A320 and Noisy Hydraulic Pumps

From:         aheatwole@telogy.com (Tony Heatwole)
Date:         31 Mar 95 02:41:29 
Followups:    1 2 3 4
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Yesterday I flew from Detroit to Washington on a Northwest A320 (yes,
I like to live dangerously). It had snowed a little, so the plane was
de-iced. Just before the de-icing started, the flaps were deployed.
A *terrible* noise preceded the flap deployment and continued for the
next 10-15 minutes. 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.

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. Since they had to deploy the
flaps slightly for the de-icing operation and the engines were off,
they had get hydraulic pressure for the flaps from the motor. The
captain noted that DC10s, especially, have a reputation for noisy
hydraulic pumps.

This brings a few questions to mind:

  1. In the cabin the noise of this motor cycling was *horrible*. Do
     airlines and aircraft designers tolerate this because it's
     hard to avoid or simply because it's a configuration which isn't
     often required?

  2. The pilot used the APU to generate the power for this motor. Could
     this motor have been powered from the jetway? We may have had to
     push back slightly to allow the de-icing, so that could explain the
     use of the APU.

  3. Why didn't the pilot turn off the hydraulics after deploying the

  4. Is the DC-10, in fact, notorious for the noise of its hydraulics?
     What other airliners have this reputation?

Tony Heatwole

TELOGY Networks, Inc.