Air Asia A320 at Perth on Feb 19th 2016, descended below safe height on approach
Last Update: January 16, 2018 / 14:59:36 GMT/Zulu time
The Australian Transportation Safety Board (ATSB) rated the occurrence a serious incident and opened an investigation.
Radar data suggest the aircraft descended to or below 2000 feet more than 9nm before the runway while on a VOR approach to runway 06 (MSA 2500 feet on VOR approach).
On Jan 16th 2018 the ATSB released their final report concluding the probable causes of the serious incident were:
- The flight crew’s diagnosis of the captain’s failed flight management guidance computer was accurate, but after they did not find the procedure to follow, the failure was not appropriately managed. This resulted in degraded systems capability for the approach.
- The flight crew had a limited understanding of how the captain’s failed flight management guidance computer would affect the use of the aircraft’s automated systems during the instrument landing system approach. This meant that their decision to engage autopilot 1 resulted in the frozen data stored in the failed guidance computer being utilised by the auto flight system, leading to an unexpected increase in engine thrust and prompted the crew to conduct a missed approach.
- The unresolved system failures, combined with the conduct of a missed approach procedure and the subsequent runway change increased the flight crew's workload. This likely reduced their ability to analyse the actual extent to which their automation was degraded, and effectively manage the subsequent approaches.
- During the first runway 06 non-precision approach, the flight crew’s focus of attention was outside the aircraft, attempting to locate the runway. This distraction, along with their unfamiliarity with the approach procedure, inhibited their ability to monitor and maintain the correct flight profile and altitude during the approach. The flight crew did not detect that the aircraft had descended below the segment minimum safe altitude for that stage of the approach.
Other safety factors
- During the flight, multiple dual control inputs occurred, which in other circumstances have resulted in aircraft responding in an unexpected manner.
- The aircraft's flight path profile was not adequately monitored or communicated between the flight crew during the non-precision instrument approaches to runway 06. This reduced the captain’s awareness of any deviation from the prescribed approach and limited his ability to correct it.
- The second runway 06 non-precision instrument approach did not meet the stabilised approach criteria for a short period during the final approach, increasing the safety risk of the landing.
The ATSB reported that the flight crew had prepared for an ILS runway 21 approach and was about to intercept the localizer descending through 5000 feet when the crew received a "CAB PR LDG ELEV" warning (indicating the FMCG1 did not have the elevation of the landing airport and thus could not maintain the cabin pressure profile) after engaging AP1 (in addition to AP2) in approach mode. With both APs engaged AP1 and FMCG1 take priority over AP2/FMCG2. The crew set the cabin pressure control to manual and continued the approach. The warning however signified that FMCG1 had failed.
Earlier during the flight, while in cruise ,the captain's navigation display turned blank with a red error message "MAP NOT AVAILABLE", while the first officer's ND remained available with the message "INDEPENDENT OPERATION". The crew correctly identified the failure of the FMCG1 however were unable to locate the proper procedures to resolve the issue.
The first officer (CPL, 4,200 hours total, 3,100 hours on type) was pilot flying, the captain (ATPL, 13,500 hours total, 5,200 hours on type) was pilot monitoring.
The ATSB described the following sequence of events: "At 2144:38, after reducing the aircraft’s selected speed to 160 kt and establishing the aircraft on the ILS, the selected altitude on the flight control unit (FCU)16 was set to the go-around altitude, consistent with the operator’s standard operating procedure. A short time later, the flight crew elected to select a managed speed mode. This change resulted in the auto flight system attempting to capture the speed target contained in FMGC1 when it failed (which was 253 kt) and the autothrust system commanded an increase in engine thrust. The flight crew recognised the increasing engine thrust and airspeed but they did not understand why it occurred. The captain told the first officer, ‘make a go-around’ and then advised ATC that they were conducting a go-around."
The captain took control of the aircraft, re-engaged autopilot and autothrust, but disengaged autothrust again after about 10 seconds due to uncertainty about the previous unexpected thrust increase. The crew requested another ILS approach runway 21, vectors were provided, however, about 4 minutes later ATC advised that cross winds had increased to 22/25 knots cross wind component and offered a VOR approach to runway 06. The crew accepted and prepared for the VOR approach concluding no holding was needed to prepare the systems as there was sufficient time to prepare the cockpit. ATC advised a new ATIS was in effect indicating moderate to severe turbulence below 3000 feet. The first officer programmed his FMCG2 for the VOR approach and cross checked with the paper charts.
The ATSB described the second approach, first VOR runway 06 approach:
At 2200, ATC asked the flight crew twice if they were established on the 248 radial (which was the inbound track for the VOR procedure) and the first officer responded that they were 10 NM from the runway. ATC again asked the crew to confirm they were established, and the first officer replied stating, that they were ‘established on the inbound radial 068’. In response, ATC cleared the crew to conduct a VOR runway 06 approach, with 10 NM to touchdown. Shortly after, the captain recalled the first officer asking, ‘do we descend now captain?’ In response, captain initiated a descent from 2,500 ft when the aircraft was at 9 DME. Soon after, the landing gear was extended and the first officer selected 1,600 ft on the FCU for the next descent altitude limit. However, the operator’s Flight Crew Training Manual (FCTM) required that the go-around altitude be set on the FCU when established on final approach. Coincidentally, 1,600 ft was the corresponding segment minimum safe altitude for the runway 03 VOR approach, whereas the published altitude for that stage of the runway 06 approach was 1,900 ft.
The captain elected to continue manually flying the aircraft using his primary flight display (PFD) and the first officer’s ND, and manually controlling the engine thrust due to the apparent automation failures. The first officer continued to monitor the descent gradient and vertical speed, and later recalled believing that they were on the correct descent profile. However, for most of the descent, the aircraft’s rate of descent exceeded the recommended rate (700 ft/min) that was published on the approach chart for the aircraft’s groundspeed. The maximum recorded rate of descent briefly reached 1,550 ft/min.
As the approach continued, the flight crew became concerned that they could not see the runway, and both crew became focused on locating the runway. The first officer later reported that because of this, he was no longer monitoring the approach segment minimum safe altitude
The ATSB reported the third approach, second VOR approach to runway 06, was flown by the first officer again. The final descent started late causing a high rate of descent, there were dual control inputs at 3 occasions, the aircraft however landed safely on runway 06.
The ATSB annotated that fatigue did not play a role in the occurrence.
The ATSB analysed:
During the flight, the captain’s flight management guidance computer (FMGC1) failed. The flight crew’s response to this and their utilisation of FMGC1 during the runway 21 instrument landing system (ILS) approach, resulted in an unexpected increase in engine thrust and subsequent go-around. After conducting the go-around, they were required to change runways due to increased crosswind and conduct a VHF Omni Directional Radio Range (VOR) approach onto runway 06. During this VOR approach, air traffic control (ATC) received a minimum safe altitude warning, prompting the controller to alert the crew of their low altitude and instructed them to conduct a go-around. Subsequently, the flight crew made another approach for runway 06 and the aircraft landed safely.
The following analysis discusses the crew’s understanding and management of the FMGC failure, their systems knowledge, and the human performance factors that affected the management of the approaches, which resulted in two go-arounds.
Crew response to the flight management guidance system failure
During the cruise, when the captain identified that his multipurpose control and display unit (MCDU1) had frozen and the navigation display (ND) map became unavailable, the flight crew correctly identified an FMGC1 failure. However, they could not locate any information about how to resolve it in the aircraft’s manuals. Had they found the MCDU and FMGC reset procedures in the Quick Reference Handbook (QRH), the crew may have been able to rectify the failure. This would have provided normal operation of the captain’s navigation display ND and MCDU. With an operational MCDU, the captain would have been able to input the approach and aerodrome data into the FMGC1 and the unexpected increase in engine thrust would not have occurred.
Additional information on the failure mode was available in the Flight Crew Operating Manual (FCOM), but the flight crew did not find this information in that manual. The investigation was unable to determine why the crew did not locate the relevant information. This failure to find and action the QRH and FCOM, meant the aircraft systems remained degraded for the rest of the flight.
Understanding of system interactions and subsequent crew decision making during the runway 21 ILS approach
The flight crew discussed the FMGC1 failure during their first approach briefing and decided to use the first officer’s functioning MCDU (MCDU2) and ND on the descent. Because the crew had not previously found the information in the FCOM relating to the failure, they did not understand how it could affect the interactions between the aircraft systems and hence the conduct of the descent. They were therefore unaware that both autopilots, and in particular autopilot 1 (AP1), should not have been engaged during the instrument approach. The reason for not engaging AP1 was that FMGC1 took primacy over the first officer’s FMGC (FMGC2) when both autopilots were used and the data in FMGC1 had frozen at the time of failure.
When the flight crew engaged AP1 during the first approach, they did not recognise that the subsequent cabin pressure fault was related to the engagement of AP1 and its utilisation of FMGC1 data. Additionally, the crew did not realise that the FMGC1 target speed at the time of failure (253 kt) would be utilised when the speed mode was changed from a selected mode (which had a target speed of 160 kt at the time), to a managed mode after AP1 was engaged.
The issue of flight crew understanding systems interactions is not limited to this occurrence. In response to the increase in incident and accident reports of flight crew experiencing difficulties using flight path management systems, a United States Federal Aviation Administration-led Flight Deck Automation Working Group analysed several data sources to produce findings and recommendations for the use of automation on modern flight decks. The working group found that operators had concerns with the level of flight crew skills required for managing automated system malfunctions and/or failures. The working group was cognisant that it was impossible to train pilots in all possible malfunction situations or failure scenarios. They stated that pilots needed to be prepared to recognise the results of partial and complete system failures and intervene appropriately (PARC/CAST Flight Deck Automation Working Group, 2013).
Crew’s understanding of systems interactions and dual control inputs
In this case, the flight crew’s lack of understanding of how the systems interacted led to inappropriate system selections and resulted in the increase in engine thrust. Although the failure of FMGC1 had not been resolved by the flight crew prior to the aircraft commencing descent, FMGC2, MCDU2, autopilot 2 (AP2) and engine autothrust were all capable of normal operation and could have been used to complete the approach normally.
Although the flight crew elected to conduct a go-around when the engine thrust unexpectedly increased, there was a period of 25 seconds where dual sidestick control inputs occurred, prior to the captain taking over control of the aircraft. These dual control inputs indicate a level of confusion and lack of communication regarding conduct of the go-around and about which pilot was in control of the aircraft. There were other instances of dual control inputs of short duration (less than 5 seconds). While these inputs did not affect the flight, there have been other instances where sustained dual control inputs have had a detrimental effect on the control of the aircraft.
Reaction to automation functionality and decision making
Following the go-around and without understanding the reason for the increase in engine thrust or the effect the FMGC1 failure had on the system, the flight crew briefly re-engaged AP1. Due to doubts about the functionality of the automation, the captain then elected to reduce the level of automation, and manually fly the aircraft. This decision presented as being intuitively derived from patterns of behaviour the captain had used successfully in the past. The decision had the effect of removing the potentially problematic automation, but it also increased the crew’s workload.
Researchers (Klein 2008, Kahneman, 2011) have stated that, in time-constrained environments, individuals can make decisions using intuitive reasoning where the steps are often unconscious and based on pattern recognition. For intuitive or naturalistic decision-making, an experienced individual will identify a problem situation as similar or familiar to a situation they have dealt with before and will extract a plan of action from memory. If time permits, they will confirm their expectations prior to initiating action. If time does not permit, actions will need to be initiated with uncertainty that may result in a poor decision.
The ATSB subsequently entered an extensive analysis of workload factors:
Workload has been defined by Orlady and Orlady (1999) as:
…reflecting the interaction between a specific individual and the demands imposed by a particular task. Workload represents the cost incurred by the human operator in achieving a particular level of performance (p.203).
The available cognitive resources are finite and will vary depending on the experience and training of the individual as well as the level of stress and fatigue experienced. Workload is managed by balancing task demands such that, when workload is low, tasks are added and when workload becomes excessive, tasks are shed (Orlady and Orlady, 1999). Tasks, such as internal and external communication, can be shed in an efficient manner by eliminating low priority tasks or they can be shed inefficiently by abandoning important tasks. The task demands can be influenced by the mental and physical requirements of the task, as well as the time available (Wickens and Hollands, 2000).
Factors increasing crew workload
After the first approach, the flight crew’s workload increased substantially with the following conditions:
- although the engine autothrust, AP2 and FMGC2 were still operating normally, the crew became uncertain about the automation’s functionality and elected to manually control the engine thrust and fly the aircraft using raw data.
- the crew had limited experience, outside of simulator sessions, flying approaches manually.
- the turbulent conditions increased the attention required by the captain to maintain desired heading, pitch attitude and airspeed.
- the unexpected runway change meant the crew needed to program the approach into the first officer’s MCDU, review the approach and conduct a briefing prior to the approach.
- the unexpected runway change and reduced timeframes limited the time available for the crew to review the approach charts.
- the captain’s ND was still in operating in a degraded mode and was not displaying lateral tracking guidance for the VOR approach nor the information from the distance measuring equipment (DME). For that information, the captain needed to refer to the first officer’s ND on the other side of the cockpit instrument panel.
- the captain’s flight director (FD) was still referencing the frozen FMGC1 and was not providing valid FD attitude guidance targets.
- while the crew were experienced in flying non-precision VOR approaches, they had limited experience flying the Perth runway 06 VOR approach at night. The first officer reported never having flown the approach before.
Recorded data indicates that there was 8 minutes from making the decision to conduct the VOR approach to when the flight crew confirmed they were established on the approach, at 10 NM. The limited time available to prepare for the approach, combined with the degraded systems, would have further increased their workload.
Effect of increased workload
Workload and time pressure can lead to a reduction in the number of information sources an individual may access, and the frequency or duration of time these sources are checked (Staal, 2004). Amongst other effects, a high workload can result in individuals not understanding the implications of the information they are presented with.
In this occurrence, the captain’s workload was increased due to the decision to hand-fly the aircraft using the first officer’s ND for lateral tracking guidance and distance information during the VOR approaches. This increased workload made it more likely that, in the visual conditions, he would try and continue the approach using external visual reference.
The flight crew’s workload impacted their ability to manage the approaches and was evidenced by the shedding of tasks, such as:
- reviewing the approach charts,
- monitoring of the flight profile,
- descending the aircraft without confirming the aircraft position,
- neither crew member observing the DME distance,
- breakdown in the crews’ use of standard operating procedures, such as selecting altitudes other than the missed approach altitude on the flight control unit, while conducting the non-precision instrument final approach.
Had the flight crew elected to hold and prepare prior to conducting the approach, they may have reduced their workload, improved their preparations and conducted a thorough briefing prior to conducting the unfamiliar approach. (editorial note: this last emphasis added by the editor)
Actual vertical profile during first VOR 06 approach (Graphics: ATSB):
Perth VOR runway 06 approach (Graphics: AIP Australia):
This article is published under license from Avherald.com. © of text by Avherald.com.
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