Re: DC-9 Flight Control/ hydraulic systems

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         10 Jun 96 12:22:25 
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In article <airliners.1996.846@ohare.Chicago.COM> kls@ohare.Chicago.COM (Karl Swartz) writes:
>>There was the Turkish Airways DC-10 out of Paris, and the UAL at Sioux
>>City.  What was the third crash?
>I believe he's referring to AA 191 (Chicago; 1979), though I would
>disagree with that reference.  The engine separation damaged one of
>the hydraulic systems, which in turn caused the leading edge slats on
>the left wing, which unlike other designs lacked any sort of mechnical
>lock, to retract.  The non-redundant stall warning and slat disagree-
>ment systems were powered only by the engine which separated from the
>airframe, and thus the pilots were not aware of the asymmetric lift,
>nor of the left-wing stall, once they reduced airspeed in compliance
>with AA operational procedures.  With these warning systems intact,
>simulations showed that the plane was still flyable -- there was no
>total hydraulic failure and the flight controls were largely intact.

I would argue that the slats are considered a flight control, and that the
failure of the hydraulic system did, indeed, trigger a failure mode which
resulted in the crash of the airplane.  We can argue over a few knots,
but the fact is, those knots made a difference according to how McDonnell
Douglas designed the airplane to be flown, and how the FAA decided to
certify it.

Note that I'm not terribly focused on total hydraulic system failures.
It has undoubtedly happened on the 727, and others; however, we don't
hear about them because of mechanical redundancy.  What matters is control
system failure: and if the loss of one hydraulic system or all  hydraulic
systems directly contribute to a crash, that failure mode is what has to
be taken into account.

In the case of AAL 191, the sole changes to be implemented were a re-
evaluation of the use of forklifts to hoist engines, and the modification
of takeoff speeds--not any dramatic re-implementation of the slat actuation
mechanism.  The latter may have been sufficient to avert further tragedies;
we'll never know.

Robert Dorsett                         Moderator, sci.aeronautics.simulation