Re: 757 Crash / Airspeed Indicator Question

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         21 Mar 96 02:38:00 
References:   1
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In article <airliners.1996.388@ohare.Chicago.COM> (John Witherspoon) writes:
>I don't understand why a malfunctioning airspeed indicator would
>cause a 757 to crash.  At the point in the flight when the pilots
>discovered that there was a discrepancy, weren't they at full-throttle
>anyway?  If they weren't at maximum power, why didn't they go
>to it immediately and put the nose down some when the stick started
>shaking (didn't it shake for over a minute?)?

Worse than stalling an airplane is going too fast.  Stalls in of themselves
are generally not an "end game" situation (if you have enough altitude) but
if you're going too fast, you can overstress the airplane, incur extreme
aerodynamic loads, and start to break off important surfaces.

The Vmo pointer on this airplane would have been set to about 350 knots
at 7,000 feet.  According to AvLeak, the DFDR was indicating they were
flying at 330 knots.

So you have a crew at night, in clouds, flying on instruments, with two
sets of instruments contradicting one another and two sets of warnings
contradicting.  If they did feel they were flying too fast, then lowering
the nose would have bounced the airspeed needle past Vmo in a couple of
seconds.  Maybe they were getting buffeted, and didn't have the experience
to distinguish between low- and high- speed buffet. Maybe they thought the
stick shaker was indicating high-speed buffet and stall.

If they were a typical crew of an advanced aircraft, most of their training
and attention is spent on figuring out the FMS.  Systems can suffer.

Who knows?

Robert Dorsett                         Moderator, sci.aeronautics.simulation