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.
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- 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.
- Block level: a section of airspace with specified upper and lower limits on a specific track, in which cleared aircraft are permitted to manoeuvre.