A320 - which electric/hydraulic motor is making this sound?

[baigar]

Hi tech experts - travelled some times with an A320 now and I wonder what
motor makes that sound clearly audible after engine shut down in parking
position, best audible from above the wings? It is not the freight doors - they
come later and flaps and slats are already retraced of course. And I have the
feeling it is an electric motor and not something hydraulic.

I recorded the sound in a video: Engines are shut off at 0:30 and the sound of
the interruptive motor appears 1:16-1:25. Some feedback loops seems to
be active here - may some part of the trimming system still active?

For comparison, here I assume you can hear the motor of a freight door which
stops at 0:25 and this sound comes after the one above and after the ground crew
approached.

Sometimes this ehh-ehh-ohh-ehhhhhh-ohh sounds lasts for much longer and you
can sometimes hear this sound after taxiing started (but than it is more difficult to hear as
the engines are running).

Just curious - any tips?

Thanks, Erik.

P.S. I know that it is silly to link to a video if we are just talking on sound and maybe
some people may even have problems with the dropbox-link. My apologies for this...

[Thrust Augmentation]

It's the dog;

http://www.askthepilot.com/questiona...airbus-noises/

[baigar]

Many thanks - that is exacly what I mean. Here a youtube example...

youtube link

...of such a (very monotonic) sound for other readers! Cool that you solved this puzzle so fast...

[Thrust Augmentation]

It wasn't hard when she was six or so to convince my daughter that Easyjet carried a gaurd dog on every flight....

[alcan60283]

Silliest thing on an aircraft really. Don't know why they couldn't have just gone with a regulated pump that ran full time instead of the on demand thing they came up with. If you look at the PTU in service, and watch the lines, EVERYTHING moves drastically when it kicks on too. Odd design that thing

[baigar]

Originally Posted by alcan60283

Silliest thing on an aircraft really. Don't know why they couldn't have just gone with a regulated pump that ran full time instead of the on demand thing they came up with. If you look at the PTU in service, and watch the lines, EVERYTHING moves drastically when it kicks on too. Odd design that thing

Yes you are right, after being sensitized to this sound and knowing its origin it hurts the engineering/physics
part of my brain each time it kicks in ;-)

[B2N2]

Hydraulic Transfer Unit

You Tube

[Uplinker]

Yes, the sound is the hydraulic PTU running as the hydraulic pressure in the Green and Yellow systems decays. It is fitted in the main gear bay area and runs one way or the other to support the pressure in whichever (Green or Yellow) system has the lowest pressure. It cuts in and out at certain pressure drop values* with some hysteresis applied. However, the PTU in that you-tube clip (#3), seems to be cycling excessively. I wonder if the various hydraulic accumulator charge pressures were correct?

As to why they don’t use an electric pump instead of a PTU; well there are electric pumps in the Blue and Yellow hydraulic systems, and I dare say there are redundancy and diversity considerations for having a mechanical instead of electrical back-up in the event of certain failures, such as Emergency electrical config, where electrical power might be very limited or absent.


*Cannot remember the A320 figures, and can’t be bothered to look them up!

[baigar]

Well, looking at the video the PTU is explained like a simple on/off valve to let hydrolic oil from the
higher pressure system (e.g. yellow correctly at 3000psi) flow to the lower pressure system
(eg. green at 2700psi) and thus supplying energy to it. But the dog like sound heared is that
of a motor running forward/backward/forward/backward and so on. So is the PTU a pump acually?
But this would not make much sense as the failed system should have a pressure lower than the
OK system leading to an automatic flow in the right direction...

[Epsomdog]

There is no transfer of hydraulic fluids between any of the 3 hydraulic systems, (in order to provide protection in the event of fluid loss). Any graphic suggesting otherwise is simply wrong! The PTU is a hydraulic pump connected to a hydraulic motor by a mechanical shaft. The pump and motor are identical and can in fact switch function, depending on which hydraulic system has the lowest pressure.

[baigar]

Many thanks Epsomdog - this explanation is perfect and now I fully understand it! The setup makes sense as
it does not allow oil getting lost ;-) Thanks!

[Max Angle]

Also worth pointing out that this normally only occurs when one engine has been shut down during taxi in after landing. The PTU only runs when there is differential pressure between the two systems and during the single engine taxi (always No.2 shutdown) the electric pump on the yellow system is switched on to keep the system pressurised which means the PTU won't run. It is done to prevent wear on the PTU which is considered a critically important system, so important it is automatically tested at every departure during engine start. Once the aircraft is on stand the yellow pump is switched off and the No1 engine shutdown leaving differential pressure across the green and yellow system which runs the PTU while No1 engine pump runs down.

At a previous airline I used to just reach up and switch the PTU off during the shut down scan to stop the noise in the cabin, it is rather frowned upon at my current airline so now my passengers get the barking dog.

[baigar]

Hi Max Angle - thanks for your answer... Really astonishing for an non-insider, that the procedures between different airlines are that different even on the same type of equipment... Thanks for your comment!!!

[Epsomdog]

I Think you'll find there is only one Airbus FCOM standard procedure. However crews do have discretion to adapt procedures to suit their needs at the time. As in the case of the VS A340 with the landing gear hang up at LHR.