Incident
At 0528 on 20 November 2021, a De Havilland Canada DHC-8, registered VH‑QQD, departed on a charter flight from Perth to Port Hedland, Western Australia. As the aircraft approached 10,000 ft, the flight crew observed that the cabin had not pressurised. They decided to return to Perth, and the aircraft landed uneventfully.
Findings
The ATSB found that the aircraft had been undergoing maintenance the previous day, and for operational reasons, the recirculation fan had been removed for fitment to another aircraft. VH‑QQD was not removed from the schedule, and its unserviceability was not detected before the flight. The absence of the recirculation fan prevented the aircraft from pressurising.
The operator’s operations department incorrectly interpreted a message from its engineering department regarding the serviceability status of VH‑QQD, and the aircraft remained assigned to the charter flight. This allocation remained on the flight crew’s roster and the flight manifest, which contributed to the flight crew’s confidence that the aircraft was serviceable.
As there were no allowable defects against the aircraft, the captain did not check the maintenance log before flight and did not detect the fan removal. However, during pre-flight activities, the captain observed a circuit breaker that had been opened to facilitate the fan removal and reset it without reviewing the maintenance log for recent or open defects, as required by the operator’s flight crew operating manual.
The aircraft’s unserviceable status was not entered into the operations management system, so it remained on the flight manifest. The flight crew had no reason to believe the aircraft was unserviceable based on the information available. The document suite’s unusual location provided an indication that something was wrong, but this was not normally interpreted as unserviceability. The captain likely experienced expectation bias from the initial information showing VH‑QQD assigned to the flight.
The first real opportunity for the flight crew to discover the problem was the maintenance log, which contained entries for the fan removal and a propeller de-ice defect. The captain did not check the log because there were no pink pages indicating permissible unserviceabilities. The first officer did not check the documentation, as there was no requirement to do so. The opened circuit breaker was another prompt for the captain to review the maintenance log, but due to expectation bias, the prompt was not successful.
The flight crew ultimately detected the problem when they conducted the transition altitude checklist, demonstrating the importance of checklist items. Had that not occurred, they would have been alerted by a cabin altitude warning. Although the occurrence was unlikely to result in an adverse outcome, it demonstrated the importance of rigorous processes for ensuring aircraft serviceability.
Actions Taken
After the occurrence, the operator refined the terminology used by engineering to communicate aircraft serviceability and reviewed internal communication methods. An internal memo was distributed to all staff advising that if the aircraft technical logs were not in the crew room when receiving the aircraft, it was to be considered unserviceable. Another memo reiterated existing paperwork and circuit breaker resetting requirements. Additionally, a dedicated compartment was installed for each aircraft’s documents in the crew room, with a placard instructing flight crews to contact maintenance watch if the scheduled aircraft’s documents were missing.