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
Incident Facts
Date of incident
May 15, 2015
Classification
Report
Airline
Skytraders
Flight number
SND-2
Departure
Perth, Australia
Destination
Melbourne, Australia
Aircraft Registration
VH-VCJ
Aircraft Type
Airbus A319
ICAO Type Designator
A319
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
Departure
Perth, Australia
Destination
Melbourne, Australia
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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