Re: DC-9 Flight Control

From:         James Buongiovanni <>
Organization: Good John's  Shaperite Concepts
Date:         23 May 96 10:37:33 
References:   1
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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...
>     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
> D.Warren

 The engine driven Hydraulic pumps on the DC-9 are mounted on the gear
box of each engine. Even with total electrical failure the would still
have had hydraulic power, they would'nt have had indication though. Also
depending on how far out they were from the airport the crew would have
select "low" for the engine pumps which is 1500 psi. Of course after an
emergency they (the crew) would have immediately gone to high(3000psi)
along with turning on the Aux pump and the Alt Gear Pump.

 The Ailerons are fly by tab. The only play hyd have is the flight
spoilers once you turn the control wheel past a certain range.

 The rudder goes to manual once hyd press drops below a certain psi.
Manual is harder but not impossible.

 The only hyd on the elevator is when you push the stick full forward(to
recover a stall) the hyd power will drive the tabs down. That is know as
Elevator power.

 One news article noted that the steps in the E&E compt were scorched.
These steps are located next to the main aircraft batteries. If these had
over heated from excessive charging could have cause an explosion its
self. Battery temp sensor may have failed in that case. Just speculation.
Hope it was of help.