Re: DC-9 Flight Control

From: (RD Rick)
Organization: Netcom
Date:         23 May 96 10:37:33 
References:   1 2
Followups:    1 2
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> (billie durso) wrote:
>>I am new to this subscription so please bear with me.  I was wondering
>>about the recent crash of the Valujet DC-9, and arrived at the
>>following question.  This question applies to all of the elder
>>transports 727, 737, DC-9, DC-10...
>Now, it goes without saying that some aircraft have more redundancy than
>others- the L-1011 has 4 hydraulic systems, but the DC-10 (of similar size,
>power, range, etc.) only has 3, for example.

Unlikely as it may seem, that fourth hyd system on the 1011 has made a
difference.  An EAL flight was at 10K feet when the front spool on the
number two engine broke loose and augured forward.  It destroyed lines
to three of the four hyd systems, and damaged the fourth. They landed
on the fourth.  AIR, there was one other similar case.  It was not
really good design that put all those systems on the bulkhead ahead of
the #2 fan.

For clarification, hyd transfer pumps transfer power, not fluid.  It is
a hyd motor in one system driving a hyd pump in the other, so one is
kept intact in case the other loses its fluid.

I have been told that the DC-9 does not use hydraulics to move the
control surfaces.  Cables from the yoke move just the trim tabs.
Sometimes you will see a DC-9 at the gate with one elevator up, and the
other side down.  They are moved by independent trim tabs.  The aux hyd
pump is turned on for the approach, but I believe that is only to move
the flaps and gear.