A Saab 340's stall warning activation during the aircraft's descent in turbulent conditions into Merimbula, NSW, highlights the importance of effectively monitoring aircraft systems and performance during the approach and landing, an ATSB final report notes.
Shortly after 1500 on 8 June 2023, the Rex Airlines-operated Saab 340 was on descent into Merimbula at the end of a scheduled passenger flight from Sydney, with 3 crewmembers and 22 passengers on board.
Icing conditions were expected during the descent, and so the first officer, who was pilot monitoring, selected the engine and wing anti-ice ON.
About 7 minutes later, the crew commenced a visual approach to Merimbula's runway 21, and selected the flaps to 20 degrees for the landing.
During the approach, in turbulent conditions, the airspeed reduced and the aircraft's stall warning activated, at a speed of 109 kt.
The captain then re-established the required approach flight path and speed, and continued the landing without further incident.
An ATSB investigation found prior to the stall warning activation, the captain, who was pilot flying, had reduced engine power to flight idle to avoid an inadvertent flap overspeed.
"However, when the flight crew had earlier turned on the anti-ice systems, this also activated the ice speed system, which reduced the angle of attack activation angle for a stall warning," ATSB Director Transport Safety Stuart Macleod explained.
"Due to the autopilot mode active at the time, the reduced thrust resulted in a continuous reduction in airspeed that required pilot intervention to prevent a stall warning."
Due possibly to distraction associated with a windscreen wiper setting, the airspeed continued to decrease undetected by the crew, until the stall warning activated, at a higher than normal margin above the stall speed due to the anti-ice systems being activated.
"The approach and landing phases can be a time of substantially increased flight crew workload," Mr Macleod observed.
"As such, effectively monitoring aircraft and approach parameters, including performance associated with autopilot modes, and managing any distractions is essential to ensuring that an approach is safely completed."
