What happened
While performing an approach to Merimbula Airport, the crew of the aircraft encountered turbulence and a rain shower, likely linked to a passing gust front. To prevent the aircraft from exceeding flap speed limits during this turbulence, the captain reduced engine power to flight idle. Because of the active autopilot mode, this reduction in thrust caused the aircraft to pitch up and increase its angle of attack to maintain the programmed descent rate, leading to a continuous loss of airspeed.
During this period, the crew engaged in a brief discussion regarding the setting of the windscreen wipers. This interaction, along with the reduction in power, resulted in the crew failing to notice that the airspeed had dropped significantly below the required 136 kt minimum for that segment of the approach. The airspeed eventually decreased to 109 kt, triggering the aircraft's stall warning system. The crew successfully intervened to recover the aircraft and completed the landing without further incident.
The investigation
Investigators examined the state of the aircraft's anti-ice systems prior to the approach. Although the aircraft had recently passed through icing conditions, the ambient temperature at the time of the occurrence was well above freezing and no ice was present on the airframe. However, because the engine anti-ice system had been activated during the descent, the ice speed system remained active according to operator procedures.
This active system had modified the stall warning parameters, setting the stick shaker and aural warnings to trigger at an angle of attack of only 6°, rather than the standard 12.5°. Consequently, the warning activated at a much higher airspeed margin than usual, which provided the crew with an earlier indication of the approaching stall.
Findings
- The captain's decision to reduce power to flight idle to avoid flap overspeed, combined with the autopilot's response to reduced thrust, caused a continuous deceleration.
- The crew's attention was likely diverted by a discussion regarding windscreen wiper settings, which prevented the timely detection of the airspeed decay.
- The aircraft's stall warning system was operating with a reduced activation angle of attack due to the active ice speed system following recent flight through icing conditions.