High workload contributed to the triggering of a Rex Saab 340’s stall warning stick shakers while in a turn, the ATSB has found.
According to a report from the Australian Transport Safety Bureau, the flight crew of a Rex Saab 340 did not detect the aircraft’s reducing airspeed, prompting the stick shaker warning, while attempting to respond to a separate fault.
The aircraft was travelling from Perth to Albany on 6 July 2021 when, during climb, the flight crew were alerted to a fault in the wing de-ice system.
After levelling off at 7,000 feet, the crew completed the relevant abnormal checklist, however, were unable to clear the fault with the system, the ATSB report said.
The flight crew then began a descent to 5,000 feet to exit icing conditions and decided to divert back to Perth Airport.
At this point, air traffic control reached out to the flight crew and instructed them to make a right turn. About 20 seconds into the turn, the aircraft’s stick shakers activated, warning of a potential aerodynamic stall.
“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 of transport safety Stuart Macleod.
Responding to the warning, the pilot flying, which was the first officer at the time, initiated the stall recovery procedure before the captain took control to complete the recovery.
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 per cent torque when flying level at 7,000 feet, to about 15 per cent torque during the descent.
However, engine power remained at 15 per cent torque when levelling off and turning at 5,000 feet.
“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,” Macleod said.
“This airspeed reduction went unnoticed 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.”
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,” 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.”
Since the incident, Rex has 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 manoeuvring, the ATSB said.
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.