Why the ATSB did this research
Stall warning events have always been an area of interest for airlines and aviation safety investigators as they indicate that an aircraft is operating at the margins of safe flight. As these occurrences are reportable to the ATSB, the ATSB can analyse trends across airlines and Australia. By publishing such analysis, it is hoped that the wider aviation industry will be able to learn from the experience of others.
What the ATSB found
A review of 245 stall warnings and stall warning system events reported to the ATSB over a 5–year period showed that almost all were low risk events which were momentary in duration, and were responded to promptly and effectively by the flight crew to ensure positive control of the aircraft was maintained. No occurrences resulted in a stall or an irrecoverable loss of aircraft control, and only a few were associated with minor injuries to passengers or crew (generally those that occurred in severe turbulence) or a temporary control issue.
About 70 per cent of stall warnings reported to the ATSB were genuine warnings of an approaching stall, with the remainder being stall warning system problems. In only a minority of cases were system problems reported that resulted in false or spurious stall warnings such as a stick shaker activation.
Stall warnings (and in particular stick shaker activations) were well reported by Australian air transport operators, and occurred in a range of flight phases and aircraft configurations, not exclusively those related to low speed, high pitch attitude flight, or flight in poor meteorological conditions. Fifty-five per cent occurred in visual (VMC) conditions, and those in instrument (IMC) conditions mostly occurred in cruise. In typical stall warning events during cruise, the aircraft was operating at an altitude where there was a narrow band (about 20 knots) between the maximum operating speed and the stall warning speed (VSW). Common precursors to these events were rapid changes of pitch angle or airspeed. In about one-fifth of these occurrences, the stall warning system was activated when the autopilot tried to correct the aircraft’s speed or flight path due to a disturbance. Stall warnings during VMC flight were most common on approach, often involving aircraft being affected by turbulence while manoeuvring around weather.
The ATSB identified 33 serious and higher risk incidents in which a stall warning occurred. The majority of these involved brief stick shaker activations, and were associated with moderate or severe turbulence. Most happened on approach to land, when aircraft were in a low speed, high angle of attack configuration, and in several cases the stall warning speed was higher than normal (due to a higher wing loading (g) factor in a turn, or an incorrect reference speed switch setting). In these cases, the risk of a stall developing was increased by a lack of awareness of decreasing airspeed and increasing angle of attack prior to the stall warning, and/or an approach to land where the flight crew were focused on trying to correct the approach prior to the stabilised approach height instead of conducting a go-around.
Stall warnings occur in normal operations, and are normally low risk events. In Australia, even the most serious events have not resulted in a loss of control, and have been effectively managed by flight crew to prevent a stall from occurring. To avoid higher risk stall warning events, pilots are reminded that they need to be vigilant with their awareness of angle of attack and airspeed, especially during an approach on the limits of being stable.