The Australian Transport Safety Bureau (ATSB) has released a research report examining every incident of stall warning activation between 2008 and 2012 in transport-category aircraft operating in Australian airspace. The incidents recorded in the October 31 report include both local aircraft as well as those of foreign registry.
During this five-year period, 245 stall warning incidents were reported, most of which were categorized as low-risk, short-duration events in which the outcome was never in doubt. Thirty-three incidents, however, were classified as serious and occurred during the approach phase. Seventy percent of the warnings were deemed actual alerts rather than mechanical failures. Fifty-five percent of reported stall warnings occurred in VMC, while most of the IMC-related alerts took place during cruise, often when the aircraft was near the high end of its altitude-operating envelope.
Findings showed common precursors to stall activations were rapid changes in pitch or airspeed. In one-fifth of IMC events, the stall warning activated when the autopilot tried to correct the aircraft’s speed or flight path in response to a disturbance such as turbulence. Scrutiny of the serious events revealed that the crews’ failure to recognize decreasing airspeed and increasing angle of attack before the stall warning increased the risk of a stall developing. The crew was often focused on correcting the approach path before the stabilized approach height when a go-around would have been the more prudent option.