Re: Dominican Republic 757 crash (RISKS 17.82)

From:         "peter (p.j.) ashwood-smith" <petera@bnr.ca>
Organization: Bell-Northern Research Canada
Date:         21 Mar 96 02:37:59 
References:   1
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In article <airliners.1996.344@ohare.Chicago.COM>,
Robert Dorsett <rdd@netcom.com> wrote:
>I don't think the investigators have concluded anything--they may have
>completed their first sweep of the DFDR and CVR recovered earlier this week,
>but the accident investigation's just starting.
>
>More precisely, the comments today suggested that an erroneous speed
>indication is an issue under investigation.  There can be a significant
>difference between that and saying the indicator is bad.
>
>For the "indicator" to be bad, both the pilots' airspeed indicators would
>need to have failed.  And the standby speed indicator.  A highly improbable
>event.

 Agreed but there is another possibility. What if the ONLY thing that went
wrong was ONE of the angle of attack indicators.

 I'm just guessing here but I suspect the algorithms used to turn on a
stick shaker are something like "if any AOA indicator shows close to
critical angle ... shake stick" etc. I.e. the design is prudent, it
assumes it is safer to warn of a stall when there isn't one than to not
warn of a stall when there is one. Only problem is that at night when
you are low and all of a sudden you get stall warning the natural
reaction is to push forward. I doubt you'd even think to suspect the
stall warning indicator, the first reaction would be to doubt the
airspeed indicators .. all the time adding more and more forward
pressure to try to stop the shaker ... 7000 feet is only a few 10's
of seconds under these circumstances.

 Anyway could somebody who has more info on the stall warning inputs
and how they are processed let us know if this is out to lunch or not?
Its been bugging me all night that a single point of failure could
result in this kind of event.

--
Cheers,

--
Peter Ashwood-Smith   	  |  email: petera@bnr.ca
Bell Northern Research    |  work : (613)763-4534
Ottawa, Ontario, Canada	  |  home : (819)595-9032