Sequence of Events
Shortly after departing Perth, the flight crew received cockpit indications of a de-ice system fault. The fault was likely triggered by a delaminated de-ice boot on the underside of the right wing that failed during its inflation cycle shortly before encountering icing conditions. With a low freezing level and forecast icing along the planned route, the crew decided to return to Perth.
While manoeuvring towards Perth, engine power was reduced to descend the aircraft from 7,000 ft to 5,000 ft. However, when the aircraft levelled off at 5,000 ft, engine power was not increased. As a result, airspeed decreased due to the low engine power, and the autopilot maintained the selected altitude by pitching the nose up. This increased the angle of attack and further reduced airspeed. The condition continued unnoticed until one of the angle-of-attack sensors reached the level required for stick shaker activation.
Crew Actions
The first officer, who was the pilot flying in the time leading up to the stick shaker activation, likely became task saturated while managing flying tasks (changing flight state to descend, level flight, and turn) as well as communicating with and following air traffic control instructions. This task saturation reduced the attention the first officer paid to managing airspeed and engine power. At the same time, the captain (pilot monitoring) was also not monitoring these key parameters due to a focus on other communication tasks related to the return to Perth. The crew's reduced awareness of airspeed resulted in the potential stall going unnoticed until the stick shaker activated.
In addition, during that rapid sequence of events, neither pilot recognized the first officer's high workload and task saturation. Consequently, no attempt was made to alleviate the situation, such as by discontinuing the approach or deferring air traffic control requests for information until the aircraft was straight and level. Such actions would have provided additional time and opportunity for the crew to refocus on flight instruments and key parameters.