Re: DC-9 Flight Control

From:         Steve Lacker <>
Organization: applied research laboratories
Date:         21 May 96 11:10:54 
References:   1
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  or MIME structure (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...

>Assuming a total power failure in
>the aircraft, are these hydraulic boosters shutdown?

Aircraft hydraulic systems have, by necessity, a lot of redundancy and failsafe
capabilities. Generally, hydraulic systems are split, so that one part of the
system is powered by a pump on one engine, another part of the system is
powered by another engine, and so that the plane is controllable even with one
or more systems disabled. A good description of the 4 hydraulic systems used on
the L-1011, including which engine powers each and what components are powered
by each, for example, can be found at:

The description for the L-1011 also shows that the hydraulic systems are
generally "re-configurable" to a certain extent. For example, if one engine
shuts down, the hydraulic system it powers can be powered by another engine via
a transfer pump.  Some aircraft provide an electrical pump as a backup, and
usually the APU can provide hydraulic power, or even a ram-air driven turbine
can power a system in the event that none of the engines nor the APU are
functioning and the plane is gliding.

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. Still, loss of all electrical
power should not, by itself, totally disable aircraft hydraulics.

Since the question was originally directed at the ValueJet crash, I should
state that I think its wrong to speculate much on what happened. It will take
quite a while to sort it out (given the degree of the destruction) and no doubt
MANY rumors will circulate before the real answer is found (if it is ever

Steve Lacker	/	Applied Research Laboratories, The University of Texas
512-835-3286	/	PO Box 8029, Austin TX 78713-8029