Skytraders A319 at Melbourne on May 15th 2015, push buttons, human perceiption and a descent below minimum safe altitude

Last Update: November 24, 2017 / 18:23:45 GMT/Zulu time

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Incident Facts

Date of incident
May 15, 2015

Classification
Report

Airline
Skytraders

Flight number
SND-2

Aircraft Registration
VH-VCJ

Aircraft Type
Airbus A319

ICAO Type Designator
A319

A Skytraders Airbus A319-100, registration VH-VCJ performing flight SND-2 from Perth,WA to Melbourne,VI (Australia) with 18 passengers and 5 crew, was on approach to Melbourne's runway 16 via the WENDY 1A standard arrival route, the captain (ATPL, 17,250 hours total, 2,835 hours on type) was pilot flying, the first officer (also ranked Captain, ATPL, 12,290 hours total, 2,200 hours on type) was pilot monitoring. Autopilot 1, modes NAV and (managed) DESCENT, and autothrust were engaged, the aircraft turned left at waypoint NEFER descending towards cleared 3000 feet (3000 feet active in the FCU). While descending through 4700 feet the flaps were selected to and reached stage 1. The aircraft rolled wings level after the turn heading towards BOL. When descending through 3600 feet the captain announced to "arm the approach", which would have resulted in pushing the APPR push button on the FCU, however, accidentally the captain pushed the button EXPED. The autopilot changed to expeditious open descent mode, the vertical speed increased from 800 to 1600 fpm with 220 KIAS remaining constant. A few seconds later the first officer spotted the changed flight mode and pointed out the incorrect mode change. In an attempt to correct the mistake the captain now pressed the A/THR push button effectively selecting the autothrust function off resulting in a THRUST LOCK condition with according announciation on both PFDs, ECAM, an aural alert and a master caution. The first officer recognized the condition and pointed it out. At the same time the autopilot changed to ALT* (altitude acquire). The captain recognized the Thrust Lock condition and pressed both "instinctive disconnect" buttons on the side stick and thrust levers with the intention to reduce the airspeed and retard the thrust levers. The disconnect disconnected the autopilot, the cavalry sound activted for one second, the THRUST LK condition disconnected. As the thrust levers were still in the CLB detent, the commanded thrust changed from idle to climb, the engines began to accelerate. The captain re-engaged the autopilot, however, did not reconnect autothrust. Due to the rapidly accelerating thrust the captain began to provide nose down inputs on the side stick (in post flight interviews he stated he did not recall any nose down input but recalled he had the impression the aircraft was pitching up), due to the nose down input the autopilot disconnected again, the cavalry charge sounded for one second. The captain moved the thrust levers to idle, the aircraft accelerated through 240 KIAS and the first officer queried whether he should retract the flaps affirmed by the captain, the flaps were retracted. The rate of descent rapidly increased, the altitude alerter sounded (C-Chord) for 15 seconds, the engines spooled down and the captain began to provide nose up inputs, the rate of descent stabilized, then began to decrease. EGPWS sounded "SINK RATE" twice, the thrust levers were placed fully forward and the captain announced they were going around. The engines responded and began to accelerate, the EGPW sounded "TERRAIN AHEAD, PULL UP, TERRAIN AHEAD", due to the now rapidly increasing thrust the captain again changed to provide nose down inputs, the aircraft continued to descend. EGPWS sounded "TERRAIN! TERRAIN! PULL UP!", the captain now provided nose up inputs and the aircraft began to climb while the first officer radioed ATC, that they were going around. The lowest altitude was recorded at 2200 feet MSL, 1100 feet AGL, during the flaps retraction the aircraft reached its maximum speed at 314 KIAS. At the ATC desk the minimum safe altitude warning (MSAW) activated, the controller saw the aircraft descending through 2300 feet and starting to climb again at 2200 feet. ATC cleared the aircraft to climb to 4000 feet, then informed the crew the MSAW had activated. While the aircraft positioned for another approach the aircraft was cleared to climb to 5000 feet, subsequently joined the ILS for runway 16 and landed safely.

Australia's ATSB released their final report concluding the probable causes of the serious incident were:

- The pilot flying inadvertently selected the EXPED pushbutton instead of the APPR pushbutton, and, in an attempt to correct the error, pressed the A/THR pushbutton, creating a thrust lock condition.

- In attempting to remove the thrust lock condition, the pilot flying pressed the instinctive disconnect pushbutton but did not move the thrust levers to match the locked thrust setting. As the thrust was locked at idle while the thrust levers were set to climb thrust, this resulted in an unexpected, significant thrust increase.

- The pilot flying likely experienced pitch-up illusions during two rapid thrust increases and responded to these illusions with pitch-down sidestick input.

- Pitch-down inputs by the pilot flying, combined with a very high thrust setting, resulted in a very high rate of descent with rapidly increasing airspeed. This led to the breach of the cleared minimum descent altitude, as well as triggering a number of Enhanced Ground Proximity Warning System alerts.

- The rapidly changing aircraft state led to the crew experiencing a high workload. This was likely to have limited their capacity to identify mode changes and to respond to the aircraft’s undesired high airspeed and rate of descent.

- The pilot monitoring’s ability to identify and influence the rapidly changing situation was likely affected by the non-routine actions of the pilot flying, the reduced communication between flight crew and an apparent focus on the flap speed exceedance as the aircraft started to accelerate.

Other factors that increased risk

- At the time of the occurrence, the pilot flying was likely experiencing a level of fatigue known to have a demonstrated effect on performance, predominantly due to the time of day and time awake.

- The aircraft’s rapidly increasing airspeed resulted in the limit speed for the extension of the aircraft slats being significantly exceeded.

The ATSB also released following safety message:

A pitch-up illusion can affect the most experienced pilot. Ideally, adherence to instrument scan techniques, setting and maintaining known aircraft attitudes for specific phases of flight, and using flight aids such as autopilots and/or flight directors, are all strategies to reduce the risk of responding inappropriately to pitch-up illusions. However, when pilots are experiencing a high workload this can be difficult to achieve. In this case, there are benefits in increasing crew communication, to enable more time to identify issues and consider solutions as well as to facilitate the pilot monitoring’s ability to monitor the situation.

Aviation operators conduct non-technical skills training for their pilots. An occurrence such as this demonstrates the way in which topics such as human error prevention and detection, information processing, decision making and communication continue to be relevant.

The ATSB analysed:

While conducting an arrival procedure, prior to commencing an approach into Melbourne, Victoria on 15 May 2015, the Skytraders Airbus A319 descended to about 2,200 ft, which was below the ATC-assigned altitude of 3,000 ft. The crew broke off the arrival procedure and climbed to the new ATC cleared altitude of 5,000 ft before returning to land at Melbourne.

During the descent below 3,000 ft, the aircraft’s Terrain Avoidance and Warning System (TAWS) initiated a number of warning alerts, the speed limit for the aircraft flaps was exceeded, and the Minimum Safe Altitude Warning System (MSAW) initiated an alert to the ATC controller. Critically, during the 26 seconds from the time that the PF pressed the instinctive disconnect pushbutton on the thrust levers to when the aircraft reached its minimum altitude, the aircraft descended just over 1,000 ft and increased speed by about 100 kt.

The event was initiated by an inadvertent switch selection by the pilot flying (PF). This was followed by a combination of errors, rapidly changing events, high workload and an apparent response to a pitch-up illusion, resulting in the aircraft quickly developing a very high rate of descent and increasing airspeed.

The ATSB analysed the inadvertent button pushes:

As the aircraft was approaching the localiser for Melbourne runway 16, the PF recalled intending to arm the aircraft’s autoflight system (AFS) to capture the localiser for the approach. This required the PF to press the APPR pushbutton on the Flight Control Unit (FCU). Instead, the PF mistakenly pressed the EXPED pushbutton and the AFS entered the expedite descent mode. In an apparent attempt to cancel the expedite descent mode, the PF inadvertently pressed the A/THR pushbutton, which was adjacent to the EXPED pushbutton.

The acts of pressing the EXPED and then the A/THR pushbuttons were both predicated by a prior intention to act, but neither action went as planned. In this case, this prior intention was the pressing of the APP push button, which was part of a routine set of actions. Routine actions are generally characterised as requiring less attention.

The pressing of the A/THR was an apparent instinctive reaction to realising that an error had been made. Both selections were consistent with unintentional slips. Furthermore, the similar size, shape and colour of the EXPED and APPR buttons on the FCU, as well as their close proximity, may have contributed to the error. The lighting conditions in the flight deck may have increased the difficulty for the pilot monitoring (PM) to monitor the actions of the PF.

The ATSB analysed the reaction to the Thrust Lock condition:

On becoming aware that the engines’ thrust had been locked, the PF reacted by pressing the autopilot and autothrust instinctive disconnect pushbuttons, thereby removing the thrust lock condition. The likely intent of disconnecting both autopilot and autothrust was to revert to a fully manual flight mode. This is supported by the simultaneous disconnection of the autopilot and autothrust systems through the use of the instinctive disconnect buttons, an automatic action to complete the apparent intent.

However, in disconnecting the autothrust, the PF did not match the thrust levers to the current power, or set a desired power. This was likely to be a lapse, which is ‘simply omitting to perform one of the required steps in a sequence of actions’ (Harris, 2011). As to why this lapse occurred, the PF’s incomplete response to the ‘thrust lock’ condition may have been a result of a response consistent with a perceived urgency to handle an undesirable state, particularly as the instinctive disconnect pushbuttons were designed for a quick response

The ATSB analysed the work load of both crew:

Pilots who encounter abnormal or emergency situations experience a higher workload with an increase in performance errors compared to pilots who do not experience these situations (Johannsen and Rouse, 1983). During the occurrence, the attention of the flight crew was likely divided between a number of different information cues and task requirements, from the time the PF made the inadvertent selections on the FCU, through to when the aircraft began to climb.

These included:
- multiple aural warnings and alerts
- identifying and responding to mode changes, including appropriate actions to address the THR LK ECAM message
- disengagements and re-engagement of the autopilot
- focus on airspeed (mostly by the PM)
- interactions with ATC towards the end of the occurrence sequence. At the time, the aircraft was in the descent phase, which inherently has a higher workload. The PM recalled that the workload became very high after the inadvertent FCU selections occurred. The high workload experienced by the PM was demonstrated in the use of an incorrect call sign during ATC communications, as the aircraft started to climb out.

The degree of recollection from both crew after the occurrence also indicated that they experienced a high workload over a short period of time, as details including the numerous aural warnings (including the EGPWS), one of the inadvertent FCU selections and autopilot changes were not recalled. Overall, the high workload the flight crew experienced appeared to have limited their capacity to identify mode changes, such as autopilot disconnections, and to respond to the aircraft’s undesired high rate of descent.
Incident Facts

Date of incident
May 15, 2015

Classification
Report

Airline
Skytraders

Flight number
SND-2

Aircraft Registration
VH-VCJ

Aircraft Type
Airbus A319

ICAO Type Designator
A319

This article is published under license from Avherald.com. © of text by Avherald.com.
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