VA469 - Incident: Virgin Australia B738 at Perth on Jul 31st 2021

A Virgin Australia Boeing 737-800, registration VH-YIE performing flight VA-469 from Perth,WA to Brisbane,QL (Australia), was climbing through about 2000 feet out of Perth's runway 24 when the crew detected an uncommanded descent, disengaged the autopilot with autothrottle remaining engaged, and trimmed the aircraft manually while descending through 1536 feet. Shortly afterwards a GPWS warning "Don't Sink!" sounded. The aircraft finally climbed again, the autopilot was re-engaged. While climbing through about 18000 feet autopilot and autothrottle both disconnected automatically. The crew tested whether this was a problem of the left hand autopilot and engaged the right hand autopilot and re-engaged autothrottle. ATC queried whether their operation was normal. The crew spotted the "STAB OUT OF TRIM" light had illuminated. While working the related checklists autopilot B disengaged. The followed the resulting checklist requiring to fly the aircraft manually with autopilot and autothrottle off. The crew advised ATC they were not compliant with reduced vertical separation minima and requested a block altitude of FL380 to FL400. The crew consulted with maintenance and chief pilot, who advised another company pilot travelling as a passenger to assist in the cockpit. The crew continued the flight to Brisbane for a safe landing on runway 01 about 3:50 hours after departure.

On Sep 16th 2021 the ATSB announced they have discontinued the investigation into the occurrence rated an incident as no systemic findings were to be expected. A post flight inspection revealed the circuit breaker for the automatic flight control system stab trim had tripped and the STAB OUT OF TRIM light was faulty. The circuit breaker was reset, the faulty light was replaced but considered unrelated to the tripped circuit breaker. The reason, why the circuit breaker tripped, could not be established. A verification flight went without further problems, the problem could not be replicated.
Related Flight: VA469, Twitter: #VA469, Virgin Australia News