Re: A320 and Noisy Hydraulic Pumps

From:         geohull@ditell.com (George Hull)
Organization: DirecTell L.C. - Park City, UT. - 1.801.647.0214
Date:         15 Apr 95 11:38:19 
References:   1 2 3
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> I'll add the 767 to the list with noisy pumps. I was seated about halfway
> down, aisle seat in the middle row on an Air Canada flight from SFO to
> YYZ.  When we were on our descent to Toronto, the hydraulic pumps would
> make a low-pitched but LOUD "ZZZZSSSSHHHHHH" noise when we were
> manuevering, seemingly only when we were banking. When the flaps were
> extended for landing, a much higher-pitched, but quieter pump seemed to be
> working. Are there different pumps for different control surfaces??

We're talking about a bunch of different pumps and motors and transfer
units here.  The electric hydraulic pumps typically make a squealing
noise, but they're on all the time from just prior to pushback to just
after the brakes are set at the destination gate.  The power transfer
units (B757s) are in operation if the aircraft is taxied on the right
engine only . . if we taxi with the left engine the PTU will not operate.
The hydraulic motor sounds that you hear during configuration for takeoff
and landing occur during the operation of the flaps.  The hydraulic
pressure from the noisy pump and/or the noisy PTU is ported to the noisy
hydraulic flap drive motor.  The other control surfaces, including the
primary flight controls and the spoilers do not have hydraulic motors . .
they have hydraulic actuators which port pressure to the appropriate side
of the module which displaces the controls.  Those processes are not noisy
in the cabin, but you can definitely hear a lot of commotion when you
stand outside during a control check.

George