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

ATSB – AR-2012-172
Safety analysis
In the 5 years from 2008 to 2012, 245 1 stall warnings and stall warning system issues were
reported to the ATSB by high capacity 2 air transport operators and flight crews. These included
both Australian civil (VH-) registered and foreign-registered high capacity air transport aircraft
operating within, to, or from Australia. Of these, four were classified as serious incidents (involving
circumstances indicating that an accident nearly occurred). These serious incidents are discussed
in further detail later in this report. There was also a stall warning associated with an Airbus A380
accident in November 2010, where an uncontained engine failure occurred on a flight departing
from Singapore that resulted in a number of aircraft system malfunctions and failures. 3
A number of modern aircraft types have fly-by-wire control systems that include flight envelope
protection functions. Protections such as alpha prot (protection) and alpha floor prevent the aircraft
from approaching the stall and so these aircraft do not provide stall warnings. As these protections
provide a similar function to stall warnings on other aircraft, the data below includes alpha prot and
alpha floor occurrences as equivalent to stall warnings.
These 245 occurrences represent about 1 per cent of the 27,000 occurrences reported to the
ATSB over the same period involving high capacity air transport aircraft. Reporting of safety
incidents from Australia’s airlines is very robust. As the ATSB and airlines work closely to
continually improve the level and quality of reporting, there has been a gradual increase in the
number of all reported safety occurrences over time that is independent of growth in flying activity.
Figure 2 shows changes in the number of stall warnings reported to the ATSB in the 5 years since
2008. The growth observed since 2009 is reflective of increased reporting levels by operators, and
improved capture by the ATSB of all stall warnings and stall warning system issues, not just those
that resulted in activation of a stick shaker.
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3
Including 37 reports which were determined not to be transport safety matters as defined by the Transport Safety
Investigation Act 2003.
A high capacity aircraft refers to an aircraft that is certified as having a maximum seating capacity exceeding 38 seats,
or having a maximum payload capability that exceeds 4,200 kg.
The engine failure caused significant damage to many aircraft systems, and led the flight crew to make a return to
Singapore Changi Airport. One effect of this damage was the aircraft’s control system reverting from Normal to
Alternate 1A law. Under the Alternate 1A law, alpha floor protection was lost, and the stall warning function (not
normally required due to flight envelope protections provided by Normal law) was restored. On the final approach and
immediately before the aircraft touched down on the runway, the stick shaker activated. Post-accident analysis by
Airbus identified that, as a result of the control system laws and damage to the aircraft, the stall warning was genuine
and that the flight envelope margins were maintained (ATSB investigation AO-2010-089).
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