Aviation safety investigations & reports

Control issue involving Boeing 737-8FE, registered VH-YIE near Perth Airport, Western Australia, 31 July 2021

Investigation number:
Status: Discontinued


Section 21 (2) of the Transport Safety Investigation Act 2003 (TSI Act) empowers the ATSB to discontinue an investigation into a transport safety matter at any time. Section 21 (3) of the TSI Act requires the ATSB to publish a statement setting out the reasons for discontinuing an investigation. The statement is published as a report in accordance with section 25 of the TSI Act, capturing information from the investigation up to the time of discontinuance.

Overview of the investigation

The ATSB commenced an investigation into a flight control issue involving a Virgin Australia Boeing 737-8FE aircraft, registered VH-VIE, which occurred near Perth Airport, Western Australia (WA) on the afternoon of 31 July 2021. The flight was a scheduled passenger service from Perth WA to Brisbane, Queensland.

During initial climb, the crew detected that the aircraft had commenced an uncommanded descent. The Captain disconnected the autopilot, leaving the autothrottle engaged while descending through 1,536 ft and manually trimmed the aircraft using the electrical stabiliser trim. Shortly afterwards, the crew received an enhanced ground proximity warning system (EGPWS) ‘DON’T SINK’ alert. The aircraft was re-established in a climb configuration.

At 2,144 feet, the Captain re-engaged the autopilot. Six minutes later, at 17,888 feet, the autopilot automatically disengaged. The autothrottle then also disengaged for reasons undetermined. To test if there was a problem with the A system autopilot, the crew engaged the B system autopilot and re-engaged the autothrottle. Shortly afterwards, air traffic control (ATC) queried the crew if their operations were normal. While responding to ATC, the crew detected the ‘STAB OUT OFF TRIM’ light illuminated and advised ATC to standby for further details. The crew then began the associated Quick Reference Handbook Non-Normal Checklist and while doing so, the B system autopilot automatically disengaged. The crew followed the checklist by not re-engaging the autopilots and autothrottle, and continued to manually fly the aircraft.

Once in cruise, the crew contacted ATC advising that they were unable to fly at reduced vertical separation minima (RVSM)[1] and requested a block level [2] clearance of flight level (FL) 380 to FL400 due to stable atmospheric conditions. The crew then discussed the risks of continuing the flight to Brisbane. After liaising with company engineers, referring to company documentation and examining the weather at the departure, alternate and arrival airports, the crew decided to continue to Brisbane. The crew also assessed their fitness to fly, and distributed different phases of flight to each other to manage workload and fatigue. They communicated with their Chief Pilot and requested an off-duty company pilot, who was flying to Brisbane as a passenger, to join them in the cockpit to provide additional support if required.

A post-flight engineering inspection found that the circuit breaker for the automatic flight control system (AFCS) stabiliser trim was in the tripped position and the STAB OUT OF TRIM light was faulty. Engineers reset the circuit breaker, but were unable to replicate the fault during testing. The faulty light was replaced, but was considered unrelated to the tripped circuit breaker. The reason for the tripped circuit breaker was not established. A verification flight ensured that the AFCS was commanding the stabiliser without any issues. No further faults were detected.

As part of the investigation the ATSB:

  • interviewed the flight crew
  • examined the recorded flight data and the operator’s and aircraft manufacturer’s procedures
  • examined the crew’s in-flight decision making.

The ATSB found that at all stages of the flight, the flight crew acted in accordance with operator’s and aircraft manufacturer’s procedures, and had considered and managed the risks associated with continuing the flight.

Reasons for the discontinuation

Based on a review of the available evidence, the ATSB considered it was unlikely that further investigation would identify any systemic safety issues or important safety lessons. Consequently, the ATSB has discontinued this investigation.

The evidence collected during this investigation remains available to be used in future investigations or safety studies. The ATSB will also monitor for any similar occurrences that may indicate a need to undertake a further safety investigation.


  1. Reduced vertical separation minima (RVSM): the reduction of vertical space between aircraft from 2,000 to 1,000 feet at flight levels from 29,000 feet up to 41,000 feet.
  2. Block level: a section of airspace with specified upper and lower limits on a specific track, in which cleared aircraft are permitted to manoeuvre.
General details
Date: 30 July 2021   Investigation status: Discontinued  
Time: 14:41   Investigation level: Short - click for an explanation of investigation levels  
Location   (show map): near Perth Airport    
State: Western Australia   Occurrence type: Control issues  
Release date: 13 September 2021   Occurrence category: Incident  
Report status: Discontinued   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 737-8FE  
Aircraft registration VH-YIE  
Serial number 38708  
Operator Virgin Airlines Australia (VAA)  
Type of operation Air Transport High Capacity  
Sector Jet  
Damage to aircraft Nil  
Departure point Perth Airport  
Destination Brisbane Airport  
Last update 14 September 2021