Re: A brief commentary

From:         mikem727@aol.com (MikeM727)
Organization: America Online, Inc. (1-800-827-6364)
Date:         29 Jul 96 13:24:41 
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>> Of course the nut of the problem for Birgenair (pending the NTSB
>> report ;-) is deciding which intstruments are right, and to what
>> extent.  My point was that in this case the decision criteria seemed
>> straightforward:  one instrument, the ASI, said "too fast", and
>> another, the stick-shaker, said, "too slow".  This impasse can be
>> resolved through an instrument all pilots carry with them:  "their
>> ass strapped to the hardware", as someone else put it.  At a given
>> throttle setting and nose attitude, a B757 is not about to suddenly
>> accelerate past its airframe speed limit.  That eliminates "too
>> fast" as a threat.  "Too slow" can be taken care of by a moderate
>> throttle setting/pitch, and then one can start worrying about which
>> instrument(s) are wrong, and what one should do next.
>
>Aaah, but how do you know that your attitude indicator is not faulty?
>What if the various attitude indicators on board also didn't agree?
>See the problem? They have to identify which instruments they can
>trust.

Well there's three attitude indicators (ADI's), and there's no evidence
that any of them were inoperative or showing erroneous information.
Therefore, if all three are showing the same thing, and all other
performance instruments agree (except one airspeed indicator), you can
believe the ADI's.

Stall warning is provided by angle of attack (AOA) vanes, whereas
overspeed warning is provide by pitot input.  I would think that the AOA
vanes are more reliable.  Also, there should have been two AOA indications
along with the three airspeed indications, so determining which is
defective is not impossible.

Of course this is all Monday morning quarterbacking, and it's a lot easier
to think of this in front of the computer than when the sh** is hitting
the fan.  However there's one thing they should have known without much
contemplation.  There is, at least in my airplane, a procedure for "Flight
with Unreliable Airspeed".  It involves, as someone wrote above,
maintaining moderate power / pitch settings.

Mike,
ATP/FE, Boeing 727
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