Re: Stall Warning Systems on Commercial Aircraft

From:         eeyore@dcs.qmw.ac.uk (Mark Anthony Brown;E200)
Organization: Computer Science Dept, QMW, University of London
Date:         24 Apr 93 09:28:49 PDT
References:   1
Followups:    1
Next article
View raw article
  or MIME structure

In <airliners.1993.357@ohareChicago.COM> sharpes@c-17igp.wpafb.af.mil (Civ Daniel G. Sharpes) writes:

>  I'd like to get some info on how the stall warning systems on various
>commercial big airplanes (and the biz-jets, too) work.  What sort of
>anticipation is included?  How often do nuisance warnings occur?  How
>were nuisance warnings eliminated?  Do the systems key on velocity and
>aircraft configuration or do they track alpha?

Don't claim to be an expert, but I think some at least use angle of attack
sensing vanes (some made by Rosemount, probably, if you want data from a
manufacturer), usually times two (one mounted each side of fuselage).
A recent crash (TriStar?) was attributed to a stuck angle of attack vane
causing premature stick shaker at rotate - the a/c landed back on almost
immediately grossly over max. landing weight and cracked the main spar.
All survived, fortunately.

You also get some stall warning through airframe buffet; its designed that
way. 

The alpha at which the stall warning is tripped would almost certainly be
modified by configuration sensing (flap & u/c settings).

The Panavia Tornado spin prevention and incidence limiting system has dual
alpha sensing vanes; this is supposed to prevent departure (inc. stalls)
by limiting control authority of the command/stability augmentation system.
This has caused some fun and games with pilots running out of control
authority in manoeuvres if they didn't predict what the system would do,
but you can disengage the SPILS.

On an early Avro Jetsteam I had stick-pusher operation demonstrated to me.
There were two audible warnings from separate sensors, (port & starboard),
followed by stick shake & finally stick push. I don't know if both sensors
have to show high alpha before stick shake. The Jetstream has a stick pusher
due to the tailplane configuration; it may cause deep-stall with the 
elevators in the turbulent flow off the stalled wings (so no pitch control and
you follow it all the way down).

Stick pusher was I believe removed from the F-104G which spent a lot of time
at low levels (F-104 is a T-tail and hence also prone to deep stall). If
you're hacking along on the deck, the last thing you want is for the stick
pusher to operate ruining your day. 

Hope this is of some use, and not too apocraphal (or at least wildly
inaccurate)..

Mark.