Stall warning revisited

From: (Robert Dorsett)
Date:         02 Jun 93 00:16:23 PDT
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Someone some time ago asked how airliner stall warning systems work.  I
just ran a description of the 727's system; this is a condensed version.

1.  Stall warnings are designed to signal an *impending* stall.  Inputs
are the angle of attack, flap position, and the air/ground switch.  The
system is comprised of an angle of attack sensor, flap angle transmitter,
a stall warning module, a stick-shaker, and the air/ground sensor.  The 
system is 28 VAC; the stick-shaker operates 28 VDC.

2.  The angle of attack sensor is a vane, mounted behind the first officer's
aft window, beneath the airplane centerline.  It takes the form of 
a V-shaped wedge, acute portion pointing into the wind.  There are dowel 
pins to keep it from reversing (perhaps for mechanical reasons).  This 
is mounted on a little lever assembly, which enters the aircraft through a 
small, flush port.  The port is headed, to alleviate the possibility of 
ice hindering the effectiveness of the system.  The lever is used to generate 
an electrical signal, which is communicated to two synchro transmitters.  
One is used by the autothrottle system, if installed; the other sends the raw 
electrical signal to a device on the left inboard trailing edge flap 

3.  This device takes the signal and electrically "rotates" its phase, as 
a function of actual inboard flap *position*, thus compoensating for 
configuration-based stall-speed changes.  This rotated signal is then 
sent to a stall warning module.

4.  The stall warning module is a simple logic-circuit mounted on the 
overhead panel in the cockpit.  It compares the phase angle to a reference
28VAC value.  If it figures out that a warning situation exists, it will 
issue a signal which activates the stick shaker motor on the pilots' 
control column(s).  

5.  The stall warning module interface consists of an OFF light (illuminated 
in case of electrical failure or major fault), a test annunciator, and a 
three-position control switch.

6.  The control switch has three position: normal, test (activates motor
and shows the test annunciator), and heater off (used in ground maintenance).

7.  The gear air/ground switch comes into play when the oleo struts are
compressed: this suppresses the stall warning during roll-out on the ground.

Robert Dorsett!!rdd