Re: A320 and Noisy Hydraulic Pumps

From:         slacker@arlut.utexas.edu (Steve Lacker)
Organization: Applied Research Laboratories
Date:         09 Apr 95 22:37:12 
References:   1
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In article <airliners.1995.347@ohare.Chicago.COM>, aheatwole@telogy.com
(Tony Heatwole) wrote:

> The sound was like someone trying desperately to
> start a car with a nearly-dead battery followed by a thump that you
> could feel through the floor. This noise/thump cycle had a period of
> 5-10 seconds with a short silent period.

Excellent description! :-)
  
> I learned from talking to the captain after the flight that the sound
> (which they couldn't hear in the cockpit) was the cycling of an electric
> motor maintaining hydraulic pressure.

I last heard this sound while on board a Delta 757 waiting (a long time)
for takeoff at DFW. The captain actually made a PA announcement saying that
it was 'a characteristic of this aircraft during single engine taxi' and
that it was a hydraulic pump that pressurized the hydraulic system normally
powered by the engine that was shut down for the long taxi and wait for T/O.
I had *assumed* that this was a hydraulic-to-hydraulic motor/pump, but maybe
its electric. (???) Sure enough, the sound stopped when we heard the second
engine spool up.

Why do they tolerate such loud noises? Maybe its a waste of weight for sound 
deadening material for a mechanism that is infrequently used? I've heard a 
similar sound on MD-80's too, so apparently Boeing, Airbus, and McDD solve the
problem in a similar way.

Steve (sorry about the .sig, its obviously for another newsgroup ;-)

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