Executive summary

What happened

On the morning of 13 May 2024, a Beechcraft King Air B200, registered VH-XDV and operated by Eastern Air Services was being used for a passenger transport flight from Williamtown Airport to Lord Howe Island, New South Wales. On board the aircraft were the pilot and 2 passengers. Shortly after the aircraft departed Williamtown, the landing gear malfunctioned and jammed in a partially retracted position. The pilot was unable to extend the gear using the emergency procedures.

After approximately 4 hours of holding the pilot completed a wheels-up landing at Williamtown Airport with emergency services standing by. The aircraft sustained minor damage from the landing incident. The pilot and passengers were uninjured.

What the ATSB found

The ATSB identified that during either the take-off roll or landing gear retraction sequence the steering link fractured from a pre-existing fatigue crack, resulting in a mechanical disconnect within the nose landing gear steering system.

When the landing gear retracted, the fractured steering link allowed the nose gear shimmy damper to contact the side of the wheel well and jam the nose landing gear in a partially retracted position. This prevented extension of the landing gear using the published emergency procedure, necessitating a wheels-up landing.

A review of the ATSB database did not identify any similar previous occurrences and no other failures of this type were identified by the aircraft manufacturer.

What has been done as a result

Eastern Air Services reported that in the days following the incident it conducted a maintenance inspection of the landing gear system on the other King Air B200 aircraft (registered VH-MVP) within its fleet. No defects were found. On 28 August 2024, VH-MVP underwent a scheduled maintenance check, during which the nose gear steering link was removed for an additional non‑destructive dye-penetrant crack inspection. No defects were found in the steering system of that aircraft. 

The operator further advised that it is reviewing its B200 maintenance program to consider including a non-destructive inspection of the steering link, which would be additional to the manufacturer’s existing maintenance requirements.

Safety message

The ATSB advises King Air B200 operators and maintainers that in this incident, fatigue‑related fracture of a steering link in the nose landing gear system led to the landing gear becoming inoperative. Although scheduled maintenance inspections required general inspection of the nose steering parts, the inspections did not call for a detailed inspection for cracks. 

This incident also highlights the value of aircraft system knowledge and resource management in resolving malfunctions and in-flight emergencies. The pilot established that the available fuel endurance allowed time to carefully consider the circumstances and attempted to resolve the issue. They engaged company personnel to provide system troubleshooting information and sought the assistance of the air traffic control personnel to inspect the aircraft.

The pilot also liaised with emergency services and prepared the passengers for the wheels‑up landing. This minimised the risk of injury and ensured the evacuation was conducted safely.

The investigation

The occurrence

On the morning of 13 May 2024, a Beechcraft King Air B200, registered VH-XDV and operated by Eastern Air Services, was being prepared for a multi-sector passenger transport flight from Williamtown Airport, New South Wales. The flight was to transport 2 passengers from Williamtown to Lord Howe Island, with an intermediate stop at Port Macquarie to collect an additional 6 passengers. 

Both passengers boarded at Williamtown and at about 0830 local time the pilot taxied the aircraft from the departure bay to runway 30. A passenger video recording captured the pilot’s actions and cockpit area throughout the departure. No problems were identified with the aircraft during taxi. The recording also identified that, as the pilot retracted the landing gear during the initial climb, mechanical crunching noises were audible. As the aircraft continued to climb, the pilot commenced a right turn and noted that the red indicator lights on the landing gear control handle remained illuminated, signifying that the landing gear remained in transit or was not locked. 

The pilot then contacted air traffic control (ATC) and reported a landing gear indication fault. The pilot then cancelled their airways clearance to Port Macquarie and requested clearance from ATC to remain in the Williamtown circuit to complete functional checks of the landing gear system. 

The pilot was directed to operate in a southern circuit so that the aircraft would remain visible to the tower controller. The controller visually identified that the landing gear was partially retracted, which they conveyed to the pilot. In response, the pilot declared PAN PAN[1] and the controller initiated the airport emergency plan.[2] Additional controllers were then called to the tower to help manage the emergency. 

Emergency management and response

While in the Williamtown circuit, the pilot consulted the abnormal procedures contained in the flight crew operating manual to manually extend the landing gear (see the section titled Landing gear system and nose wheel steering). Only 2 or 3 pumps of the alternate extension lever could be achieved before maximum resistance was encountered. The lever had no effect on moving the gear to the down and locked position, despite additional force applied by the pilot. The red indicator lights on the gear handle also remained illuminated, which provided further indication that a malfunctioned had occurred.

At about 0930, while the aircraft was still in the circuit, the pilot confirmed the problem with ATC and that a wheels-up landing would be required. ATC then upgraded its response to a Level-1 emergency and notified the civil emergency services of the situation. An image of the aircraft as it flew overhead Williamtown Airport was recorded by an observer, it showed the landing gear partially retracted and the nose wheel tilted to the left (Figure 1). 

Figure 1: VH-XDV with landing gear partially retracted as it overflew Williamtown Airport

During a normal retraction cycle of the landing gear the nose wheel automatically centres. Source: Department of Defence

The pilot maintained a holding pattern in the southern circuit over Williamtown Airport for approximately 3 hours and 17 minutes. With rain approaching from the south, and a lowering cloud base, the pilot’s desire to remain in visual meteorological conditions[3] and consume additional fuel prompted them to track along the coastline to the north-east while the weather passed (see Figure 2 and the section titled Pilot’s commentary of the emergency). Upon arrival over Hawks Nest, ATC advised the pilot that the Williamtown Airport weather conditions had improved.

While the pilot positioned the aircraft at the instrument approach waypoint[4] for Williamtown Airport, the weather conditions deteriorated and the aircraft entered cloud and heavy rain. During this period the pilot identified that on landing the fuel quantity onboard the aircraft may reduce below the required final reserve.[5] ATC tower recordings identified that the pilot declared a MAYDAY FUEL[6] at 1206.

At 1219 with the gear still jammed in the partially retracted position, the aircraft touched down on runway 30 and came to a sliding stop after about 20 seconds. Airport rescue and firefighting and other waiting services were then cleared to attend and entered the runway. 

No injuries were sustained by the pilot or passengers on board. The aircraft sustained minor damage from the landing incident and there was no fire.

Figure 2: VH-XDV completed numerous circuits at Williamtown before transiting to Hawks Nest, then returning to Williamtown

The aircraft track was obtained from the automatic dependent surveillance-broadcast data transmitted from the aircraft. Source: Google Earth, annotated by the ATSB

Context

Aircraft information

The Beechcraft King Air B200 is a pressurised, low-wing, twin turbine-engine aircraft with retractable landing gear. The aircraft had a certified maximum take-off weight of 
5,670 kg and could be flown by a single pilot. The aircraft, serial number BB-1100, was manufactured in the United States in 1982 and subsequently registered in Australia in 2008. Eastern Air Services had been the registered operator of the aircraft since February 2018.

Operator’s examination of the aircraft

The operator’s post-incident examination of the aircraft identified that it had sustained minor damage during the landing. Both main landing gear, the radio antennas and the underside airframe panels had been abraded. Additionally, both propellors had sustained rotational contact damage from striking the runway. 

During their examination of the landing gear system, the operator identified that the shimmy damper that was attached to the nose gear leg had become bent and jammed against a door hinge within the wheel bay (Figure 3). A link in the nose steering system had also fractured, resulting in a disconnection between the pilots steering input and the nose landing gear (Figure 4). The steering barrel had bent where it connected to the steering link. The operator’s examination of the electrical system identified that a circuit breaker had opened during the incident flight. They commented that this had likely occurred automatically to prevent electrical overload of the landing gear system.

Following replacement of the fractured steering link, functional testing of the landing gear was completed as part of the operator’s fault-finding investigation. That testing showed that, under normal circumstances during a landing gear retraction sequence, the nose steering shimmy damper remained clear of all components of the wheel bay area. They further identified that the shimmy damper could only contact the wheel bay area if the nose wheel became mechanically disconnected from the steering system. That disconnection enabled the nose wheel to rotate left beyond its operational limit sufficiently for the shimmy damper to contact the undercarriage door bay area during the nose gear retraction sequence. 

The operator advised that there were no binding or other defects present in the remainder of the steering or landing gear components fitted to the aircraft. The operator also found that once the circuit breakers were reset, the landing gear system became electrically functional, allowing the gear to lower. One of those required circuit breakers was not accessible to the pilot in flight.

Figure 3: The nose landing gear tilted to the left with the shimmy damper jammed against a hinge in the undercarriage bay

Source: Eastern Air Services, annotated by the ATSB

Figure 4: Damaged nose gear steering components

Source: Eastern Air Services, annotated by the ATSB

Landing gear system and nose wheel steering 

The aircraft has a mechanical landing gear system controlled through a landing gear control handle on the right side of the pilot’s centre panel. When the handle is positioned to either extend or retract the landing gear, an electric motor drives the landing gear gearbox assembly. The main landing gear actuators are driven by torque tubes from the gearbox. The nose gear is driven by a duplex chain from a sprocket on the gearbox torque shaft. Circuit breakers protect the system from electrical overload. 

Emergency manual extension of the landing gear is controlled by a floor‑mounted lever centrally located between the left and right pilot seats. When the lever is manually operated, the landing gear electric motor and gearbox drive mechanisms are overridden, allowing extension of the landing gear system. The following procedure for manual extension of the landing gear was listed in the King Air pilot operating handbook:

LANDING GEAR MANUAL EXTENSION (MECHANICAL SYSTEM)

1. Airspeed - ESTABLISH 130 KNOTS

2. Landing Gear Relay Circuit Breaker (Pilot's subpanel) - PULL

3. Landing Gear Control - DOWN

4. Alternate Engage Handle - LIFT AND TURN CLOCKWISE TO THE STOP TO ENGAGE.

5. Alternate Extension Handle - PUMP UP AND DOWN UNTIL THE THREE GREEN GEAR-DOWN ANNUNCIATORS ARE ILLUMINATED.

Note: Additional pumping when all three annunciators are illuminated could damage the drive mechanism and prevent subsequent electrical gear retraction.

If all three green gear-down annunciators are illuminated:

6. Alternate Extension Handle - DO NOT STOW (Proceed to step 8.)

Should the landing gear fail to extend, the next steps in the procedure stated:

If one or more green gear-down annunciators do not illuminate for any reason and a decision is made to land in this condition:

7. Alternate Extension Handle – CONTINUE PUMPING UNTIL MAXIMUM RESISTENCE IS FELT, EVEN THOUGH THIS MAY DAMAGE THE DRIVE MECHANISM

8. Landing Gear Controls – DO NOT ACTIVATE 

Nose wheel steering is through direct mechanical actuation of the linkages in the system from the rudder pedals that connect to an arm near the top of the nose gear shock strut (Figure 5). A spring mechanism in the steering barrel dampens the transmission of excessive shock loads to the rudder pedals. A strut-mounted roller engages with a centring ramp to automatically centre the nose wheel during retraction of the gear. 

Figure 5: King Air B200 nose steering system schematic 

Source: Textron Aviation, annotated by the ATSB

Aircraft maintenance 

Scheduled maintenance for VH-XDV was based on a 200-hour phased inspection program, the details of which were specified by the aircraft manufacturer and contained within the King Air B200 maintenance manual. The operator’s maintenance records identified that a Phase-4 check was completed on 18 March 2024 at 18,298.4 hours. During that check the landing gear system was overhauled. Parts relating to the nose wheel steering were inspected but not replaced as their continued serviceability was based ‘on-condition’, rather than having a prescribed ‘life-limit’. On that basis the service life of the steering link was not required to be tracked and its service history was unable to be established. 

The Phase-4 inspection requirements listed in the maintenance manual for the nose gear components were non-specific and all parts were to be inspected for ‘wear damage and surface corrosion.’ For the steering linkage, it was required to be: 

STEERING LINKAGE - Inspect nose gear steering mechanism and attaching hardware for wear, damage and corrosion..

There was no specific requirement to inspect the steering link for cracks.

Pilot information 

The pilot held an air transport pilot licence (aeroplane), issued in September 2015, with a multi‑engine aeroplane instrument rating. In addition, they held a current grade 1 flight instructor rating and a multi-engine class flight test examiner rating. They reported approximately 8,500 hours total flying experience, of which 2,500 hours were accrued on the King Air B200. 

Pilot’s commentary of the emergency

After the pilot identified that the abnormal procedure for extending the landing gear was ineffective, they assessed that the fuel quantity on board the aircraft was sufficient to troubleshoot the malfunction and prepare for a wheels-up landing by completing or considering the following:

• Throughout, the pilot remained focused on the aircraft fuel quantity and their intention was to land with about 600 lb of fuel remaining onboard. Their reasoning was that a significantly reduced fuel load would minimise the potential fire hazard on landing. They provided numerous status updates to ATC that included remaining fuel quantities.
• They contacted the operator’s chief engineer on several occasions for technical advice and spoke with another pilot colleague to assist with planning the landing.
• They relocated both passengers to the seats aft of the main spar nearer to the main cabin door then briefed them on its emergency operation.
• They communicated with the airport fire and rescue personnel on the most suitable place on the runway to land the aircraft.
• During the final approach to Williamtown Airport they configured the flaps to the approach setting and immediately prior to the aircraft contacting the runway surface, they depressurised the aircraft cabin, shut down both engines, set the propellors to fine pitch, and isolated the aircraft electrical system. Their reasoning was to minimise any potential for the aircraft to remain pressurised and reduce the likelihood of ignition of the fuel onboard after landing.
• Upon tracking to the north-east to Hawks Nest, the pilot was cognisant that instrument meteorological conditions[7] might be entered during the approach back to Williamtown and requested an instrument approach for runway 30. The pilot recalled that the instrument approach required additional distance to be flown and estimated that it could result in the aircraft entering the 45-minute fixed reserve, which then prompted them to declare MAYDAY FUEL. 

Williamtown weather

When the aircraft was initially directed to the Williamtown southern circuit, the ATC controller estimated that the cloud base was between 2,000 ft and 2,500 ft AGL. Meteorological observations at Williamtown Airport identified that from 1130 through to 1400, rain showers and drizzle persisted. The ATC controller reported that during that period the cloud base began to lower, making it difficult to sight the aircraft.

In the minutes prior to the wheels-up landing, the meteorological observations indicated visibility greater than 10 km, drizzle in the vicinity of the airport, and a cloud amount of 1‍–‍2 oktas[8] at 1,300 ft AGL. 

Pilot guidance for wheels-up landing 

There was no specific guidance for a wheels-up landing published within the B200 operating handbook. The manufacturer advised that in this emergency situation, it would be expected that a pilot would use their aeronautical decision-making skills, training, knowledge of the aircraft, and knowledge of the situation to determine the best course of action.

The Airplane Flying Handbook produced by the United States Federal Aviation Administration (FAA) was intended to assist pilots to improve their flying proficiency and aeronautical knowledge. Chapter 18 of the Handbook, Emergency Procedures, provides generic advice to pilots for performing a gear up landing. The Handbook advised pilots to:

• select an airport with fire and rescue facilities
• request emergency equipment to stand by
• select a smooth, hard runway surface rather than an unimproved grass strip
• consider burning off excess fuel to reduce fire potential
• consider that the safest course of action may be to land with all three gears in the retracted configuration.

ATSB component examination

The following parts of the damaged nose wheel steering system were removed from the aircraft by the operator and sent to the ATSB’s laboratory facilities in Canberra for metallurgical examination:

• steering link
• shimmy damper
• steering barrel.

The ATSB’s visual examination of the shimmy damper identified that it had sustained bending of the centre shaft. The body of the shimmy damper also contained a significant gouge. The location of that damage was consistent with the operator’s reporting and the post‑occurrence photographs that the shimmy damper had become wedged against a door hinge in the nose wheel bay. 

The aircraft manufacturer indicated the steering link had been cast from an aluminium alloy, which was confirmed through chemical analysis of the part by the ATSB. General visual examination of the steering link fracture surfaces was followed by detailed high-magnification examination using a scanning electron microscope. A semi-circular feature was identified on the steering link fracture surface and its aspects were consistent with that of a pre-existing defect (Figure 6). Surface staining on the defect area was also consistent with the development of corrosion. 

High‑magnification imagery confirmed the presence of very fine crack progression striations throughout the defect area (Figure 7). The striations confirmed that a fatigue crack had progressed into the steering link prior to its complete fracture. 

The origin of the fatigue cracking was associated with a region of porosity at the component surface. The porosity was a manufacturing flaw that had been introduced during the casting process. The remainder of the fracture surface was comprised of brittle features produced during overstress of the steering link. 

Figure 6: A semi-circular pre-existing defect (labelled) was identified on both halves of the steering link fracture surface 

Source: ATSB

Figure 7: Microscopic striations (arrowed) were identified on the steering link fracture surface within the region of the pre-existing defect

The above image was captured using a scanning electron microscope at x4,500 magnification. Source: ATSB

Other occurrences

The ATSB's aviation occurrence database was searched for landing gear malfunctions leading to a wheels-up landing involving King Air B200 aircraft between 2014‍–‍2024. No such instances were identified. There were, however, 130 records of a wheels-up landing for all aircraft types, none of which resulted in injuries or fire. 

The review of occurrence database records also identified 92 reports of 'gear unsafe' indications for the King Air B200. There was a broad range of reasons attributed for those indications, however, none of them led to a wheels-up landing. There were 12 instances where the emergency manual extension was required to be utilised and in each of those the landing gear was able to be extended. 

Additionally, the aircraft manufacturer (Textron Aviation) advised that it was not aware of other instances of this specific malfunction where the mechanical landing gear became jammed leading to a wheels-up landing. It was also unaware of other instances where fatigue cracks had been identified in steering links. 

Safety analysis

The ATSB identified that the aircraft was able to be steered by the pilot using the rudder pedals as it was taxied to the departure runway and during the take‑off roll. This indicated that the nose wheel steering system, including the steering linkage, was intact during the taxi and the initial stage of the take-off roll. 

ATSB’s laboratory examination of the steering components identified the presence of a fatigue crack on the fracture surfaces of the steering link that had initiated from a surface flaw. The pre‑existing, high-cycle fatigue crack was small in comparison to the overall steering link cross section, indicating that the final fracture likely occurred over a relatively short number of loading cycles.

The nature of the loading that resulted in the final fracture was not identified. However, casting alloys are generally regarded as having relatively low fracture toughness when compared with wrought alloys, predisposing them to brittle fracture. Additionally, it is generally the case that the increase in stress intensity from a pre-existing fatigue crack will lead to fracture at much lower stresses due to the inherently low fracture toughness of a casting alloy. 

As the pilot was able to conduct the take‑off without issue, the fracture of the steering link must have occurred either late in the take-off roll or as the gear was retracted. Once the steering link fractured, the nose wheel was able to rotate beyond its normal operational limits. In this instance, the nose wheel rotated significantly to the left. That movement led to the nose gear shimmy damper also rotating beyond normal limits sufficiently to become jammed against a door hinge within the nose wheel well. 

When the pilot consulted the procedures for a manual gear extension, their efforts were ineffectual because the shimmy damper had mechanically jammed the operation of that system. Despite numerous attempts, the pilot was subsequently unable to extend the landing gear using the published emergency extension procedure, necessitating a wheels‑up landing.

Findings

From the evidence available, the following findings are made with respect to the wheels-up landing involving a Beechcraft King Air B200, VH-XDV, at Williamtown Airport, New South Wales on 13 May 2024. 

Contributing factors

• During the take-off roll or landing gear retraction sequence, the steering link fractured from a pre-existing fatigue crack, resulting in a mechanical disconnect within the nose landing gear steering system.
• When the landing gear retracted, the fractured steering link allowed the nose gear shimmy damper to contact the side of the wheel well and jam the nose landing gear in a partially retracted position. This prevented extension of the landing gear using the published emergency procedure, necessitating a wheels-up landing.

Safety actions

Safety action by Eastern Air Services

The operator reported that in the days following the incident, it conducted a maintenance inspection of the landing gear system on the other King Air B200 aircraft (registered VH‑MVP) within its fleet. No defects in the landing gear or steering system were found during that inspection. On 28 August 2024, further maintenance checks were completed on VH-MVP, including the removal of the nose gear steering link for non-destructive crack inspection. No defects were found. 

The operator further advised that it is reviewing its B200 maintenance program to consider including a non-destructive inspection of the steering link, which would be additional to the manufacturer’s existing maintenance requirements.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot of VH-XDV
• Eastern Air Services
• Bureau of Meteorology
• Royal Australian Air Force – 453 Squadron
• Textron Aviation
• ADS-B data from the aircraft. 

References

United States Federal Aviation Administration, Airplane Flying Handbook FAA-H-8083-3C

Beechcraft King Air B200 Maintenance Manual

Beechcraft King Air B200 Pilot’s Operating Handbook

Submissions

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

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

• the pilot of VH-XDV
• Eastern Air Services
• Royal Australian Air Force – 453 Squadron
• Textron Aviation
• Civil Aviation Safey Authority

Submissions were received from:

• the pilot of VH-XDV
• Eastern Air Services
• Royal Australian Air Force – 453 Squadron

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

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

Investigation summary

What happened

On 9 May 2024, an Embraer ERJ 190‑100 IGW aircraft, registered VH‑UZI and operated by Alliance Airlines, departed Cairns, Queensland (Qld) for Brisbane, Qld with 29 passengers and 2 flight crew on board. 

As the aircraft approached Brisbane in darkness, and with the autopilot engaged, air traffic control cleared the aircraft for the instrument landing system (ILS) approach to runway 19 left. The captain (pilot flying) disconnected the autopilot and enabled the flight path reference (FPR) line on their primary flight display to assist with manually flying the approach. The captain asked the first officer (pilot monitoring) to adjust the FPR line to the ILS glideslope angle for the runway (3.0°). Shortly after, the aircraft’s automated ILS flight mode unexpectedly disengaged. 

Over the next 10 seconds, and with the aircraft becoming unstable below 500 ft above aerodrome level, the flight crew focused on troubleshooting the unexpected change and recapturing the ILS flight director mode, rather than conducting a go‑around. During this time, the aircraft's glideslope deviation exceeded the stabilised approach criteria limit of 1.0 dot glideslope deviation. After recognising that the aircraft was low, the captain began to increase the aircraft pitch, and immediately after, the enhanced ground proximity warning system (EGPWS) generated a glideslope warning. The captain arrested the aircraft’s descent and re‑established the aircraft on the glidepath, before continuing the approach and landing. 

What the ATSB found

The ATSB found that, in response to a request from the captain to adjust the FPR line on their primary flight display, the first officer inadvertently pushed the flight path angle (FPA) button which selected the FPA mode and disengaged the aircraft’s ILS approach mode. The first officer’s action constituted a ‘slip’ type error where an individual’s understanding of the situation is correct, but the wrong action is performed.

Following the unexpected change to the aircraft’s flight modes, the flight crew diverted their attention to recapturing the ILS approach mode and did not effectively monitor the aircraft's flight path. Consequently, the aircraft exceeded the glideslope limit requirement of the stabilised approach criteria undetected by the flight crew. 

The aircraft continued to descend below the glideslope, resulting in the EGPWS glideslope alert activating. Subsequently, the flight crew did not perform the required terrain avoidance manoeuvre, and instead continued the approach.

What has been done as a result

 In response to the occurrence, Alliance Airlines has: 

• added a discussion in the pre‑brief of the cyclic training program to include the EGPWS ‘glideslope’ activations and required procedures
• issued an Operational Notice to remind crew of the stabilised approach criteria and go‑around requirements
• conducted a thematic review of unstable approaches and analysed data for further review.

Safety message

When flight crew are faced with the unexpected, effective crew resource management, with each crewmember performing their procedurally assigned roles of flying and monitoring, is essential to ensuring the continued safety of flight while the disruption is investigated and managed. Additionally, in the case of aircraft equipped with auto flight systems, immediate reference to the flight mode annunciation display offers the best opportunity to promptly identify and resolve instances of inadvertent mode selection.  

This incident highlights how quickly a disruption can result in an aircraft transitioning from a stable to unstable approach. If the disruption results in the exceedance of stabilised approach criteria, early recognition of the situation and prompt execution of a go‑around, rather than continuing the approach, will significantly reduce the risk of approach and landing accidents. Furthermore, flight crew must execute the correct response to ground proximity warning systems glideslope alerts without hesitation to ensure obstacles or terrain are avoided.

The investigation

The occurrence

On 9 May 2024 an Embraer ERJ 190‑100 IGW aircraft, registered VH‑UZI, was being operated by Alliance Airlines on flight[1] QF1887 from Cairns, Queensland (Qld) to Brisbane, Qld with 29 passengers and 2 flight crew on board. The captain was the pilot flying, and the first officer was the pilot monitoring.[2] 

The aircraft departed Cairns at 2101 local time. By about 2243, the aircraft was stabilised on the Brisbane runway 19L instrument landing system (ILS) approach, descending below 1,000 ft above aerodrome level (AAL) – 1,015 ft above mean sea level (AMSL) – in darkness and in visual meteorological conditions.[3] At 2243:49, air traffic control provided a landing clearance, and shortly after, the captain disengaged the autopilot to manually fly the approach.

To assist with following the approach glidepath, the captain enabled the flight path reference (FPR) line on their primary flight display (PFD) (see the section titled Flight guidance system and displays). The FPR displayed the aircraft’s flight path angle[4] reference line and digital readout on the PFD’s attitude indicator. The reference line was initially presented at 3.2° down, and the captain asked the first officer to adjust the line to present 3.0° (the ILS glideslope angle for Brisbane runway 19L). To do so, the first officer first needed to press the FPR button on their display controller panel to display the reference line on their attitude indicator, and then turn the flight path angle select (FPA SEL) knob to the requested value of 3.0°. At 2244:12, with the aircraft at about 460 ft AAL (475 ft AMSL), the first officer inadvertently pressed the flight path angle (FPA) button, which selected the FPA mode and changed the lateral and vertical navigation guidance for the flight director from localiser (LOC) and glideslope (GS) to aircraft roll angle (ROLL) and flight path angle (FPA). 

The captain reported that the mode change was unexpected for the flight crew, while the first officer reported experiencing ‘startle’.[5] After the mode change, the flight crew focused on troubleshooting the unexpected change and recapturing the ILS flight director modes. With the captain still manually flying, the aircraft’s pitch angle began to decrease, with an associated increase in the descent rate. A few seconds later, the captain selected the approach (APP) navigation mode which armed the ILS approach mode but did not capture the LOC or GS navigation modes. One second later, at 2244:16, the aircraft’s glideslope deviation reached 0.5 dot below the ILS glidepath, and a second later, the vertical rate of descent exceeded the operator’s stabilised approach criteria limit of 1,000 feet per minute, reaching a maximum of 1,139 feet per minute at about 300 ft AMSL. At 2244:19, 7 seconds after the FPA button was pressed, the glideslope deviation increased to about 1.0 dot below, which was the stabilised approach criteria limit, and the lateral guidance (LOC) for the ILS approach mode was captured. 

Over the next few seconds, as the aircraft descended to about 295 ft AAL (310 ft AMSL), the glideslope deviation increased to 1.5 dots. During this time, the captain felt that the aircraft’s nose was low and observed the low glideslope indications on their attitude indicator, as well as the precision approach path indicator (PAPI) system[6] showing 3 red lights, indicating the aircraft was below the glideslope. In response, the captain began to increase the aircraft pitch, and immediately after, with the aircraft still descending, the enhanced ground proximity warning system (EGPWS) generated a glideslope warning (see the section titled Enhanced ground proximity warning system (EGPWS)). The first officer reported calling out ‘slope’ at some point before the EGPWS activation (see the section titled Flight crew task sharing and standard calls). A maximum glideslope deviation of 1.8 dots was reached while the excessive descent rate was being arrested. 

As the aircraft descended to about 235 ft AAL (250 ft AMSL), the vertical guidance (GS) for the ILS approach mode was captured. A few seconds later, when the aircraft was about 1 dot below the glideslope, the descent rate reduced to less than 100 feet per minute. The aircraft then levelled at 233 ft AAL (248 ft AMSL) and the glideslope warning deactivated. Over the next 5 seconds, the captain re‑established the aircraft on the glidepath, and then continued the approach, with the aircraft subsequently landing within the touchdown zone[7] at an appropriate speed without further incident. The circumstances of this occurrence meant that there was no air traffic control alert issued to the flight crew for the glideslope deviation and excessive descent rate.

Context

Flight crew

The captain and first officer both held an air transport licence (aeroplane) and class 1 aviation medical certificates. The captain had over 12,100 hours of flying experience, of which 2,100 hours were on the E190 aircraft type, with 110 hours accrued in the previous 90 days. The first officer had almost 8,200 hours of flying experience, of which 975 hours were on the E190, with 154 hours accrued in the previous 90 days. 

Fatigue

At interview, both pilots reported that they obtained poor quality sleep the night before the day of the incident flight. While the captain was uncertain about why they slept poorly, the first officer reported that they went to sleep about 2.5 hours past their usual bedtime and generally did not sleep well outside of their usual pattern. The captain reported obtaining 8‍–‍9 hours of sleep in the previous 24 hours and 17.5‍–‍19.5 hours in the previous 48 hours while the first officer reported 5.5‍–‍6 hours and 13.5‍–‍14 hours respectively.

The crew also reported feeling ‘moderately’ tired towards the later stage of the flight and that flight crew fatigue was identified in the approach briefing as a threat to be managed. While the incident occurred during the approach, both crew remarked that it was not a high workload situation at the time.

The ATSB conducted an assessment of the flight crew’s sleep opportunity, actual sleep obtained, and quality of sleep leading up to the flight as well as other fatigue‑related factors, identifying that:

• the flight crew had an adequate rest opportunity of about 14 hours before the incident flight
• the rest opportunity was overnight which coincided with the circadian rhythm cycle and was unlikely to increase the risk of fatigue
• although the flight crew reported poor sleep quality, the conditions at the hotel accommodation where they spent the night were suitable and therefore conducive to obtaining restful sleep
• biomathematical modelling[8] of the flight crew’s roster data for the 2 weeks leading up to the flight indicated a low likelihood of fatigue.

In addition, research indicated that:

• the crew’s reported hours of sleep in the previous 24 and 48 hours were within limits that were unlikely to increase the risk of fatigue
• the time the flight crew had been on duty at the time of the incident was unlikely to have increased the risk of fatigue
• the time the flight crew had been awake at the time of the incident was not associated with significant performance degradation
• the time the incident occurred (2245 local time) would not have increased the risk of fatigue as it was outside the window of circadian low.[9]

The assessment concluded it was unlikely the flight crew were experiencing a level of fatigue known to adversely affect performance. 

Instrument landing system

An instrument landing system (ILS) is an instrument approach procedure that provides lateral (localiser) and vertical (glideslope) position information using angular deviation signals from the localiser antennas (located past the upwind end of the runway) and the glideslope antennas (located approximately 1,000 ft from the runway threshold). Aircraft systems detect these radio signals and provide instrument indications which, when utilised in conjunction with the flight instruments, enable an aircraft to be manoeuvred along a precise final approach path.

The Brisbane runway 19L ILS approach provided the typical 3° glideslope to the runway (Figure 1).

Figure 1: Brisbane runway 19L ILS approach chart

Source: Airservices Australia, annotated by the ATSB

Flight guidance system and displays

The E190 featured an integrated automatic flight control system (AFCS) that processed inputs from several aircraft systems and sensors. The AFCS supplied this processed data to the flight guidance control system (FGCS), which provided visual and aural information to the flight crew. 

The E190 FGCS also provided flight guidance information to the primary flight display (PFD) flight director (Figure 2) and the autopilot. The flight mode annunciation (FMA) display was located at the top of the PFD and displayed autothrottle, autopilot, approach status, and flight director lateral and vertical mode indications.

The attitude director indicator (ADI) was located below the FMA display on the PFD, and presented the following:

• flight director represented by a magenta diamond providing lateral and vertical guidance
• glideslope and localiser deviation pointers with scales (1 dot spacing), independent of flight director guidance
• flight path angle symbol that showed the current flight path angle in reference to the horizon line
• flight path reference line (FPR) and readout which indicated a manually selected flight path angle for reference.

Figure 2: E190 flight mode annunciation and attitude director indicator displays

Source: Alliance Airlines, annotated by the ATSB

The flight director provided guidance based on pilot selections on the guidance panel (Figure 3). When a mode change was selected by the flight crew, the selected mode was armed and, when certain conditions were met, became active. This active mode was displayed on the FMA display which temporarily flashed in reverse video (black text on green background) to highlight the change. 

Pressing the ‘APP’ button armed approach navigation modes, and when on an ILS approach, activated the ILS approach mode, providing vertical (glideslope) and lateral (localiser) flight director guidance. This navigation mode was displayed on the FMA as ‘LOC’ and ‘GS’. 

Figure 3: Guidance panel and display controller panel

Source: Alliance Airlines, annotated by the ATSB

Pressing the FPA button selected the flight path angle vertical mode and the aircraft roll hold (ROLL) lateral mode. When the FPA mode was active, it commanded the flight director to a flight path angle reference and the flight path reference (FPR) line was displayed as a solid line. The FPA SEL control knob was then used to manually select the desired flight path angle, represented by the FPR line. 

The FPR line feature could also be used by pilots to assist with flight path management when manually flying an ILS approach and was activated by pressing the FPR button located on the display controller panel. The FPR line was then presented as a dashed line when activated and adjusted using the same FPA SEL control knob on the guidance panel (Figure 3). When the FPR button was pressed, the line and numerical flight path angle value presented was that of the aircraft’s flight path angle at that time.

The first officer reported that they had previous experience using the FPR function, but it was not a feature commonly used by the operator’s flight crew. 

The aircraft manufacturer advised the ATSB that it had not received any previous flight crew reports of a similar inadvertent selection of FPA instead of FPR as occurred during this incident. 

Enhanced ground proximity warning system (EGPWS)

The aircraft was fitted with a Honeywell EGPWS, which used aircraft position and configuration information, along with a radio altimeter and a terrain database, to provide flight crew with increased awareness of the terrain along the projected flight path via aural and visual alerts and warnings. These included a mode that alerted pilots to excessive glideslope deviation during an ILS approach (Figure 4), and excessive descent rate (Figure 5).

When the aircraft descended more than 1.3 dots below the glideslope while at a radio altitude less than 1,000 ft, an aural ‘GLIDESLOPE’ would be generated and an amber ‘GND PROX’ alert displayed on each PFD (‘soft’ glideslope alert). If the descent continued to less than 300 ft and the glideslope deviation was 2 dots below, the ‘GLIDESLOPE’ aural alert would be louder and faster (‘hard’ glideslope alert). The aural and visual alerts continued until the aircraft exited the alert envelope.

Figure 4: EGPWS descent below glideslope alert

Source: Alliance Airlines

When the aircraft altitude was lower than 2,450 ft above ground level, aural and visual alerts were generated when the EGPWS calculated that the aircraft had an excessive descent rate towards terrain (Figure 5). When the outer limit of the descent rate envelope was breached, an aural ‘SINKRATE’ would be generated with an amber ‘GND PROX’ alert displayed on each PFD. If the inner limit was breached, a ‘PULL UP’ aural and visual alert was generated.

Figure 5: EGPWS excessive descent rate alert

Source: Alliance Airlines

Recorded data

The ATSB was notified of the incident 4 days after it had occurred. By this time, the cockpit voice recorder audio covering the time of the incident had been overwritten and was unavailable to the investigation. 

The flight data from the aircraft’s quick access recorder was analysed by the ATSB and the aircraft manufacturer, Embraer (Figure 6). The data showed that at 2243:57, the autopilot was disconnected, and the aircraft closely followed the glideslope until 2244:12, when the first officer inadvertently selected the FPA mode. At that time, the aircraft’s flight path angle was 3.3° down with a pitch angle of about 2.5° up. Over the next 10 seconds, the control column pitch up input reduced, with a subsequent reduction in aircraft pitch up angle, and the descent rate and deviation from the glideslope both increased.

Between 2244:18 and 2244:23, the descent rate exceeded 1,000 feet per minute, reaching a maximum of 1,139 feet per minute at about 300 ft above ground level. This was outside of the EGPWS excessive descent rate activation envelope and, therefore, no ‘SINKRATE’ alert was generated. At 2244:21, the glideslope deviation reached 1.0 dot, and 2 seconds later, the captain pitched the aircraft up, with the EGPWS ‘GLIDESLOPE’ alert activating immediately after. Three seconds later, a maximum glideslope deviation of 1.8 dot was recorded and the EGPWS alert deactivated after a further 5 seconds. The glideslope deviation reduced below 1.0 dot at 2244:31, and about 5 seconds later, the aircraft recaptured the glideslope at around the approach minima (220 ft AMSL). The maximum deviation below the glideslope was approximately 60‍–‍70 ft during the EGPWS activation.

Figure 6: VH-UZI recorded flight data during the approach

Source: ATSB

Operator procedures 

Approach briefing

The objective of an approach briefing is to ensure all flight crew understand and share a common mental model for the proposed plan of action. The approach briefing was normally performed by the pilot flying with the pilot monitoring reviewing and checking the information. 

The operator’s procedures required that the flight crew cover several topics during the briefing such as the expected manoeuvring to the initial approach fix, nomination of navigation aids required for the approach (for example, ILS), terrain, weather, obstacles, and any threats. 

The captain stated that the approach briefing for the occurrence flight was ‘normal’ other than fatigue being identified as a threat (see the section titled Fatigue). The captain stated that they did not brief the use of the FPR line feature during the approach briefing as they only decided to use it after the approach had already commenced and following the autopilot disconnection. 

Stabilised approach criteria

An approach is stable when all of the stabilisation criteria specified by the operator are met and an unstable approach is any approach which does not meet these criteria. According to an International Air Transport Association (IATA) report published in 2017, historical commercial aviation accident data indicated that many accidents occur during the approach and landing phase of flight, with frequent contributing factors being an unstable approach together with a subsequent failure to initiate a go‑around. Failure to maintain a stable approach could result in a landing that is too fast or too far down the runway, leading to a hard landing, runway excursion, loss of control, or collision with terrain. 

The operator’s standard operating procedures required all flights conducting instrument approaches to be stabilised by 1,000 ft above aerodrome level, and an immediate go‑around was required for any approach that did not meet the following stabilised approach criteria:

a) the correct flight path;

b) only small changes in heading/pitch are required to maintain the correct flight path;

c) the aircraft speed is not more than VAPP + 10 knots indicated airspeed and not less than VREF;

d) the aircraft is in the correct landing configuration;

e) sink rate is no greater than 1,000 feet per minute

f) thrust or power setting is appropriate for the aircraft configuration;

g) all briefings and checklists have been completed;

h) specific types of approached are stabilized if they also fulfil the following

i. instrument landing system (ILS) approaches must be flown within one dot of the glideslope and localizer

ii. a Category II or Category III ILS approach must be flown within the expanded localizer band

i) unique approach procedures or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing to have been completed prior to beginning the approach.

• Note 1: A momentary excursion is permitted for points (c) & (e). A momentary excursion is defined as a deviation lasting only a few seconds and where every indication is that it will return to the stabilised criteria as listed in points (c) & (e).

• Note 2: Where the nominal descent path for a particular approach requires a descent rate greater than 1000 fpm. This is only permitted when expected rates of descent have been briefed prior to the approach being commenced.

Flight crew task sharing and standard calls

During a manually flown approach, the pilot flying was responsible for controlling the aircraft flight path. The pilot monitoring was responsible for performing actions requested by the captain and monitoring the aircraft status (for example, configuration, altitude, speed, and flight path). For precision approaches, such as an instrument approach, the pilot monitoring was required to call out flight path deviations, such as glideslope deviations, below the stabilisation height:

Any time the [pilot monitoring] calls deviations from 'on slope' the PF should make corrections to avoid flight path excursions towards full scale.

The [pilot monitoring] should continue slope deviation calls until the glideslope indicator stops moving toward full scale and whenever the indicator is at full scale.

Defined phraseology was used to standardise communication of critical items in high workload situations. Deviation calls were to be made if a deviation limit was exceeded, and no corrective action had been observed (Table 1).

Table 1: Relevant standard calls

EGPWS

The operator’s E190 EGPWS policies and procedures required the crew to take the following action in response to an EGPWS alert:

If an EGPWS alert is associated with a PFD AMBER visual message of ‘GND PROX’, the EGPWS WARNING CORRECTIVE MANEUVER must be performed unless on daylight operations with clear visual conditions (not IMC), and a positive visual verification ensures that no obstacle or terrain hazards exist.

…During daylight in VMC, with terrain and obstacles clearly in sight, the alert may be considered cautionary. Take positive corrective action until the alert ceases or a safe trajectory is ensured. 

Perform the appropriate GPWS warning or alert procedure at all other times and climb the aircraft to the [lowest safe altitude] when enroute or to the [minimum safe altitude] when in the terminal area.

The ‘EGPWS WARNING CORRECTIVE MANEUVER’ required the pilot flying and pilot monitoring to perform various actions and callouts (Figure 7). 

Figure 7: EGPWS warning corrective manoeuvre

The operator advised that flight crew received training on the EGPWS during initial type training and through recurrent cyclic simulator training sessions. The training involved different scenarios involving an EGPWS alert that required the corrective manoeuvre to be performed, with the training focus being on ensuring that the procedure was executed correctly. The operator advised that while EGPWS glideslope alerts were probably not simulated as frequently as other EGPWS alerts, the response to almost all EGPWS alerts, as specified in the operator’s procedures, was to perform the corrective manoeuvre. 

The captain reported being surprised when the EGPWS glideslope alert activated as they could see they were ‘very near the place they needed to be’ and assessed that the safest course of action was to continue the approach.

Safety analysis

Incorrect mode selection

While manually flying the Brisbane runway 19L instrument landing system (ILS) approach, the captain enabled the flight path reference (FPR) line function and requested the first officer adjust the FPR value to the runway ILS glideslope angle of 3.0°. This required the first officer to press the FPR button and then turn the FPA SEL knob to the requested value. Instead, the first officer inadvertently pressed the FPA button.

The first officer’s action constituted a ‘slip’ type error that is a failure of an execution of an action (Reason, 1990). Specifically, slips occur when an individual’s understanding of the situation is correct, but the wrong action is performed (Wickens et al, 2022). Characteristics of this error also occur when people accept a match for the proper object, something that looks like it, is in the expected location or does a similar job. Specifically, it can occur when some characteristics of either the stimulus environment or the action sequence itself are closely related to the wrong action. It occurs during well practiced tasks where the operator may not be carefully monitoring their own action selections (Salvendy and Karwowski, 2021).

The first officer’s prior intention was to press the FPR button, which was a routine action, but this did not go as planned. In addition, the FPA and FPR buttons were both used in conjunction with FPA SEL control knob, which the first officer would have needed to turn after pressing either button. The aircraft manufacturer reported they were unaware of any similar occurrences that would indicate that this was a significant ergonomic issue. 

The captain had not briefed the use of the FPR line function during the approach briefing and therefore the flight crew did not have a shared mental model regarding the use of this function during the approach. However, it was unlikely that this influenced the first officer’s ‘slip’ type error as the first officer knew, and intended to press, the correct button.

Pressing the FPA button disengaged the ILS approach mode and changed the active flight director modes from glideslope and localiser to flight path angle and roll mode on the flight mode annunciator display. As a result, the flight director moved to the aircraft’s flight path angle at the time of the button press (3.3° nose down), and the flight path reference line turned solid with the readout indicating 3.3°. Although the ILS approach can be flown without the flight director guidance, the change to the flight director mode was unexpected and resulted in the crew diverting their attention to correct the mode change.

Diversion of attention

Although the first officer reported being ‘startled’ when the flight director mode change occurred, they were more likely experiencing ‘surprise’, which is when a mismatch is detected between what is observed and what is expected (Rankin et al, 2013). Surprise can be described as a ‘… combination of physiological, cognitive, and behavioural responses, including increased heart rate, increased blood pressure, an inability to comprehend/analyse, not remembering appropriate operating standards, “freezing,” and loss of situation awareness’ (Rivera et al, 2014).

At the time, the flight crew did not expect a mode change and were surprised when it occurred. After the mode change, the flight crew diverted their attention from monitoring the flight path and became preoccupied with resolving the mode change and recapturing the ILS flight director modes. As a result, the aircraft descended below the glideslope and the approach became unstable with respect to glideslope deviation, which was not recognised by either flight crewmember. The descent rate also exceeded the stabilised approach criteria limit of 1,000 feet per minute for about 6 seconds, although the criteria allowed for ‘momentary excursions’ of this parameter.

The first officer reported calling out ‘slope’, however they did not make the ‘slope limit’ or ‘unstable’ call to indicate that a go‑around was required. Furthermore, the time of the ‘slope’ call in relation to the glideslope deviation could not be determined. However, if these calls were made, they were unlikely to have affected the outcome given that the captain began to recover the aircraft’s descent shortly after the 1.0 dot glideslope criteria was exceeded. 

Research has highlighted the difficulties in understanding the aircraft’s present state following a surprising event, which includes an inadvertent mode change (Rankin et al, 2016). As a result, there are also challenges in identifying which response is appropriate. In such circumstances, immediate reference to the flight mode annunciation display offers the best opportunity to promptly identify and resolve the situation. When focus is diverted to a primary task such as manual flying or emergency actions, attention narrows to that task, and so monitoring of other sources degrades (CAA, 2023). This degradation of monitoring often occurs without the flight crew realising it.

Response to ground proximity warning system alert

Shortly after the aircraft exceeded 1.0 dot glideslope deviation, the captain recognised that the aircraft was too low and initiated a pitch up manoeuvre to correct the deviation. Immediately after, the enhanced ground proximity warning system (EGPWS) glideslope alert activated, which was heard by the flight crew. As the alert occurred at night, procedures required that the EGPWS corrective manoeuvre be performed. The EGPWS glideslope alert also indirectly indicated to the flight crew that the aircraft had exceeded the stabilised approach criteria for glideslope deviation. However, the flight crew did not perform the required EGPWS corrective manoeuvre and continued the unstable approach until the aircraft landed. The decision not to perform the corrective manoeuvre and to continue the approach increased the risk of landing too fast or too far down the runway, which in turn increased the risk of a hard landing, runway excursion, loss of control, or collision with terrain.

Findings

From the evidence available, the following findings are made with respect to the unstable approach involving Embraer 190, VH‑UZI, about 4 km north‑east of Brisbane Airport, Queensland on 9 May 2024. 

Contributing factors

• In response to a request from the pilot flying to adjust the flight path reference line on their primary flight display, the pilot monitoring inadvertently disengaged the aircraft’s instrument landing system approach mode by mis‑selecting the flight path angle mode.
• Following the unexpected change to the aircraft’s flight modes, the flight crew diverted their attention to recapturing the instrument landing system approach mode and did not effectively monitor the aircraft's flight path. Consequently, the aircraft exceeded the glideslope limit requirement of the stabilised approach criteria undetected by the flight crew.
• The aircraft continued to descend below the glideslope, resulting in the Enhanced Ground Proximity Warning System ‘GLIDESLOPE’ alert. Subsequently, the flight crew did not perform the required terrain avoidance manoeuvre, and instead continued the approach.

Safety actions

Safety action by Alliance Airlines

Following the occurrence, the operator conducted an internal review of the incident, including interviews with the flight crew and analysis of flight data to assess procedural adherence and identify contributing factors. 

In response to the occurrence, Alliance Airlines implemented the following to enhance safety and learning:

• a discussion was added in the pre‑brief of the cyclic training program to include the EGPWS ‘glideslope’ activations (hard and soft) and required procedures
• issued an Operational Notice to remind crew of the stabilised approach criteria and go‑around requirements
• conducted a thematic review of unstable approaches and analysed data for further review.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the flight crew
• quick access recorder data
• Alliance Airlines
• the aircraft manufacturer (Embraer)
• Airservices Australia.

References

Civil Aviation Authority (CAA) (2023) Flight-crew human factors handbook, Civil Aviation Authority, United Kingdom Government.

IATA (2017). Unstable Approaches – Risk, Mitigation Policies, Procedures and Best Practices (3^rd ed.). Retrieved from https://www.iata.org/contentassets/7a5cd514de9c4c63ba0a7ac21547477a/iat….

Rankin A, Woltjer R, Field J and Woods D (25–27 June 2013) ‘Staying ahead of the aircraft’ and managing surprise in modern airliners [conference presentation], 5th Resilience Engineering Symposium, The Netherlands, accessed 16 October 2024.

Rankin A, Woltjer R and Field J (2016) ‘Sensemaking following surprise in the cockpit—a re-framing problem’, Cognition, Technology & Work, 18:623–642, doi: 10.1007/s10111-016-0390-2.

Reason J (1990) Human error, Cambridge University Press, Cambridge, United Kingdom. 

Rivera, J., Talone, A. B., Boesser, C. T., Jentsch, F., & Yeh, M. (2014). Startle and Surprise on the Flight Deck: Similarities, Differences, and Prevalence. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 58(1), 1047-1051. https://doi.org/10.1177/1541931214581219

Salvendy G and Karwowski W (2021) Handbook of human factors and ergonomics, 5th edn, John Wiley & Sons, New Jersey.

Wickens CD, Helton WS, Hollands JG and Banbury S (2022) Engineering psychology and human performance, 5th edn, Routledge, New York.

Submissions

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

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

• the flight crew
• Alliance Airlines
• Qantas Airways
• Embraer
• United States National Transportation Safety Board
• Brazilian Aeronautical Accidents Investigation and Prevention Center
• Civil Aviation Safety Authority
• Airservices Australia.

Submissions were received from:

• Alliance Airlines
• Civil Aviation Safety Authority.

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

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

Executive summary

What happened

On 11 April 2024, a Cessna 404 Titan was being operated on an aerial survey flight north of Emerald, Queensland, with 3 crew members onboard. During the survey run, the crew was affected by fumes inside the cabin and chose to return to Emerald. At 1538, the aircraft landed successfully at Emerald Airport, where all 3 crew members received medical treatment for impairment symptoms. The aircraft was temporarily withdrawn from service for examination.

What the ATSB found

The ATSB found that operating crew members were all affected by fumes in the cabin and, possibly due to the associated impairment, the pilot returned the aircraft to Emerald rather than diverting and landing at a closer alternate airport. Despite extensive ground and in‑flight examination after the occurrence, the source of the fumes could not be established.

What has been done as a result

On 30 April 2024, the operator issued a mandatory requirement for all pilots on all flights – including those below flight levels – to secure the onboard oxygen equipment within seated reach of the pilot-in-command. As an additional risk control, the operator also required photographic evidence of this to be forwarded to the Head of Flight Operations (HOFO) prior to departure. 

On 2 May 2024, the operator issued guidance to all pilots via Notices to Aircrew (NOTACs) regarding the circumstances in which oxygen should be used in‑flight. The operator recommended that supplementary oxygen be used in the following circumstances:

• discharge of a fire extinguisher in aircraft cabin
• smoke or fumes in cabin
• suspected CO in cabin
• any other occasion where oxygen may assist the health or wellbeing of a crew member. 

Pilots were also advised to follow existing standard operating procedures and conduct a precautionary landing as soon as possible in the event of smoke, fumes, or gas in the cabin. This guidance was also incorporated into the operator’s emergency training modules.

Safety message

Fumes and airborne contaminants can result in the rapid onset of incapacitation that significantly affects crew decision‑making, communication and aircraft handling ability. The degree of physical or cognitive incapacitation can also vary widely between individuals, which may make it difficult to detect and respond to fume events.

Operating crews should therefore be alert to the potential hazards posed by odours and fumes and not hesitate to use supplemental oxygen and all other available means to ventilate the cabin. It is also important to be aware of alternate airports en route, and consider diverting to reduce the airborne exposure time. Crews should also communicate the presence of fumes, and any symptoms being experienced, to air traffic control at the first available opportunity as this will maximise the assistance available to crews both in the air and on the ground.

The investigation

The occurrence

On 11 April 2024, a Cessna 404 Titan was being operated by Aero Logistics on an aerial survey flight north of Emerald, Queensland. The flight was being operated under visual flight rules,[1] with one pilot and 2 task specialists operating the survey equipment. At 1317 local time, the aircraft departed Emerald Airport to the north-east before turning towards Moranbah and climbing to an altitude of 5,000 ft (Figure 1).

Figure 1: Survey area location and recorded flight path between Emerald and Moranbah

Source: Google Earth and operator ADS-B data, annotated by the ATSB.

At around 1349, the aircraft approached the Moranbah area and was descended to 4,000 ft to begin the first survey run. This required the aircraft to maintain a constant altitude and speed while travelling along specific parallel ‘lines’ overhead the target area as instructed by the task specialists. Approximately 20 minutes into this run, all 3 crew members started to notice a sporadic smell in the cabin, although their recollection of the smell differed.

The pilot advised that, as the flight progressed, they noticed that they had increasing difficulty setting the aircraft up and aligning it correctly. One of the task specialists also began to feel affected by the fumes, and at 1449 the pilot cancelled the survey and commenced a return to Emerald Airport. The first task specialist moved to the rear of the aircraft due to the extent of their respiratory symptoms. The second task specialist was not experiencing any symptoms at this stage, and repositioned to the cockpit to assist the pilot if needed.

Return to Emerald

During the return to Emerald, the crew opened the windows, vents, and cabin door to ventilate the cabin. They also disconnected the survey equipment and checked several aircraft systems, including ensuring that the autopilot was selected off, in an attempt to control or reduce the fumes. None of these measures resulted in any improvement, and the pilot and task specialists reported experiencing worsening symptoms as the flight progressed, but the nature and extent of these symptoms varied between each person. Although several diversionary airports were available en route, the pilot chose to continue to the base at Emerald.

Approximately 15 minutes from Emerald, the pilot considered conducting a precautionary landing in a field, due to worsening symptoms. They initiated a descent, and made a broadcast to Brisbane Centre, advising they were ‘landing somewhere in a field hopefully’.

Recorded flight data showed that, during this descent, the aircraft reached a maximum descent rate of 2,664 ft/minute. The pilot levelled the aircraft off at around 2,200 ft, approximately 14.5 km north‑east of Capella Airport. However, the pilot did not consider a diversion to Capella at that time. The pilot advised that they had managed to get some fresh air and decided to continue.

Several minutes later the pilot observed that the trim wheel started what they assessed as an uncommanded nose down input, and that ‘the autopilot was actually now trying to push us into the ground.’ In response, the pilot asked the task specialist seated beside them to hold the trim wheel, which rectified the issue. The task specialist confirmed that the trim wheel was moving but that they did not require significant force to stop it. They also advised that it was possible the pilot accidently activated the electric trim switch on the control column.

Landing 

The pilot advised air traffic control that they were experiencing fumes in the cabin, but they did not mention any control issues with the aircraft. They declined an offer for emergency services on arrival, and no MAYDAY[2] or PAN PAN[3] calls were heard or recorded by air traffic controllers at any point during the flight. The pilot continued to Emerald where, after confirming the wind direction on their electronic flight bag, and informing other traffic of their intentions, tracked for a right base leg of the circuit for runway 24,[4] and landed at 1538.

Although the pilot had not requested emergency services, the Brisbane Centre controller called the aerodrome reporting officer (ARO) at Emerald Airport to advise them of the aircraft’s approach. The ARO then called emergency services at around 1530, and several Queensland Fire and Emergency Service (QFES) appliances were waiting at the parking bay when the aircraft landed. The crew was able to disembark unassisted, but one task specialist exited the aircraft and lay down on the grass next to the aircraft due to nausea. The second task specialist reported having a headache towards the end of the flight. 

The ARO did not detect any fumes or smoke when they opened the rear door of the aircraft. The operator’s chief engineer entered the aircraft and similarly could not identify any smells, fumes, or smoke inside the aircraft. QFES crews then attended the aircraft, conducting a thermal scan and gas sampling of the interior with nil results. All of the internal panels and the flooring were removed for inspection and the QFES crews did not detect any fluid leaks internally or externally, but the aircraft was isolated overnight as a precaution.

The crew was attended to by paramedics at the scene before being transported to Emerald Hospital. They were given several hours of high-flow oxygen as a standard treatment and cleared to leave hospital later that evening. Blood samples were not taken from the crew as Emerald Hospital did not have the equipment required for blood gas testing. The aircraft was temporarily withdrawn from service for additional examination and testing.

Context

Crew information

Pilot

The pilot was experienced with piston and turboprop aircraft, and possessed the relevant qualifications and competencies for the work being conducted. The pilot had several decades of experience conducting freight, passenger, and recreational flights with a number of operators across Australia and overseas. The pilot had recently joined the operator to operate their piston aircraft. The pilot had passed their most recent aviation medical examination in July 2023.

Task Specialist 1

Task specialist 1 (TS1) was experienced with aerial survey work, having worked for a number of years with the owner/operator of the aerial survey equipment installed on the aircraft. Most of their experience had been in Cessna 404 and 406 series aircraft, and they had not previously experienced fumes or smells inside the cabins of these aircraft.

Task Specialist 2

Task specialist 2 (TS2) had limited experience on the Cessna 404 and had spent most of their time on Cessna 406 and 441 series aircraft. At the time of the occurrence, they were in the process of obtaining an aeroplane pilot licence, with several hours of flight time already logged. TS2 had experienced fumes in another aircraft several years earlier caused by a fault in the air conditioning system, but had not encountered anything similar in the Cessna 404.

Response to fumes and symptoms

When the crew decided to cancel the survey run, the aircraft was around 37 km north‑west of Moranbah Airport and Emerald was around 200 km south. The pilot advised that they would have had to declare an emergency to land at Moranbah Airport, as it was a private airport, and at that stage they did not consider there was an emergency. They had also experienced a flap issue the day before with resultant fumes in the cockpit and they considered this to be a similar event.

The pilot had completed emergency response and hypoxia awareness training in November 2023 and had been issued a pulse oximeter as part of the operator’s standard induction process. The aircraft had oxygen equipment onboard and the pilot and both task specialists were trained in how to use this equipment. The task specialists confirmed that the pilot had given an emergency briefing prior to departure. They could not recall whether the onboard oxygen equipment had been mentioned specifically, however there had been no plan to fly above 10,000 ft. 

The crew confirmed that during the return to Emerald, they used a supplied pulse oximeter to assess the oxygen saturation levels in their blood several times – however, none of the crew considered using the onboard oxygen equipment. 

Aircraft information

The aircraft was a Cessna Aircraft Company 404 Titan, manufactured in 1978 and equipped with 2 Teledyne Continental GTSIO-520-M engines. It was one of 7 such aircraft in the operator’s fleet. The operator told the ATSB that these aircraft were not modified from the original Cessna 404 design apart from the floor cutouts for the survey equipment unit. This unit was placed above the fuselage cutout at the rear of the cabin and secured to the floor with the cameras and sensors facing downwards.

The survey operators would usually place aluminium tape or cardboard around the unit to seal any gaps between the unit and the cutout in the floor. The operator also confirmed that the cable routing and electrical harnesses for the equipment and aircraft systems had been arranged and routed in a standard configuration around the cutouts in the fuselage.

Maintenance information

On 10 April, the day prior to the incident, the same aircraft with the same crew was conducting similar survey runs north of Emerald. The pilot said that during this flight they experienced an uncommanded extension of the flaps to the full-down position after the flaps were extended to Flap 10. The pilot attempted to troubleshoot the problem by moving the flap lever back and forth, but the flaps did not respond to movements of the flap lever. The pilot also began to notice a smell in the cabin when the flap issue commenced, and that their airspeed was lower than expected. The pilot decided to return to Emerald, but did not advise the maintenance personnel of the presence of fumes.

An engineering inspection found that the wires on the micro-switch had detached from the flap select lever. The operator said that this inspection also found that ‘the wire hadn't touched any surrounding areas, hadn't tripped, hadn't burned or anything like that’. The wires were checked and reconnected on the morning of 11 April, and the flap lever was tested before the aircraft was released back into service that day. No flap issues were detected after this repair was made.

Following the incident flight, the operator undertook flight tests, scheduled maintenance, and strip‑down examinations of aircraft wiring and componentry, including inspections and testing of:

• panels and flooring, which were removed to inspect for electrical wiring damage
• the autopilot and electrical trim systems
• around and underneath the cockpit dash and pilot side electrical panel and relay boxes
• the battery and lighting systems
• the heater and ventilation systems
• the engines, including alternators and electrical looms
• the hydraulic system
• the nose and main landing gear bays and wiring
• the tail interior and flight controls
• the left and right inner wings
• the flap coves and flight controls
• the nose locker and wing lockers including interior lighting.

No mechanical or wiring issues that could have been related to fumes were detected. The survey operator found no wiring defects or component faults in their survey equipment unit, which was then reassembled and installed on a different aircraft. The operator also confirmed that there was a card-based carbon monoxide and a digital aural CO detector on the aircraft, and that neither of these detectors had activated during the incident flight or the test flights conducted after the occurrence. Additionally, neither the pilot nor task specialists recalled either CO monitor activating during the occurrence flight.

Environmental

Weather

Weather data was requested from the Bureau of Meteorology (BOM) for winds aloft and automated METARs[5] in the area between Emerald and Mackay. This data confirmed that visual meteorological weather conditions[6] existed, with temperatures around 25 ˚C and cloud cover[7] ranging between few and scattered up to altitudes of around 5,000 ft. These conditions were consistent for the duration of the flight. 

Bushfires and mining activity

The crew did not observe any blast fumes or mining activity in the Moranbah area during the incident flight. BOM data also confirmed no known plumes and smoke in the area. Queensland government data on mining activity and blast fumes was requested but this was not provided. The pilot did not indicate the presence of any bushfires or smoke that could have generated fumes, and TS2 stated that there were some minor grass fires in the area, but these were not in the immediate vicinity of the flight path.

The operator confirmed through post-flight testing that ground-based fumes and smells could ‘be picked up within the cabin with the environmental settings open in the venting configuration’. The operator also described a smoke smell in the cabin during the post-incident test flight when the aircraft flew directly overhead a local grass fire. However, this smell was only temporary and limited to the specific fire area and was markedly different to the smell described by the crew.

Safety analysis

During the return flight to Emerald following the onset of impairment symptoms, the crew could not identify the source of the fumes inside the cabin. Based on the available evidence, no definitive cause or source for the fumes could be established. There were no visible signs of smoke or leaks, and there were no environmental conditions that could have consistently generated fumes inside the cabin. The onboard CO detectors did not activate, and post-incident testing by QFES did not identify noxious gases or thermal hotspots. The operator was also unable to identify any faults or defects in the wiring or componentry of the onboard systems and could not replicate the fumes in test flights.

Although the decision to cancel the survey run was prudent, flying back to Emerald, rather than diverting to closer suitable airports exposed the crew to the fumes for longer than necessary and may have worsened the impact. However, this decision may have been influenced by the flap‑related incident on the previous day. In addition, although the crew was trained to use the supplemental oxygen equipment onboard, and repeatedly used the pulse oximeters, the crew did not consider using the available oxygen. 

It is probable that all the crew members were affected by the fumes, although these symptoms presented differently in each crew member. TS1, sitting at the rear of the aircraft, reported nausea and respiratory symptoms. TS2 reported experiencing a headache. The pilot did not experience any physical symptoms but may have been experiencing cognitive impairment in terms of their decision-making and aircraft handling, even though the pilot navigated and controlled the aircraft as per the established procedures and communicated effectively on the radio.

The pilot’s decision not to declare a MAYDAY or PAN PAN, or to have emergency services in attendance for the landing, may have been a result of their impaired decision‑making. Fortunately, the controller proactively initiated the emergency response by alerting the ARO.

Findings

From the evidence available, the following findings are made with respect to the fumes event involving Cessna 404, VH-LAD, near Moranbah, Queensland on 11 April 2024.

Contributing factors

• The operating crew was affected by fumes in the cabin and the pilot returned the aircraft to Emerald rather than landing at a closer suitable alternate airport.

Other findings

• Despite extensive post‑occurrence ground and in‑flight examination, the source of the fumes could not be established.

Safety actions

Safety action by Aero Logistics

Prior to the commencement of the ATSB investigation, the operator proactively took several steps in response to the incident.

Placement of oxygen equipment

On 30 April 2024, the operator issued a mandatory requirement for all pilots on all flights – including those below flight levels – to secure the onboard oxygen equipment within seated reach of the pilot-in-command. As an additional risk control, the operator also required photographic evidence of this to be forwarded to the Head of Flight Operations (HOFO) prior to departure. 

Use of oxygen equipment

On 2 May 2024, the operator issued guidance to all pilots via Notices to Aircrew (NOTACs) regarding the circumstances in which oxygen should be used in‑flight. The operator recommended that supplementary oxygen be used in the following circumstances:

• discharge of a fire extinguisher in aircraft cabin
• smoke or fumes in cabin
• suspected CO in cabin
• any other occasion where oxygen may assist the health or wellbeing of a crew member. 

Pilots were also advised to follow existing standard operating procedures and conduct a precautionary landing as soon as possible in the event of smoke, fumes, or gases in the cabin. This guidance was also incorporated into the operator’s emergency training modules.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the operating crew
• Aero Logistics
• Airservices Australia
• Bureau of Meteorology
• recorded data from the ADS-B unit on the aircraft. 

Submissions

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

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

• the operating crew
• Aero Logistics
• the Civil Aviation Safety Authority.

No submissions were received.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2024 Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the 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.