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ShuttleDweller
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High-pitched Whine While A319 Taxis?

Mon Aug 16, 2004 10:45 am

I recently took a roundtrip on the USAir shuttle DCA-LGA-DCA, all on the A319. I noticed this happening on both segments of my flight:

During pushback, the plane began making a very high-pitched whine that was almost painful to hear. This continued unchanged until a few minutes before takeoff, after which it stopped. The sound did not happen at all while we were in the air. After we landed and turned off the active runway, it started again, and continued until we were stopped and parked at the gate.

I fly a lot and I know exactly what most sounds are. I can promise you that it was not the flaps or the baggage compartment. Any ideas what it is?????
 
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Starlionblue
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RE: High-pitched Whine While A319 Taxis?

Mon Aug 16, 2004 11:33 am

There have been several threads on this, and IIRC it's some sort of hydraulic pump.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
320tech
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RE: High-pitched Whine While A319 Taxis?

Mon Aug 16, 2004 9:37 pm

Hi, Shuttledweller.

I can tell you precisely what it is. It is the Power Transfer Unit, located on the aft wall of the main wheel well. It operates automatically whenever there is a pressure differential between the green and yellow hydraulic systems (500 psi diff). It sounds variously like a loud whining or grinding noise coming from under the cabin floor.

It operates on start up because one engine is running and the other isn't. The system detects the pressure diff and the PTU operates to bring the other hyd system up to pressure.

The PTU shouldn't be operating if both hyd systems are at full pressure (eg, when both engines are running). However, if a system is operating that draws heavily on one of the hyd systems, the PTU may operate now and then to keep the pressure up. I suppose it's also possible that the pilots may have taxied on one engine for some reason, which would of course make the PTU operate.

There have been a lot of threads on this subject. Some day I'm going to make a recording of the thing, and put it on the internet somewhere. Maybe A.net could start a sound file archive, too.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
ShuttleDweller
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RE: High-pitched Whine While A319 Taxis?

Tue Aug 17, 2004 12:27 am

Thanks 320Tech - that makes PERFECT sense. We definitely taxied out on one engine, because I could hear the second engine being started just before we took off. Also, my seat was at the back edge of the wing, so I was probably sitting directly above the main wheel well, and thus directly above the PTU (making it even louder for me).
 
modesto2
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RE: High-pitched Whine While A319 Taxis?

Tue Aug 17, 2004 12:56 am

I departed on a early evening JetBlue flight on Friday. If anyone has departed JFK in the early evening, you know the lines are crazy. The pilots made an announcement prior to engine start that only 1 engine would be used for taxi. They described the sound of the hydraulic PTU as a "barking dog". There were times when it was a constant high-pitched sound while other times, it was more intermittent and varied.

320Tech, what about the blue system? Why does the PTU only detect discrepancies between the yellow and green? And what are some systems that would draw heavily on one hydraulic system? Thanks.
 
NKP S2
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RE: High-pitched Whine While A319 Taxis?

Tue Aug 17, 2004 3:13 am

what about the blue system? Why does the PTU only detect discrepancies between the yellow and green? And what are some systems that would draw heavily on one hydraulic system?

Whay doesn't the PTU detect blue system pressure? That's just the way it is Smile Really, though the blue system is a major sytem ( in that it's used full time whenever the engines are running ) it is the smallest system and cheifly operates the primary flight controls ( along with the yellow and green systems ) and uses a single source of one elctric driven pump. It can operate the flaps, but it is regarded as more of a last-chance back-up system in the event of a toal failure of the other 2. The only other blue system source is the RAT.

The green and yellow system are larger systems that do more of the "heavy lifting", as well as primary flight controls and flaps. Since the green system does not have its own electric pump ( as the yellow does ) the only way to get pressure from it without the left engine running is through the PTU. Even with both engines running, high demand can cause a drop in volume enough to need assistance from the PTU. Landing gear retraction/extension is handled by the green system and this is quite a hydraulic volume hog. After lift-off, retracting the L/G may cause the green to need assistance from the PTU. Landing can also cause a situation where you may be using the flaps and extending the L/G simultaneously ( perhaps stab trim as well ), requiring the use of the PTU. Normal wheelbrakes are also powered by the green system, and that too requires demand, as to the T/R's, powered by both green and yellow.
 
radelow
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RE: High-pitched Whine While A319 Taxis?

Tue Aug 17, 2004 3:18 am

The amount of knowledable people here is great! Thanks for the great explanations 320Tech & NKP S2!

Mark
 
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Starlionblue
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RE: High-pitched Whine While A319 Taxis?

Tue Aug 17, 2004 4:37 am

You've all got it wrong. It's the squirrels winding up the big elastic band  Big grin

[Edited 2004-08-16 21:38:10]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
320tech
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RE: High-pitched Whine While A319 Taxis?

Tue Aug 17, 2004 11:38 am

A little more detail on the blue system . . .

The blue system is basically the backup system for when everything else goes away. It does help out with the flaps and slats in normal operation. The no. 3 spoiler on each wing is driven by blue hyd. Each primary flight control has a blue servo and either a green or yellow servo (one servo is in damping mode, the other moves the control).

If everything goes bad (eg Air Transat A330 which ran out of fuel and had to glide), the Ram Air Turbine (RAT) extends and spins (about 4000 rpm). It makes an incredible racket - a sound you don't ever want to hear, for a couple of reasons. The RAT pressurises the blue system, and also provides some electrical power. There is enough hyd power to operate the flight controls normally. Landing gear would be extended manually. Flaps and slats could be extended if necessary, but would be a heavy load on the system.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
lowrider
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RE: High-pitched Whine While A319 Taxis?

Tue Aug 17, 2004 2:29 pm

Most whining on any given airliner come from the flight crew, with passengers being the second largest source.
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