Virgin Australia B738 enroute on Sep 6th 2023, inflight upset on opening the cockpit door

Last Update: November 29, 2024 / 08:01:40 GMT/Zulu time

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Incident Facts

Date of incident
Sep 6, 2023

Classification
Report

Airline
Virgin Australia

Flight number
VA-336

Departure
Brisbane, Australia

Destination
Melbourne, Australia

Aircraft Registration
VH-YQR

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Virgin Australia Boeing 737-800, registration VH-YQR performing flight VA-336 from Brisbane,QL to Melbourne,VI (Australia), was enroute when a flight attendant requested access to the cockpit. The captain, pilot monitoring, attempted to open the door but instead of pressing the relevant button activated the rudder trim causing an inflight upset requiring the first officer to apply significant right aileron to prevent the aircraft from rolling left until the trim exceeded aileron authority. The aircraft rolled left to a maximum of 42 degrees left bank before the captain, upon the first officer's request, checked the rudder trim and discovered the problem. The aircraft returned to normal flight and continued to Melbourne for a landing without further incident. One flight attendant received minor injuries.

The ATSB released their final report concluding the probable causes of the serious incident were:

Contributing factors

- While actioning a request for entry into the flight deck, the pilot monitoring mis-selected the rudder trim switch instead of the intended flight deck door switch and inadvertently applied rudder trim for about 8 seconds.

- The autopilot responded to the trim input and its consequential yaw and roll with application of opposing roll. The maximum roll that the autopilot could apply and maintain (the roll authority limit) was reached after 5 seconds of left rudder trim input, after which the continuing rudder trim input resulted in a rapidly increasing rate of roll and an inflight upset.

- During the period of the development and recovery from the upset, and despite the need to use a large right wing down aileron input to maintain an approximate wings level attitude, the flight crew were not able to promptly identify the significant left yaw as the primary initiator of the upset, which in turn delayed the restoration of balanced flight.

The ATSB analysed:

In response to a request for entry into the flight deck, the pilot monitoring (PM) intended to activate the flight deck door lock switch. The operator’s policy and procedures manual required flight crew to positively identify any control or switch before manipulating them. The PM visually identified the flight deck door switch, but in reaching for it, did not visually confirm selection or manipulation of the correct switch, instead mis-selecting and activating the rudder trim switch.

A human-factors analysis of the mis-selection of the rudder trim control found that the error was consistent with an unintentional slip. The action occurred during a period of possible distraction when the PM was talking to the pilot flying (PF) and monitoring the aircraft as it approached cruise altitude. The PM’s action of looking away from the panel when selecting the switch was also an example of attention diversion. The distraction and attention diversion were both likely factors that could lead to an unintentional slip. Furthermore, the act of twisting the door switch was a substitution error, predicated by a prior intention to act, and was therefore a routine action which did not go as planned.

As it was routine to operate the door switch, the PM probably did not give sufficient attention to this task. This was further compounded by the physical similarities in the switches and their operation, and their co-location on the aisle stand panel. However, a Boeing human factors examination of possible mitigations to these factors in response to a similar previous occurrence found that changing the switch design was unlikely to mitigate the mis-selection risk, and that the current generic aisle stand configuration and an emphasis on confirmation of switch selection prior to manipulation was the most effective control measure. Finally, Boeing identified the risk of unintentional rudder trim application in an FOTB issued to operators 2 years prior to the occurrence. The FOTB specifically acted as an alert to flight crew of the risk of mis-selection of rudder trim in circumstances identical to those in this incident.

On the initial application of the rudder trim, both pilots felt the aircraft’s immediate yaw/roll response, but were unable to identify the likely cause. Over the following 5 seconds, while the captain maintained activation of the switch and waited for the door to open, the rudder trim progressively increased to the left, causing the rudder to correspondingly move to the left. The autopilot was initially able to compensate for the increasing left yaw input and induced left roll through application of increasing right wing down roll input. This right wing down input was replicated on the pilots’ control wheel. After 5 seconds of trim input and increasing induced left roll, the autopilot reached its authority limit – that is, the autopilot had reached the maximum roll control input it could apply and maintain.

Up to this point, the autopilot had managed to limit the induced roll to a bank angle of less than 5° to the left. However, on reaching the roll authority limit, the increasing rudder trim resulted in the aircraft’s bank angle to the left increasing. As the trim input continued for a further 3 seconds, the aircraft responded with a rapidly increasing rate of roll to the left. The unexpected and increasing bank angle alerted both pilots to the developing aircraft upset.

The PF initially responded by attempting to control the increasing left roll through the use of the mode control panel heading selections and the autopilot. As this had no apparent effect, and with the bank angle continuing to increase, the PF applied a large right wing down control input while almost simultaneously disengaged the autopilot and autothrottle. At about the same time the bank angle alert triggered. The PM responded with an ‘upset’ call, and the PF responded by executing the upset recovery procedure. The aircraft was quickly recovered to about straight and level flight.

Having recovered the aircraft to an approximate wings level attitude, the PF was required to hold about 35° of right wing down control wheel displacement to maintain that attitude. While this large roll input required to maintain a wings level attitude strongly indicated a yaw-related issue, the crew continued to investigate the cause of the inflight upset unsuccessfully for a further minute.

About 70 seconds after the initial misapplication of rudder trim, the PF requested the PM check the rudder trim. Shortly after, the rudder trim was returned to a neutral position. While large right wing down aileron input required to maintain a wings level attitude provided a strong indicator that the upset was linked to a yaw related issue, a combination of the very small displacement of the rudder pedals at the point of maximum trim application, and the PF’s limited experience on the aircraft, probably contributed to some of the delay in identifying the unintended rudder trim.
Incident Facts

Date of incident
Sep 6, 2023

Classification
Report

Airline
Virgin Australia

Flight number
VA-336

Departure
Brisbane, Australia

Destination
Melbourne, Australia

Aircraft Registration
VH-YQR

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

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