What happened
Following departure from Perth, the flight crew identified a fault within the de-ice system. This malfunction was likely caused by the delamination of a de-ice boot on the underside of the right wing, which failed during its inflation cycle just before the aircraft entered forecasted icing conditions. Due to the low freezing level and the presence of icing along the route, the crew elected to return to Perth.
During the maneuver to return to the airport, the crew reduced engine power to descend from 7,000 ft to 5,000 ft. Upon reaching the target altitude of 5,000 ft, the engine power was not increased. As a result, the aircraft's airspeed continued to decay. The autopilot attempted to maintain the 5,000 ft altitude by pitching the nose upward, which further increased the angle of attack and decreased the airspeed. This developing condition remained undetected by the crew until the activation of the stick shaker.
The investigation
The investigation focused on the crew's management of the aircraft during the descent and the factors contributing to the loss of airspeed awareness. It was determined that the pilot flying, the first officer, likely experienced task saturation while managing the transition to a descent, level flight, and a turn, alongside communicating with Air Traffic Control. Simultaneously, the captain, acting as the pilot monitoring, was focused on communication tasks related to the return to Perth and was not monitoring critical flight parameters.
Findings
- The de-ice system fault was likely triggered by the delamination of the inboard de-ice boot on the right wing.
- The pilot flying became task saturated due to the high workload of managing flight state changes and ATC instructions.
- The crew failed to monitor airspeed and engine power, leading to a lack of awareness regarding the decaying airspeed.
- Neither pilot recognized the high workload of the first officer, and no attempt was made to reduce communication demands to allow the crew to refocus on flight instruments.