher than any nearby obstacles. In this incident, the operator had selected 300 ft due to trees about 100 ft high in the vicinity. The lower the transition altitude, the less time is required in the VTOL configuration, which requires substantially more power (and therefore battery) than the more efficient forward flight.

Post-accident inspection

Following the accident, an inspection revealed that the aircraft’s negative main power cable was unsoldered and had separated from its pin in the main power connector, which was plugged into the aircraft’s battery. The operator’s investigation found that the connector had failed before the aircraft battery drained.

Loss of electrical power

The wiring in the VTOL aircraft configuration was designed to supply vertical climb power for 2 minutes and the system had been validation-tested for 3 minutes at that power setting. The battery connector wiring in the accident aircraft failed 11 minutes into flight, resulting in total loss of electrical power and loss of aircraft control.

Flight data

According to the recorded flight data, the data link was interrupted about 1 minute and 20 seconds after launch. Two seconds later, an aircraft software lost-link contingency response automatically issued a command to latch the current altitude command, navigate to the launch point, descend and land.

However, 1 second after the lost link contingency command was issued, an additional spurious command was issued. This inappropriate spurious command caused the aircraft to remain in an extended hover and prevented the operator from further affecting aircraft operation, even after re-establishing data link communications. Extended operations at hover power, well beyond electrical power supply system design limits, overheated the wiring and resulted in a connector failure causing interruption of the battery power supply and subsequent loss of aircraft control.

Software error

The system manufacturer found that an inappropriate spurious command issued by the aircraft software was due to a coding error in the VTOL software that had not been detected during testing.

Communications

Communication between the GCS and aircraft is not required for flight. If communication between the GCS and aircraft is lost during flight, the autopilot continues to fly the aircraft according to programmed contingency logic. If communications are lost for more than 5 seconds during the VTOL launch phase, contingency logic commands the aircraft to return to the launch point, descend and land.

Landing/emergency command options

Having commanded ‘abort’, the operator thought that the command would continue to be sent to the aircraft after communications were restored until the aircraft acknowledged receipt of the command. This did not occur. In the launch phase, the ‘abort’ command should cause the aircraft to navigate to the launch point and descend vertically to the ground.

No other appropriate commands were available to the operator.

Findings

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

• Communication between the GCS and aircraft was lost for about 5.5 minutes during the launch phase, due to the aircraft flying into the null in the antenna pattern above the antenna.
• A return to launch command was issued by the autopilot due to loss of communications, but was not completed, because a spurious command was issued due to a coding error.
• After 11 minutes at vertical climb power, the high current overheated the wiring. The heat unsoldered the negative main power cable resulting in a total loss of electrical power.
• The total loss of power resulted in a loss of control and the aircraft collided with the ground.

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 safety action in response to this occurrence.

UAS manufacturer

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

Software redesign

The software is undergoing testing, redesign and a review of contingency management in VTOL modes.

Safety message

This occurrence highlights the importance of UAS software testing to cover potential non-normal scenarios prior to release into operation.

Aviation Short Investigations Bulletin - Issue 59

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. See VTOL section

What happened

On 25 October 2016, an instructor was supervising students in the operation of a Lockheed Martin Stalker eXtended Endurance (XE) unmanned aerial system (UAS) (Figure 1) at the Avoca racetrack in Victoria.

Figure 1: Typical Stalker XE aircraft

Source: UAS manufacturer

At about 1700 Eastern Daylight-saving Time (EDT), the instructor reviewed the student’s programmed mission and proceeded through the ground control station (GCS) launch wizard application. The launch wizard functions as a pre-flight checklist and ensures that all systems are ready for flight and that the mission is valid. The student then armed the aircraft for launch, pulled the aircraft back on the bungee cord and released the aircraft in accordance with the normal bungee launch procedures.

After release, the aircraft tracked straight ahead for 10 seconds according to standard operation, then climbed and commenced a right turn towards the first programmed waypoint. After passing the first waypoint, the aircraft made another right turn towards the second waypoint. About 20 seconds into the flight, the aircraft entered an uncontrolled descent from about 225 ft above ground level and collided with the ground. The aircraft sustained substantial damage, and no one was injured.

Manufacturer’s investigation

Power failure

The aircraft was powered by a lithium polymer battery. A battery adapter cable was plugged into the aircraft’s battery. The adapter cable was connected to the aircraft’s main power cable via the main power (battery) connector. The main power cable was secured into a pocket at the back of the connector pin.

After the accident, the aircraft’s negative main power cable was found to have separated from its pin in the connector.

The battery connector consisted of two mating pairs of contacts. In normal operation, all current flow passed through spring contacts on the male plug to the female contact. Spring tension increased the mating area and generated pressure upon the surfaces.

A different Stalker XE aircraft also experienced a battery connector failure about one month prior to this incident. In both incidents, the failed battery connector pins had the following problems:

• significant pitting and erosion of the outer quarter of the male contact
• heat damage to the spring contact surface
• loss of spring tension in most or all of the contact springs
• minor pitting of the outermost edge of the female contact.

It was determined that the capacitive load of the electrical system caused inrush current,^[1] and the operator payload and communication links increased both the capacitive load and the power draw. The contact damage from the inrush current initiated the failure process. Sustained current, which produced contact heating, then removed the contact temper^[2] and significantly degraded the connection. The manufacturer was able to duplicate the unsoldering failures via this mechanism.

The connector failure was likely the result of multiple factors:

• Inrush current produced pitting, erosion and metal splatter on other contact surfaces.
• The erosion and splatter then produced localised areas of very high current density.
• The current density resulted in localised hotspots that removed the spring contact temper, which produced very high resistance on that spring contact.
• The load was then distributed across the remaining spring contacts, compounding the failure rate.
• Eventually the connector resistance was high enough that the normal operating current heated the solder joint until the solder melted and the circuit was interrupted.

The first steps probably took many flight cycles to develop, while the last probably required only seconds to a few minutes of normal operating current.

The separation of the negative main power cable from its pin in the main power connector interrupted electrical power to all of the aircraft systems. This included the autopilot, motor and control surface servos.

Uncontrolled flight

The flight data stopped suddenly at 225 ft – the subsequent lack of data logging indicated a power interruption to the autopilot.

Once battery power was lost, all systems in the aircraft were inoperative and any subsequent ‘flight’ was uncontrolled. Due to the loss of electrical power to the motor and the autopilot, the aircraft was unable to glide to a safe landing.

Findings

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

• The main power connector pins had sustained degradation from arcing and high temperatures due to inrush current, resulting in increased resistance.
• During the initial climb, high current across the increased resistance of the main power connector heated the negative pin to a temperature high enough to melt the solder on the negative main power cable.
• The separation of the negative main power cable from its pin in the main power connector interrupted electrical power to all aircraft systems, including the autopilot, motor and control surface servos.
• Due to the total loss of electrical power, the aircraft was unable to maintain normal flight or conduct a safe glide landing, therefore collided with the ground.

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 safety action in response to this occurrence.

Aircraft manufacturer

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

• All battery connectors are being replaced and sacrificial pigtails are being added to the connectors, to be replaced after a number of connections.
• The aircraft battery circuitry is being modified to prevent arcing during connection and disconnection.

Aviation Short Investigations Bulletin - Issue 57

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. Inrush current, input surge current or switch-on surge is the maximum, instantaneous input current drawn by an electrical device when first turned on (Wikipedia).
2. Tempering involves heating steel to a specific temperature to achieve certain properties such as hardness and elasticity and to remove brittleness.

The aircraft departed Denham for travel to Perth via a refuelling stop at Geraldton.

At about 0806 hours the aircraft crashed approximately 770 metres, bearing 347°M from the threshold of runway 21 at Geraldton, apparently out of control and with the stall warning audible throughout the final radio transmission.

Investigation Indicated that both engines had stopped prior to Impact due to fuel exhaustion. There were no Indications of fuel leaks, and an analysis of times flown by the aircraft preceding the crash revealed that the available fuel should have been exhausted at about the time the engines were reported to have stopped. Fuel was available at Denham but, for unknown reasons, was not utilised by the pilot.

The pilot did not lodge a flight plan with the Civil Aviation Authority nor was any evidence found in the wreckage to Indicate that the pilot had used a flight plan or fuel log enroute. No conclusive determination for the loss of control of the aircraft was reached, however, the proximity to the threshold of runway 21 and the configuration of the aircraft with the wheels down, suggests that the pilot may have been attempting to turn towards that runway for a glide approach and landing.

SIGNIFICANT FACTORS

The following factors were considered relevant to the development of the accident:

1. The aircraft departed Denham with Insufficient fuel for the flight to Geraldton.
2. The pilot was apparently unaware that the fuel state was Inadequate and did not refuel at Denham.
3. Both engines stopped due to fuel exhaustion.
4. Control of the aircraft was lost at a height which did not allow the pilot to recover.

What happened

On 12 October 2016, at about 1330 Eastern Standard Time (EST), a Pilatus BN2A-20, registered VH-IOA (IOA), departed from Kubin on a charter flight to Horn Island, Queensland (Figure 1). On board IOA were one pilot and two passengers. At about the same time, a Bombardier DHC-8-202, registered VH-ZZJ (ZZJ), conducting surveillance operations, was preparing for departure from Horn Island. On board ZZJ were two pilots and three crewmembers.

Figure 1: IOA track from Kubin to Horn Island

Source: Google earth, annotated by ATSB

The Kubin authorised landing area (ALA) is within the Horn Island broadcast zone, and the pilot of IOA reported that they made their departure call on the Horn Island common traffic advisory frequency (CTAF). At the time IOA departed from Kubin, a faster company aircraft, registered VH-WOT (WOT), departed from Badu Island (Figure 1) on track to Horn Island. In addition to WOT, there was a training aircraft conducting circuits on runway 08 at Horn Island. While tracking to Horn Island, the pilot flying IOA heard a radio broadcast from WOT, which indicated they would join the runway 08 circuit as number 2 to the training aircraft. The area controller^[1] then passed IOA traffic information about a DHC-8 (ZZJ) aircraft taxiing at Horn Island for departure from runway 08. The pilot flying IOA planned to join a straight-in approach to runway 14 from their track from Kubin. The pilot reported that they made all the required radio broadcasts for an arrival to Horn Island including broadcasts at 10 NM, 5 NM and 3 NM before they joined the final approach for runway 14.

The first officer on board ZZJ made a broadcast they were entering and backtracking runway 14 at Horn Island for a departure from runway 08 (position 1 on Figure 2). Just prior to the intersection of the two runways, ZZJ held short of runway 08 to allow the training aircraft to complete a touch-and-go landing on runway 08. The first officer then broadcast ZZJ was entering and backtracking runway 08. While backtracking runway 08 (position 2 on Figure 2), ZZJ’s flight crew heard a broadcast that WOT was joining the circuit as number 2 to the training aircraft and then observed a third aircraft appear on their traffic collision avoidance system (TCAS) display. The captain of ZZJ asked the first officer to contact the third unknown aircraft.

When the pilot flying IOA heard a broadcast from ZZJ, which indicated it was directed at them, they responded that they were approaching a 3 NM final for runway 14 and ZZJ would have time to depart from runway 08 if they were quick. At about this time, ZZJ was approaching the threshold of runway 08. The captain lined ZZJ up for departure and the crew completed their pre-take-off checklist items (three items). At about this time, another two aircraft, 20 NM away from Horn Island, started communicating with each other on the Horn Island CTAF.

By the time the pilot flying IOA heard ZZJ broadcast ‘rolling for departure from runway 08’, IOA was at about 400 ft on final approach for runway 14. The pilot of IOA reported that at about 300 ft, they broadcast ‘hold short’ to ZZJ. However, the captain of ZZJ reported they heard the pilot of IOA say ‘land and hold short of runway 08’.^[2] Consequently, ZZJ continued their take-off. As IOA approached 100–150 ft on final approach, ZZJ had not crossed the runway intersection and the pilot flying IOA conducted a left climbing turn away from the runways to join the downwind circuit leg for runway 08. The captain of ZZJ looked out their left window when they were in the initial climb overhead the threshold of runway 26 and saw IOA turning through east at about the same level (position 4 on Figure 2). IOA then joined the circuit for runway 08 and landed after WOT without further incident.

Figure 2: Sequence of movements

Source: Google earth, annotated by ATSB

Horn Island Airport and CTAF

The Horn Island Airport is located at the northern end of Cape York Peninsula. The airport acts as the hub for access to the outer islands in the Torres Strait. Mainland services fly into Horn Island and passengers are then transferred onto the local operators’ smaller aeroplanes and helicopters for transfer to and from the outer islands. Runway 08/26 is the main runway and runway 14/32 is shorter and narrower. Smaller aircraft, such as IOA, operate to both runways, but larger aircraft, such as ZZJ, restrict their operations to runway 08/26.

The airport apron is located adjacent to the threshold of runway 32 and there are no taxiways to separate ground movements from aircraft taking off and landing. There is also higher terrain located to the southeast of the airport, which has resulted in the following additional restrictions to airport movements published in the Horn Island aerodrome chart:

Take-off runway 14 and landing runway 32 not permitted due terrain.

Consequently, the smaller local aeroplane operators’ have adopted the local practice of departing from runway 08 and landing on runway 14, weather conditions permitting.

The CTAF boundary extends laterally to 40 NM from Horn Island, which encapsulates the outer islands, and vertically from the surface to 8,500 ft.

ZZJ TCAS settings

The flight crew on board ZZJ had their TCAS set to the 12 NM range scale while backtracking runway 08. The captain reported that they could have improved their picture of the relative bearing and distance of IOA if they reduced the scale to 6 NM while backtracking runway 08. However, their normal procedure is to set the 6 NM range scale for controlled airspace and set the 12 NM range scale for departure from a non-controlled aerodrome to improve situational awareness of inbound traffic.

Right of way provision

Civil Aviation Regulation (CAR) 162 lists the rules for prevention of collision. CAR 162 paragraph 8 states:

An aircraft that is about to take-off shall not attempt to do so until there is no apparent risk of collision with other aircraft.

The captain of ZZJ reported that they relied too heavily on their TCAS and did not place sufficient importance on the ‘see’ element within the ‘see-and-avoid’ philosophy of operations at non‑controlled aerodromes. Consequently, they started their take-off before sighting IOA.

Safety analysis

Prior to the near collision event, the flight crew of both aircraft were aware of the presence of the other aircraft and their approximate position. When ZZJ was backtracking runway 08, the captain was conscious of the fact that they were occupying the main runway that was being used by other aircraft in the circuit.

The broadcast from IOA, that ZZJ could depart before IOA landed, supported the motivation of the captain of ZZJ to avoid delaying their departure from runway 08. The captain of ZZJ also relied on an approximate position of IOA from their TCAS when they elected to take-off. However, as reported by the captain, the range scale set on the TCAS was 12 NM, where a 6 NM range scale could have provided a more accurate picture of the relative position of IOA.

During the take-off, the flight crew probably also misheard the broadcast from the pilot in IOA, to hold short of runway 14, and therefore did not reject the take-off. However, it was not determined if the radio broadcast was before or after ZZJ reached their decision speed (V1) to safely reject the take-off.

Findings

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

• The captain of ZZJ was motivated to expedite their departure from the main runway, which in conjunction with the broadcast from IOA that they had time to take-off before IOA landed, contributed to them starting their take-off before they sighted IOA.
• The flight crew probably misheard the broadcast from the pilot of IOA to hold short of runway 14 and did not reject the take-off.
• IOA and ZZJ made the required CTAF broadcasts and were aware of the approximate position of each other prior to the near collision event.
• Concurrent operations to different runways at Horn Island are a normal local practice, which is employed to facilitate traffic movements.

ATSB comment

The ATSB notes that the horizontal and vertical dimensions for standard CTAF boundaries are published in the Aeronautical Information Publication (AIP). However, the dimensions for non‑standard CTAF boundaries are currently only published in the Enroute Supplement Chart LOW.

Safety message

This serious incident highlights that pilots and operators need to consider how best to employ and integrate the sources of information available to them in order to develop an accurate mental model of a potential traffic threat. In the ‘see-and-avoid’ environment, radio broadcasts and TCAS information can be used to hone the visual scan to sight other traffic, which might pose a threat.

Further information on safety around non-controlled aerodromes is available from the ATSB website.

Further information on operations at non-controlled aerodromes is available from the Civil Aviation Safety Authority’s website.

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1. IOA and ZZJ were both operating under instrument flight rules, which is why IOA received traffic information on ZZJ while operating outside controlled airspace.
2. Horn Island Airport experienced a temporary loss of power on the day of the incident. None of the transmissions from the incident aircraft were captured on the available CTAF recorded data.

Aviation Short Investigations Bulletin - Issue 57

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.

What happened

On 9 October 2016, a Singapore Airlines Boeing 777-312 aircraft, registered 9V-SYG (SYG), was operating a scheduled passenger service from Melbourne Airport, Victoria, to Singapore with two flight crew, 16 cabin crew and 261 passengers.

The crew arrived on board the aircraft and commenced their standard pre-flight procedures. The captain was operating as pilot flying (PF) and the first officer operating as pilot monitoring (PM).^[1] The captain commenced the flight deck pre-flight procedures while the first officer performed the exterior inspection. After completion of the external inspection, the first officer commented to the captain that while on the apron ‘they had difficulty walking straight due to the strong wind’. The flight crew received the automatic terminal information service (ATIS)^[2] using the aircraft communication addressing and reporting system. ATIS W advised wind conditions at Melbourne Airport were 325°at 25 kt, gusting to a maximum of 45 kt, and turbulence had been reported in the control zone.

Both flight crew reviewed the load sheet and independently performed take-off performance calculations in accordance with normal procedures. The figures from these calculations were correctly programmed into the aircraft’s flight management computer. The flight crew stated that, in accordance with the operator’s standard operating procedures, they briefed the use of full climb thrust after becoming airborne to mitigate the strong and gusty wind conditions.

At about 1120 Eastern Daylight-saving Time (EDT), the aircraft was pushed back and taxi was commenced. The flight crew stated that while taxiing to the runway 34 holding point, they observed two aircraft on approach to runway 34 perform go-arounds. Both flight crew recalled hearing another aircraft query the tower controller if windshear was reported by the flight crew of the go-around aircraft. The tower controller stated, ‘no windshear, just unstable conditions’. Two aircraft departed prior to SYG with the tower controller again advising the departing aircraft that no windshear was reported. At about 1145, SYG was then cleared for take-off from the full length of runway 34.

During the take-off run, both flight crew recall observing airspeed fluctuations on the airspeed indicator due to wind gusts. Both flight crew stated that, in their opinion, they considered the aircraft’s acceleration rate to be normal during the take-off run. At the calculated rotation speed (Vr),^[3] the PF initiated the aircraft rotation. During rotation, the PM observed a downward speed trend, below Vr on the ASI and called ‘SPEED’. The PF did not recall hearing this callout. The PF continued rotation, however, the aircraft did not achieve lift-off at the manufacturer’s stated lift‑off attitude (7 degrees). Flight data analysis shows the aircraft became airborne at 10.7 degrees pitch^[4] attitude (see Flight data analysis).

After take-off, air traffic control contacted the flight crew alerting them of a ‘possible tail strike’. With no TAIL STRIKE caution message displayed on the engine indication and crew alerting system^[5] the flight crew carried out the unannunciated tail strike non-normal checklist and determined the aircraft structural integrity was intact. The flight crew then referred to the operator’s supplementary procedures for further guidance.

An inspection of the runway identified contact marks, consistent with a tail skid contact. No metallic debris was observed on the runway. Air traffic control advised the flight crew that ‘only superficial concrete debris was found’ during the runway inspection. The captain communicated with the in-flight supervisor who reported back to the captain that cabin crew stationed at the rear of the aircraft heard a ‘loud bang’ during take-off.

The flight crew discussed all the available information and considered their options. With the aircraft pressurisation system indicating no abnormalities the captain made the decision to continue to the destination. This decision was supported by manufacturer’s recommended action to continue to operate normally in the case of an unannunciated tail strike in the B777-300 aircraft.

Subsequently, an uneventful landing was carried out in Singapore. Engineers conducted a post-incident inspection of the aircraft and found no damage to the aircraft fuselage. Damage was evident to the tail skid system with indications of a scraped tail shoe, compression of the crushable cartridge and one indicator pin extended (Figure 1).This damage indicated that a moderate energy skid contact had occurred during take-off.

Figure 1: Damage to tail skid

Source: Singapore Airlines, modified by the ATSB

Safety analysis

Flight data analysis

Analysis of the aircraft flight data showed multiple instances of airspeed stagnation from 77 kt computed airspeed through rotation initiation at 178 kt (Vr = 178 kt) and initial climb. Rotation was initiated at a computed airspeed of 178 kt (at Vr) at approximately 0.5 degrees per second initially before increasing to approximately 3 to 4 degrees per second. As rotation was initiated, the headwind component decreased 12 kt, the computed airspeed stagnated and reduced to 173 kt (Figure 2). Lift-off occurred at a pitch attitude of 10.7 degrees. The tail skid contact attitude is 8.9 degrees.

Figure 2: Flight data plot including computed airspeed, and rotation speed (Vr) and pitch attitude

Source: Aircraft operator analysed by ATSB

Wind and airspeed

After reaching rotation speed (Vr), the aircraft’s airspeed reduced by about 5 kt due to a reduction in headwind of about 12 kt.

Continued rotation

The PF reported not hearing the PM call of speed after the PF had rotated the aircraft. If the PF was aware of the speed reduction, the standard procedure, described in the operator’s Flight Crew Training Manual (FCTM), was to momentarily delay rotation. The PF reported that they thought the rotation was normal in the conditions.

Tail skid strike

The aircraft did not become airborne at the manufacture’s pitch attitude of 7 degrees, leading the PF to continue increasing the pitch attitude to 10.7 degrees where lift-off was achieved. This increased pitch attitude exceeded the 8.9 degrees attitude for where a tail strike will occur in the Boeing 777-300 aircraft.

Guidance to flight crew

The operator’s Flight Crew Operations Manual (FCOM) stated that the use of reduced thrust is standard procedure for take-off. The FCOM also listed the environmental conditions when take-offs with reduced thrust are not permitted.

The operator’s FCOM does not contain direct guidance regarding take-off thrust setting requirements in gusty wind and strong crosswind conditions. Guidance for considering the use of higher thrust settings and rotation speeds for take-offs under these environmental conditions is provided in the Flight Crew Training Manual (FCTM).

Findings

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

• The tail skid contact was a result of airspeed stagnation due to gusty atmospheric conditions which prolonged the time to lift-off, allowing the pitch attitude to exceed the tail skid contact attitude.
• The use of a higher take-off thrust setting would most likely have reduced the required runway length and minimised the aircraft exposure to gusty atmospheric conditions during rotation and lift-off.

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 safety action in response to this occurrence.

Flight Operations

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

Action taken by Singapore Airlines

As a result of this incident, the aircraft operator issued circulars to all company flight crew directing operation towards Boeing’s recommendation of the use of higher thrust and rotation speed for take-off in gusty wind and strong crosswind conditions.

Safety message

This incident serves as a reminder to Boeing pilots that guidance material contained in manuals outside the FCOM should be considered in all aircraft operations. The use of a higher thrust setting as recommended by the Boeing FCTM would have reduced the required runway length and minimised the airplane exposure to gusty conditions during rotation, lift-off and initial climb. Boeing also states that the use of a higher take-off rotation speed, if take-off performance permits, can increase the tail clearance margin during the rotation.

While taking the above message into consideration, this incident provides an excellent example of flight crew managing a non-normal operation. Throughout the non-normal occurrence period, the flight crew communicated with each other, air traffic control and the cabin crew, which allowed all relevant information available to be gathered. The flight crew demonstrated effective crew resource management and decision making resulting in the flight being able to continue to destination without compromising safety.

Aviation Short Investigations Bulletin - Issue 59

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. Pilot Flying (PF) and Pilot Monitoring (PM): 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 the aircraft’s flight path.
2. Automatic Terminal Information Service (ATIS). The provision of current, routine information to arriving aircraft and departing aircraft by means of continuous and repetitive broadcasts.
3. VR: the speed at which the rotation of the aircraft is initiated to take-off attitude. This speed cannot be less than V1 or less than 1.05 times VMCG. With an engine failure, it must also allow for the acceleration to V2 at a height of 35 ft at the end of the runway.
4. Pitch: the motion of an aircraft about its lateral (wingtip-to-wingtip) axis.
5. Engine Indication and Crew Alerting System (EICAS) consolidates engine and airplane system indications and is the primary means of displaying systems indications and alerts to flight crew.

What happened

On 12 October 2016, a Vee H Aviation Fairchild Industries Inc. SA227-DC, registered VH-VEU, conducted a regular public transport flight from Armidale, New South Wales, to Brisbane, Queensland. On board the flight were two flight crew and 13 passengers. The captain was the pilot flying (PF) and the first officer was the pilot monitoring (PM).^[1]

At 0755 Eastern Daylight-saving Time (EDT), the aircraft was about 170 km south of Brisbane, cruising at FL 170,^[2] when the aircraft suddenly yawed to the right.^[3] The PF re-stated they had command of the aircraft and directed the PM to identify the failure. The flight crew then employed their ‘identify and confirm’ crew resource management (CRM) procedures to confirm the right engine was not delivering power and then shut down the right engine and feathered the right propeller.^[4] During the diagnosis, the PM noted that all right engine indications were normal except for a low torque reading (10%) and low fuel flow (140 pounds per hour).

Air traffic control contacted the crew to confirm they were maintaining FL 170 and the PM responded with a PAN broadcast^[5] that they were descending due to a right engine failure. The crew reviewed their options and decided to continue to Brisbane Airport. They completed the remaining checklist actions and briefed the passengers. The crew then requested, and were given, a direct track to Brisbane Airport from air traffic control. They completed their normal and single engine landing checklist procedures and landed at Brisbane Airport runway 19 without further incident.

Maintenance findings

The engine installed in the aircraft was the Honeywell (previously Garrett) TPE331-12UHR-701G. The operator’s engine maintenance organisation found a retainer ring within the engine accessory gear assembly had failed, which allowed the main shaft (which drives the propeller) to de-couple from the engine driven reduction gearbox (Figure 1).

The retainer ring was shipped to the maintenance organisation from Honeywell in December 2008 as part of a batch of 10 with a certificate of conformance from the part manufacturer and Honeywell. It was fitted new to the incident engine in December 2009 at the last engine overhaul, about 2,429 hours prior to the failure. The maintenance organisation introduced this practice of replacing the retainer ring at each overhaul based upon their previous service experience of this part failing. December 2009 was the last overhaul of the accessory gear assembly prior to the failure.

In October 2015, Honeywell added temporary revision 72-241 to the maintenance manual procedure: removal and installation of accessory gear assembly, to direct the replacement of the retainer ring at each exposure. Figure 2 depicts the failed retainer ring.

Figure 1: Engine accessory gear assembly

Source: Maintenance organisation, annotated by ATSB

Figure 2: Retainer ring

Source: Operator

Continuing airworthiness maintenance interval

The operator set their maintenance interval for the incident engine in accordance with the engine manufacturer’s service bulletin for periodic inspections, (Honeywell TPE 331-72-0476). From the service bulletin, the operator set the inspection of the engine at the 7,000 hour continuing airworthiness maintenance (CAM) interval with gearbox inspection, for commercial operations. This included the requirement for the accessory gear assembly inspection in accordance with the maintenance manual procedures at the 7,000 hour interval.

The operator’s spectrometric oil-analysis programme (SOAP analysis) was set at 150 hour intervals at the time of the incident.^[6] The previous SOAP analysis was conducted at about 26 hours prior to the failure and did not detect any anomalies. Further information on SOAP is available from Civil Aviation Safety Authority airworthiness bulletin (AWB 79-1): Spectrographic oil analysis program (SOAP).

The gearbox is fitted with a single magnetic drain plug (chip detector).^[7] If a metallic particle is detected by the chip detector inflight, it will activate a caution light to advise the flight crew. The inspection interval for the chip detector is set at 300 hours. The last inspection was about 144 hours prior to the incident and no anomalies were found. There were no activations of the chip detector between the last scheduled inspection and the incident flight, and the chip detector did not activate during the incident flight.

The operator also conducts propeller dynamic balance checks at 600 hour intervals. There have been no out-of-limit vibration indications since engine installation. The last check was performed 17 September 2016.

Safety analysis

The operator had several preventive maintenance inspections in place, which included an overhaul of the gearbox, SOAP analysis, magnetic drain plug inspection and propeller dynamic balance. The previous overhaul was about 2,429 hours prior to the failure at which time the retainer ring was fitted new to the gearbox. During the time interval to failure of the retainer ring, the SOAP analysis, magnetic drain plug inspections and propeller dynamic balance checks did not detect any anomalies. Therefore, the failure of the retainer ring was within the required gearbox inspection intervals and without prior warning of an impending failure.

Findings

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

• The retainer ring failed within the prescribed maintenance interval.
• There was no prior warning of an impending failure of the retainer ring.

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 and subsequent to an update by the engine manufacturer to the engine manufacturer’s service bulletin, the aircraft operator has advised the ATSB that they are taking the following safety actions:

SOAP analysis

The operator reduced their SOAP analysis interval from 150 hours to 100 hours.

Damaged parts

The damaged parts from the gearbox were sent to the engine manufacturer for analysis.

Safety message

Following the aircraft yaw, the flight crew actively employed their crew resource management procedures to identify and confirm the engine fault and then shut down the right engine. The use of these procedures reduced the risk of an incorrect diagnosis of the fault or activation of the incorrect engine controls during shut down.

__________

1. Pilot Flying (PF) and Pilot Monitoring (PM): 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 the aircraft’s flight path.
2. 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 170 equates to 17,000 ft.
3. Yawing: the motion of an aircraft about its vertical or normal axis.
4. Feathering: the rotation of propeller blades to an edge-on angle to the airflow to minimise aircraft drag following an in-flight engine failure or shutdown.
5. 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.
6. SOAP is a method to test the health of engines by performing laboratory testing of the engine oil. A sample of oil showing an increase in parts per million of iron material could be a warning of impending failure. The chemical composition of any metal particles in the oil sample is compared to various engine parts to detect the location of abnormal wear.
7. A chip detector is a device, often a permanent magnet, for gathering metal chips from the engine oil to provide early warning of an impending failure. A magnetic drain plug is a removable chip detector.

Aviation Short Investigations Bulletin - Issue 57

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.

Safety summary

What happened

On 6 October 2016, a Boeing 747-438 ER, registered VH-OEH and operated by Qantas International, departed San Francisco International Airport, United States, for a flight to Sydney, Australia. Soon after departure and during the climb, the crew became aware of an abnormal vibration in the area of the left over-wing door. There were no cockpit indications or associated procedures to manage the observed vibration. Consequently, the flight crew were required to use their professional judgement to assess its impact on the flight. The crew utilised available crew resources and maintenance expertise to troubleshoot and assess that the flight could safely continue to Sydney.

What the ATSB found

The ATSB found that during departure from San Francisco International airport, and unbeknown to the flight crew, the aircraft sustained impact damage from a birdstrike. The birdstrike sheared a landing gear door strut resulting in the door not closing. That in turn led to turbulent airflow and in‑cabin vibration.

What’s been done as a result

While the birdstrike resulted in a landing gear door not closing, the overall impact on the flight was minimal. Larger doors that have the potential to cause damage have warning systems to indicate if they have not operated as selected.

There was no additional wear or damage to any of the landing gear components. The landing gear door strut was replaced and the aircraft returned to service.

Safety message

This occurrence highlights that, even with the assistance of sophisticated technology and systems, flight crew may experience situations that can only be managed using their professional judgement.

In this case, the flight crew utilised the support of both on-ground maintenance personnel and cabin crew to methodically troubleshoot the source of the vibration and assess that the flight could be safely continued to the destination airport.

 

The occurrence

At about 2335 local time on 6 October 2016, a Boeing 747-438 ER, registered VH-OEH and operated by Qantas International, departed San Francisco International Airport, United States, for a flight to Sydney, Australia. The flight crew reported that the night departure was uneventful.

Soon after take-off, the cabin services manager (CSM) was contacted via the aircraft interphone system by a cabin crew supervisor seated near door L3 (Figure 1). The supervisor advised the CSM that there was vibration and louder than normal cabin noise in their area. The CSM recalled that following receipt of that advice from the supervisor, they also became aware of the increased noise.

A training captain seated in the flight deck, also reported being aware of a louder than normal noise during the climb. The captain under training was new to the aircraft type and therefore did not notice the additional noise.

As the aircraft approached top of climb, and after the flight crew had turned off the fasten seatbelts sign, the CSM contacted the flight crew to report the additional noise/vibration. As indications on the flight deck were normal, the second officer was sent to investigate the source of the vibration.

The second officer inspected several sections of the cabin in consultation with the CSM and other flight attendants. They concluded the magnitude of the vibration was largest near door L3 (rows 47-49, seats ABC) and that it was considerably noisier than normal. The CSM recalled that the second officer suspected the vibration may have been due to a landing gear door seal. Unlike other large passenger aircraft, VH‑OEH did not have installed video cameras that permitted the crew to view external parts of the aircraft. In any event, the night conditions may have hampered visual examination of the landing gear area on this occasion. The second officer returned to the flight deck and briefed the other flight crew.

About 2 hours into the flight, the flight crew contacted the company’s maintenance operational control (MOC) centre via satellite phone seeking technical assistance to identify the source of vibration. The flight crew relayed the relevant information to the duty manager, a senior aircraft engineer, who then asked additional questions to assist with the diagnosis. The MOC duty manager advised the crew that it could be a gear door seal or wing-to-body seal as they were located in that area.

The MOC duty manager subsequently checked the:

• aircraft history for any similar faults/reports, of which there were none
• relevant aircraft system diagrams to ascertain what systems were located near door L3 and capable of generating the reported vibration/noise
• relevant maintenance manual section covering possible sources of airframe vibrations.

The flight crew, in consultation with the MOC duty manager and cabin crew, then systematically went through a process of trying to isolate the source of the vibration/noise.

That process included isolation and/or consideration of:

• the aircraft air-conditioning packs
• the air recirculation fans
• re-selection of the landing gear handle to the UP position
• checking toilets for possible vacuum leaks.

That troubleshooting was unable to isolate the source of vibration and noise. The flight crew requested that the cabin crew continue to monitor the vibration/noise and to advise them of any change. As the aircraft was otherwise performing normally, the flight was continued.

About 4.5 hours into the flight and with no reported change to the vibration/noise levels, the training captain left the flight deck to personally assess the vibration and assessed that it was still quite noisy. Consequently, the flight crew again made contact with the MOC to seek further technical input. They asked the duty manager what other aircraft systems were in the L3 door area. The duty manager advised that there were hydraulics and centre fuel tank transfer pumps. While the crew could not isolate the hydraulics, the transfer pumps were OFF.

The duty manager advised the crew that maintenance services were available at the now nearby Honolulu Airport, Hawaii and enquired if the crew wished to divert. The crew advised the MOC that they did not intend to divert at that stage. The training captain reported to the ATSB that, as the aircraft was otherwise performing normally, it was not considered necessary to divert from the original destination.

The flight crew, with the assistance of the cabin crew, continued to monitor the vibration (which remained constant) and the flight continued to Sydney. They also remained in contact with the MOC.

Figure 1: Aircraft schematic showing location of door L3 (circled red) and other services

Source: Boeing Commercial Airplanes

The CMS reported that the vibration and noise persisted up until the landing gear was extended during the approach into Sydney. The cabin crew did not receive any comments about the vibration or noise from the passengers.

After landing, ground engineers inspected the aircraft and immediately identified a protruding landing gear seal and initially thought this was the source of the vibration. On closer examination they observed that a rod, which normally attaches a small outboard landing gear door to the body landing gear shock strut (Figure 2), was sheared at the door attachment point as a result of a birdstrike.

Despite the door not being closed during the flight, there was no additional wear or damage. The attach rod was replaced, and the aircraft returned to normal service, with no further vibration or noise reported.

Figure 2 Main landing gear doors showing location of affected door and attach rod.

Source: Boeing Commercial Airplanes

Safety analysis

Due to the physical size of the aircraft and the night departure, the birdstrike was not detected by the flight crew or ground personnel. Given the vibration was first noticed following retraction of the landing gear, it could be argued that the vibration was therefore directly related to the landing gear. However, all indications were that the landing gear had retracted normally. In addition, as there is significant additional noise and vibration while the landing gear is extended, it is equally plausible that this noise may have been masking something unrelated to the landing gear.

While the training captain was aware of some additional noise during the climb, all flight deck indications were normal including those for the landing gear. Due to sterile cockpit procedures, that limited other than emergency communications during critical phases of flight, the first indication that there was something abnormal was when the cabin services manager contacted the flight deck.

Once the nature and location of the vibration had been established, the flight crew proceeded to make use of all available resources to try and identify the source of the vibration and its implication for the continuation of the flight. In consultation with both maintenance operational control and cabin crew, the crew systematically isolated systems capable of generating the vibration. While they were unable to identify the source of the vibration, the flight crew established that it was constant and that the aircraft was otherwise performing normally. In the absence of any abnormal flight deck indications or relevant checklist, the decision to continue the flight was therefore at the discretion of the flight crew based on their professional judgement.

The landing gear system on the Boeing 747 uses a combination of hydraulics and attach rods to extend/retract both the landing gear and the associated doors. Proximity switch sensors located on both the landing gear and the hydraulically operated doors provide the flight crew with indications of both normal and abnormal operation. Small doors that are attached by rods directly to the landing gear, do not have proximity sensors as they normally open and close with the landing gear.

The discovery of the sheared attach rod, evidence of a birdstrike, and the reported in-flight vibration, is consistent with the aircraft having sustained a birdstrike during the departure and while the landing gear was extended. Due to the sheared attach rod, the associated landing gear door would not have closed resulting in residual turbulence and vibration being generated in the area of door L3. The lack of an associated proximity sensor meant that the flight crew were unaware that the door was not closed.

Findings

From the evidence available, the following findings are made with respect to the birdstrike and inflight vibration involving Boeing 747, registered VH-OEH near San Francisco International Airport, United States on 6 October 2016. These findings should not be read as apportioning blame or liability to any particular organisation or individual.

Contributing factors

• During departure from San Francisco International airport, and unbeknown to the flight crew, one of the main landing gear door struts sustained impact damage from a birdstrike that resulted in the associated landing gear door not closing.

Other findings

• The noise and vibration experienced in the cabin was due to turbulent airflow associated with the un-closed landing gear door.
• The absence of cockpit indications and/or associated procedures to manage the observed vibration meant that the flight crew were required to use their professional judgement to assess its impact on the flight. The crew utilised available crew resources and maintenance expertise to troubleshoot and assess that the flight could safely continue to the destination airport.

Sources and submissions

Sources of information

• The sources of information during the investigation included the:
• flight crew
• cabin crew
• maintenance personnel
• aircraft operator and manufacturer.

Submissions

• Under Part 4, Division 2 (Investigation Reports), Section 26 of the Transport Safety Investigation Act 2003 (the Act), the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. Section 26 (1) (a) of the Act 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 aircraft operator, flight crew, cabin services manager, maintenance operational control duty manager and the Civil Aviation Safety Authority (CASA).
• Submissions were received from CASA, the aircraft operator and maintenance personnel. The submissions were reviewed and where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations & publishing information

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.