A320
 

Hello,

Recently I did a couple flights on an A320, window seat about 20% chord. Both times there has been this really strange sound like a dog barking in the hold on push back. Obviously not because it has happened twice. I think it is possibly something to do with hydraulics maybe? Can anyone explain this?

Also one other thing which I cannot explain is that I was looking out the window in the cruise (I also noted this on a B777) and noticed a sharp vertical line of what looked like the difference between two mediums of air. About 20% chord, illusion? Maybe I'm not sure. Both times the sun was at different angles.

Thanks in advance

The 'barking dog' sound is an electric hydraulic pump in the Yellow system. It is used to open cargo doors, it gets turned on if the crew carry out a single engine taxi etc. As for the air thing, I have no idea.

Thanks man. It is rather amusing listening to the pax comments when you can hear it..

The yellow hydraulic system electric pump creates a constant high pitch whine. The "barking dog" probably comes from the power transfer unit.

Could also be caused by a Power Push Unit which uses rollers pressed onto to the main gear wheels instead of using a pushback tug. It is normally attched on the left hand side in our airline and allows a single engineer to despatch you using a remote control unit. This PPU also makes a barking type sound as its hydraulics work to rotate the main gear wheels. Also used on the 737 on our domestic network only.

The "barking dog" - more like a pig grunting - is the hydraulic PTU (power transfer unit), as flyera343 has said. PTU is the Airbus term for what Boeing used to call a "reversible motor-pump" (RMP).

The A320 has two engine-driven ("ED") hydraulic pumps: one per engine. No 1 powers the Green hydraulics; No 2 the Yellow. (The other hydraulic system is Blue, powered only by an electric pump - or the RAT.) If only one engine is running, its hydraulic system will be pressurised by its ED pump. If certain conditions are met, however, the PTU will start running.

Take the example of No2 engine running, so the Yellow system is pressurised. Fluid from Yellow turns the PTU motor, which consequently runs the pump on the Green side, pressurising the Green system.

Regards,
Chris

The Yellow can also be pressurised by an AC (electric) pump, similar to the one on the Blue system. It makes a high-pitched whine.

Very good, but you missed out the reason why he heard it: the PTU undergoes an automatic self-test after the first engine is started.

Hi notaplanegeek,

I've noticed the same effect in the cruise also. I thought it was the onset of a shock wave as explained in figure 7.1 of this document:
http://www.dept.aoe.vt.edu/~mason/Ma...Transonics.pdf

Quote from fantom:
"...you missed out the reason why he heard it: the PTU undergoes an automatic self-test after the first engine is started."

Thanks fantom. Assume you are current on type, whereas I'm not, and my FCOM is 1988...Have been SLF for many years now, and have recently wondered about the running of PTU during engine start, as there is normally no demand on the opposite-side system to justify more than a single "grunt". The self-test of PTU on first engine start may have been a sensible retro-fit, so thanks for the info.

Further points for notaplanegeek. The other time for hearing the PTU is during single-engine taxiing after landing, when the opposite-side system drops more than 500psi below the engine-driven one. Have never understood quite why the A320 PTU is so audible in the cabin: other types have similar devices. Most common time to hear the Yellow AC pump is before departure, when the cargo doors are being closed.

rudderrudderrat is right about the shock-wave. It's by no means limited to the A320 and B777, of course. The VC10 used to produce a nice one in the right atmosphere. It migrates back as the Mach No. increases, as you can see from the link. On Concorde, it was behind the aircraft above M1.7, resulting in the drag reducing above that speed (cruise being M2.0).

The Kiwi aviator raises an interesting point about the power-push unit being noisy, but I haven't heard it.

Chris

Hi notaplanegeek,

Check this out.

Cheers.

I-2021,

(PTU) It's certainly less annoying (and less startling) when running continuously, but still noisier than other types, IMHO.

Easy: drop both gear doors and you can look straight through the fuselage in mid section. It's empty except for the Hyd system. When the PTU barks up, it is heard just about everywhere.

Thanks everyone, well explained

Wow, I always though that a/c flew under Mcrit because of the problems associated with transonic flight. Well I managed to get a picture of what it looked like, it was very apparent during flight but you can just make it out on the leading edge, mid way..

http://img153.imageshack.us/img153/318/shockwave1.jpg

Hi notaplanegeek,

Some more photographs:
Shockwave photos

Quote from me:
"Have never understood quite why the A320 PTU is so audible in the cabin."
And from fantom:
"Easy: drop both gear doors and you can look straight through the fuselage in mid section. It's empty except for the Hyd system. When the PTU barks up, it is heard just about everywhere."

Remember it well. The PTU is on the keel, IIRC.

You've exposed the sloppiness of my question. What I was trying to say was: why do we not hear similar noises in the cabin on other types of similar size? Presumably they have a PTU/RMP. Is it positioning, mounting, or simply a noisier PTU?

Chris

More specifically the PTU self test occurs during the second engine start.