Re: DC-9 Flight Control

From:         Chris Hall <>
Organization: Airborne Display Limited
Date:         21 May 96 11:10:52 
References:   1
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In article <airliners.1996.714@ohare.Chicago.COM>, billie durso
<> writes
>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...
>    I am pretty sure that all of these earlier aircraft had mechanical
>linkages to all of the primary flight control surfaces.  I also am
>pretty sure that these mechanical linkages were hydraulically boosted
>to easy in control of the aircraft.  Assuming a total power failure in
>the aircraft, are these hydraulic boosters shutdown?  If so the flight
>surfaces would become excessively hard to control.  If anyone out there
>has any experience with the design of these aircraft please give me an
>answer about this.  Could a fire in the main fusebox cause a total
>power fialure throughout th e aircraft.?  Let me know.  Thanks

You are right that it is normal to have a boosted mechanical linkage
from the controls to the surfaces.

The boosting is normally done hydraulicaly and is independant of the
aircraft's electrical power system. The huydraulic power is obtained
from engine mounted pumps, but depending on the aircraft type, can have
backup systems:-

A pump on the auxiliary power unit.

An electrically driven pump from the main battery bus.

A wind driven pump from a gravity operated drop down emergency

If all these fail, it is still possible to fly the aircraft manually,
although the control forces will be very high.
Chris Hall