The flight crew of a Saab 340 did not detect the aircraft’s reducing airspeed before its stall warning stick shakers activated, an Australian Transport Safety Bureau investigation details.
On the afternoon of 6 July 2021, the twin-turboprop Saab 340B, operated by Regional Express, departed Perth for a scheduled passenger flight to Albany, Western Australia, with two flight crew, one cabin crew, and 16 passengers on board.
During climb, the flight crew were alerted to a fault in the wing de-ice system. After levelling off at 7,000 ft, the crew actioned the relevant abnormal checklist, but were unable to clear the fault. They began a descent from 7,000 ft to 5,000 ft to exit icing conditions, and decided to return to Perth.
As the aircraft was levelling off at 5,000 ft, air traffic control instructed the flight crew to make a right turn. About 20 seconds into the turn, the aircraft’s stick shakers activated, providing a warning of a potential aerodynamic stall in the form of vibrations and an aural clacker sound.
“The ATSB’s investigation into this incident found the pilot flying became task saturated due to high workload and did not notice the aircraft’s reducing airspeed, which was also missed by the pilot monitoring due to a focus on other tasks until the stick shaker activated,” said ATSB Director Transport Safety Stuart Macleod.
Responding to the warning, the first officer, who was pilot flying*, initiated the stall recovery procedure before the captain took control to complete the recovery, and the aircraft returned to Perth without further incident.
ATSB analysis of the aircraft’s recorded data showed the flight crew had reduced the aircraft’s engine power from 60% torque when flying level at 7,000 ft, to about 15% torque during the descent. However, engine power remained at 15% torque when levelling off and turning at 5,000 ft.
“In order to maintain level flight at that engine power, the autopilot gradually increased the aircraft’s pitch, which led to a gradual reduction in airspeed,” Mr Macleod said.
“This airspeed reduction went un-noticed by the flight crew until the increasing pitch reached the level required for one of the Angle of Attack sensors to trigger the stick shaker activation.”
Mr Macleod noted the crew experienced a high workload in the lead up to the stick shaker activation, including multiple communications with ATC, which issued a series of vectors and requested flight information.
“During periods of high workload, where there is an increased chance of making errors, flight crews should prioritise monitoring critical flight parameters,” Mr Macleod said.
“Effective communication can help flight crews recognise a situation when their workload is becoming overwhelming, and consequently better manage the situation – for instance, giving themselves more time to complete the required tasks by discontinuing an approach, or deferring ATC requests appropriately.”
Following the incident, the operator amended flight crew training simulator sessions and related training material to include flight at minimum manoeuvring speeds – minimum airspeeds that provide a margin above a stall during aircraft manoevring.
The ATSB also found the alert from the aircraft’s de-ice system, which led the flight crew to return to Perth, was probably due to the right wing inboard de-ice boot delaminating shortly before encountering icing conditions.
*’Pilot flying’ (PF) and ‘pilot monitoring’ (PM) are procedurally-assigned roles with specifically assigned duties at specific stages of a flight. The PF does most of the flying, except in defined circumstances; such as planning for descent, approach and landing. The PM carries out support duties and monitors the PF’s actions and the aircraft’s flight path.
Read the report: Stick shaker activation involving Saab 340B, VH-ZLJ, 30.7 km south-west of Perth Airport, Western Australia, on 6 July 2021