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

ATSB – AR-2012-172
Background
What is a stall warning?
Stall warnings indicate to the flight crew that the aircraft will approach a
stall if action is not taken to reduce the angle of attack. There is significant
variation in the types of stall warning systems available on aircraft, and the
flight conditions under which they activate.
All modern air transport aircraft have automated stall warning systems that
provide visual warnings and audible alarms to the flight crew in the first
instance. In most of these aircraft, a motor (commonly referred to as a stick
shaker) is fitted to the pilot’s control column, causing it to vibrate as an
additional warning.
Some aircraft have flight control systems that monitor the flight conditions
and aircraft configuration, and automatically limit the angle of attack to
prevent the aircraft from approaching a stall in the first place.
If the pilot does not take action to reduce the angle of attack after a stall
warning, most commercial aircraft have a stick pusher system to
automatically reduce the angle of attack.
A pilot’s own senses also give important cues for recognising an
approaching stall. A reduction in airflow noise, buffeting, changes to the
‘feel’ of flight controls or a reduction of pitch and roll control, or changes to
the pilot’s sense of movement (direction of motion and speed) can also be
useful warnings of a decrease in airspeed or a loss of lift (FAA, 2004).
Stall warnings do not mean that the aircraft is in a stalled condition, or is
not under control by the flight crew. However, repeated activations of the
stick shaker or activation of the stick pusher are not normal situations.
Stall warnings (like stick shakers and stall warning horns) provide a warning to flight crew to take
corrective action to prevent a stall from developing (reducing angle of attack). While stall and stall
warning speeds are published in the aircraft flight manual (AFM) for each type of transport aircraft,
pilots must consider that stall speeds can be affected by:
• aircraft weight
• attitude changes affecting the load factor on the wing (for example, the stall speed (V S ),
increases in a turn)
• use, position, and combination of high-lift devices, which change the effective angle of attack of
the wing and the amount of lift it is generating
• the accretion of ice on the wing
• atmospheric conditions
• localised atmospheric disturbances such as turbulence.
› 3 ‹