Re: Eastwind 737

From:         wangermn@barder.Princeton.EDU (Pablo Wangermann)
Organization: Laboratory for Control and Automation Princeton University
Date:         10 Jul 96 12:47:13 
References:   1
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In article <airliners.1996.1145@ohare.Chicago.COM>,
 <dtuttle@ciesin.org> wrote:
>An Eastwind 737 on approach to Richmond VA experienced an uncommanded
>rudder input on 9 June.  Early reports were that the standby actuator was
>"mis-rigged".   I think Aviation Week has an article in its recent
>edition...but mine has (as usual) not shown up yet.  The New York Times
>has an article in Today's edition (available on the web too).
>
>Dale Tuttle
>

I've read the AvWeek and NYT reports so can adda bit of detail here.  The
Eastwind 737-200 (an ex US AIr aircraft) had apparently recently had an
almost complete rudder rebuild.  Apparently one of the few unchanged parts
was the yaw damper.  The 737 experienced an uncommanded roll - the pilot
regained control by switching off the yaw damper.  The pilot thought the
roll was about 20-30 deg, but analysis of the FDR suggests it was about 10.

On subsequent inspection it was found that the yaw damper was "mis-rigged",
so that instead of having -3,+3 deg motion, it was -1.5/+4.5 motion.  Npw
as I understand it, a reason why the yaw damper or another rudder actuator
has not been pinned as the cause of the Colorado Springs and Pittsburgh
crashes is that invcestigators could not produce the rudder deflections
required in order to achieve the departures from the flight path that
occurred.  The questions that arise from the Eastwind incident are:

i) Would a +4.5  deg yaw damper deflection be enough to cause the flight
paths seen in the two fatal accidents

ii) If it's possible to mis-rig a yaw damper 50%, could it be mis-rigged
even worse (say, 0,+6 deg)

iii) If, after all the attention to these incidents, yaw dampers are being
mis-rigged, how common is this in the 737 fleet?


Any aircraft mechanics/aircraft engineers care to comment on the yaw damper
design and its installation?

John Wangermann


[Cross posting to m.t.a-i]