Stall protection:
An aircraft Stall Warning System is that system
which provides the pilot with advance warning of an impending stall. As
is the case with most safety related equipment, the requirement for a Stall
Warning System will be found within the aviation regulations for the state of Manufacture
and the state of the operator
for any given aircraft. The required operational parameters of the Stall
Warning System on a particular aircraft will, in part, be dependent upon the
aircraft weight, capacity and purpose and the regulations under which the
aircraft Type of Certificate
was issued.
As per the regulations, the stall warning system
requirements can be satisfied by the inherent stall characteristics of the
aircraft itself or by other appropriate means. Some of the most common stall
warning systems are as follows:
Pre-Stall Buffet: In this case, the warning of the impending stall
is provided solely by aerodynamic buffet. As the aircraft approaches the stall,
the airflow across the upper cambered surface of the wing ceases to flow
smoothly, it losses contact with the wing surface and it becomes turbulent. If
the turbulent air then flows across the horizontal stabilizer, buffet results.
In many aircraft, even some as large as the locked AC-130 spectre, this buffet provides the sole warning of the
impending stall.
Audible Warning: Stall warning is provided by an electronic or
mechanical device that sounds an audible warning as the stall speed is
approached. The simplest such device is an air-frame mounted stall warning horn
which sounds when the airflow through it occurs at a specific angle. Slightly
more sophisticated audible warning devices consist of either a pressure sensor
or a moveable metal tab that actuates a switch as the stall is approached. The
switch, in turn, activates an audible warning horn. In some installations, the
audible warning is provided by a synthetic voice which helps to reduce warning
ambiguity.
Stick Shaker: A stick shaker is a mechanical device that
shakes the control column to warn of the onset of stall. A stick pusher may be installed in association with a stick
shaker system in aircraft which are susceptible to the deep stall phenomenon. A deep stall affects certain
aircraft designs, most notably those with a T-tail configuration, and results
in a substantial reduction or loss of elevator authority making normal stall
recovery actions ineffective; in many cases, a deep stall might be
unrecoverable. The stick pusher is designed to prevent the pilot from allowing
the aircraft to enter a stall.
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