Stall warnings in high capacity aircraft: The Australian context 2008 ...

ATSB – AR-2012-172
associated with a temporary loss of control after encountering severe turbulence during cruise. A
summary o fthese occurrences are as follows:
• After turning to avoid severe weather showing on the aircraft radar, the aircraft encountered
severe turbulence and was lifted 1,000 ft above its cleared level. The stick shaker operated
twice momentarily. After about 15 seconds the crew were able to return the aircraft to its
cleared level. (See case study Large altitude excursion on page 23.)
• The aircraft encountered severe turbulence for a period of approximately 30 seconds. The
crew received several stickshaker warnings. The crew declared a PAN, diverted the aircraft
from the cleared track and commenced a descent. (See case study on page 11.)
Figure 7:
Assessed risk of stall warning occurrences reported to the ATSB, 2008 to
2012
25
Number of occurrences
20
15
10
5
0
Very high risk of an accident High risk of an accident Medium risk of an accident
Low risk of an accident
No likely accident outcome
Between 2008 and 2012, there were 30 stall warnings reported where the ATSB assessed that
there was a medium risk of an unsafe outcome due to the conditions that existed at the time
(severe turbulence at low altitudes, such as on initial climb, where the aircraft is travelling close to
V Ref and at a high angle of attack). In these cases, the worst credible outcomes were considered
to be a temporary but recoverable loss of control of the aircraft, and minor injuries to cabin crew
and unrestrained passengers.
The most commonly involved aircraft types in these medium risk events were Boeing 747-400
(nine occurrences), Boeing 767 (eight occurrences), and Boeing 717 aircraft (three occurrences).
The majority of these occurrences were brief stick shaker activations, and were associated with
moderate or severe turbulence. Some were also associated with:
• additional warnings associated with the stall warning (excessive bank angle and undershoot
GPWS alerts, flap overspeed)
• large bank angle and altitude deviations (often associated with unexpected turbulence)
• overspeeds followed by underspeeds
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