Re: A320 and Noisy Hydraulic Pumps

From: (Brad Gillies)
Organization: Internex Online
Date:         15 Apr 95 11:38:20 
References:   1
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  or MIME structure (Tony Heatwole) wrote:
>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.

What you were actually hearing was the yellow-green system PTU
(power transfer unit) which takes hydraulic flow from the yellow
system and uses it to create flow in the green system. The PTU
"pulsates"  because as the pressure is built up to 3000 psi the
pump output is decreased to 0 until the pressure drops off at
which point the PTU output increases Thus you get a really annoying
pulsation. The only time you would hear this with the engines running
is if the engine driven pump fails on either engine.

>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?

The Airbus of which you speak (a-320) is notoriously noisy in the cabin
The only reason it is considered quiet is due to the quiet engines installed.
THe electric and PTU pumps are directly below the mid cabin as well

>  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.

 Yes the pumps can be powered VIA ground power if the generator on the
ground can handle the  load which many of them can.

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

This could be company procedure. If in fact the plane was pushed back
to deice he would want hydraulic power if the plane were to move for
some reason. He would need it for steering.

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

As does the L-1011. This is due to layout of the hydraulic systems.

Boeing tends to keepthe hydraaulic bays in the engine pylons away from
the passengers.

Brad Gillies
Compuserve: 74163,2040
Leading Edge Aviation Services
My opinions are mine all mine.I won't let
anyone else take credit or blame.