Re: DC-9 Flight Control

From:         Steve Lacker <>
Organization: applied research laboratories
Date:         25 May 96 14:40:18 
References:   1 2 3
Followups:    1
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  or MIME structure (RD Rick) wrote:
>> (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. <snip>

I believe that fact is pointed out on the L-1011 page. IF the DC-10 that
crashed in Iowa had had a 4th hydraulic system, the result might have been
different... but that is pure speculation.

I've never heard much about hydraulic failures in a 727... how many systems
does it have? Are they particularly well located? Or have I just not *heard*
about problems?

>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.

This also serves to isolate contaminated fluid (say from a failed pump or other
component) to one system. As a side-note, you can easily hear these pumps in
action on some aircraft. I was aboard a Delta 757 some years ago when a
passenger became somewhat alarmed at a "strange" intermittent grinding sound
during taxi.. the captain explained that due to a long taxi time, only one
engine was running and the transfer pump was powering the hydraulic system
normally powered by the other engine.

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