What happened
During the taxi to the runway, the flight crew noted unusually high temperatures within the aircraft. Shortly after takeoff, smoke began emitting from the cabin and flight deck vents on the left side. This was caused by a failure in the left air cycle machine (ACM), which allowed oil to be expelled and unconditioned hot bleed air to enter the air conditioning system.
In response to the smoke, the crew performed the memory items of the 'smoke in aircraft' checklist. The first officer attempted to don an oxygen mask but encountered difficulties with the fit and establishing communications, leading them to elect not to wear the mask. While the crew prioritized returning the aircraft to Perth, they did not complete the full Quick Reference Handbook (QRH) checklist. Specifically, the left bleed air was not selected to the 'off' position.
This omission caused the air conditioning duct to overheat, eventually triggering a 'wheel well and wing overheat' annunciation. During the return flight, the crew experienced several altitude deviations, including one instance where the aircraft descended to 639 ft below the lowest safe altitude.
The investigation
Investigators examined the mechanical failure and the crew's response to the emergency. The post-event engineering examination confirmed that the left ACM failed prior to departure. The investigation also focused on the impact of workload on the crew's performance. The aircraft was equipped with analogue instrumentation and lacked flight automation, which, combined with the immediate onset of smoke during instrument flight conditions, created a high-pressure environment for the relatively inexperienced crew.
Findings
- A failure in the left air cycle machine caused hot bleed air and smoke to enter the cabin.
- The failure to complete the QRH checklist and turn off the left bleed air led to the subsequent overheating of the air conditioning duct and the wing overheat warning.
- High workload levels caused the crew to omit required checklist items and made errors in decision-making and flight path adherence.
- The first officer's decision not to wear an oxygen mask due to equipment difficulties increased the risk of impairment.
- Air traffic control did not identify an incorrect altitude readback and failed to issue a safety alert when the aircraft first descended below the lowest safe altitude.