Re: A320 and Noisy Hydraulic Pumps

From:         geohull@ditell.com (George Hull)
Organization: DirecTell L.C. - Park City, UT. - 1.801.647.0214
Date:         07 Apr 95 03:09:14 
References:   1
Followups:    1
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In article <airliners.1995.347@ohare.Chicago.COM>, aheatwole@telogy.com
(Tony Heatwole) wrote:

> 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?

Hydraulic pumps are _loud_ and we confine their use on the ground to
necessary operations during taxi, but we try to keep them off at the
gate.  You will also hear noise from Power Transfer Units on some aircraft
(B-757 for one) when one of the engines is shut down.  Older airliners
also have noisy hydraulic pumps and they are usually turned on just prior
to pushback and they remain on until the aircraft arrives at the gate at
the destination.  Hydraulic systems typically use a combination of
engine-driven and electric pumps to provide pressure and redundancy.

>   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.

You would expect to be on ship's power . . either engine-driven generators
or the APU during any de-icing operation.  The aircraft can be powered
from the external power connector at the gate . . and usually is when the
aircraft is parked.  But if the aircraft is being deiced it is normal to
go to ship's power.

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

Because those pumps also supply pressure to pressurize the wheel brakes.

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

All of the airliners that I've been a crewmember on have used noisy
hydraulic pumps.  When you're next to a pump its noise is deafening . .
the differences usually involve placement and insulation within the
airframe.  They're noisy!!  I don't remember the DC-10 having particularly
noisy hydraulics.

As flight crewmembers we're conscious of the noises that the airplanes
make, but we still have to allow the noises to be made . . those pumps and
generators and actuators and PTUs are good things to have on your team.

George