Stall Warning System
Natural stall warning (buffet) usually occurs at a speed prior to stall. In some
configurations the margin between stall and natural stall warning is less than
desired. Therefore, an artificial stall warning device, a stick shaker, is used to
provide the required warning. The stall warning “stick shaker” consists of two
eccentric weight motors, one on each control column. They are designed to
alert the pilots before a stall develops. The warning is given by vibrating both
control columns. The system is armed in flight at all times. The system is
deactivated on the ground. Two independent, identical stall management yaw
damper (SMYD) computers determine when stall warning is required based
upon:
- Alpha vane angle of attack outputs.
- ADIRU outputs.
- Anti–ice controls.
- Wing configurations.
- Air/ground sensing.
- Thrust.
- FMC outputs.
- Mach compensation.
The SMYD computers provide outputs for all stall warning to include stick
shaker and signals to the pitch limit indicator and airspeed displays and the
GPWS windshear detection and alert.
Two test switches are installed in the aft overhead panel. Pushing either of
these initiates a self–test of the respective stall warning channel. The No.1
activates the Captain stick shaker, and the No. 2 activates the F/O stick
shaker. Either stick shaker vibrates both columns through column
interconnects.