Last Thursday 1/29, I took the Dragonair A321 flight in the morning to HK with final destination being Taipei from Nan Jing, PRC. It was a icy morning with previous night some continuous small flurries. We were informed by the pilots during pre-take off announcement that due to weather, we need to perform de-icing, which is fine.

As we were pushing back, and as the engine (IAE2500) started to spool up, the leading edge of the wing was being lowered for take off position. As I am aware, having being on board many times on A320 series aircraft. The lowereing of leading edge slots is quite an interesting experience. You first hear the start of the servo moto, and then it went kind of on/off for several times before the slots completed its configuration. Very amusing but sometime not making you feel comfortable - almost the sense that the motor struggles to push the slots to its final position for take off.

On this special occasion, the motor that drives that slots, continues to make rattling on/off med frequency noise as if the servo motor continue to drive this lowering of slots and did not get a confirmed lock-on so as to be turn off (That was just my guess) That noise, allow me to explain, sounds like a pulsating rubbing noise of pushing stop, pushing stop and it was loud enough to be heard across the whole cabin. This noise continues to persist, and I was sitting right on the section near the center and I can even feel minor vibrations. The noise was loud enough to block me from talking to my fellow colleagues and our plane continues the push back, continues the taxing to the runway and it became such a concern and worries among passengers that many of them, including myself, press the call buttons for an answer. While taxing, the FO came out of the cockpit and took a look at the wing and then went in. The taxing lasted 15 minutes to the runway and it was only at that time, right before we apply the take off thrust, then the noise abated, almost instantly.

I confer to the inflight manager and her answer, after consulting with the Captain was, it is normal!
I am not sure why it is normal, given I have never experienced this disturbing and yet hairing experience on my past 25 years of flying!

Can someone shed some lights on what really happened? The fact that the pilot choose not to make an announcement to address such concern, to me, was not an appropriate one and I have serious reservation on flying on Dragon Air again in future.

What you heard is known as the PTU, called a power transfer unit. Some people think it also sounds like a barking dog. The initial sounds you would have heard would be the the PTU operating as the hydraulic pressure increased in the engine that was starting.

Slats and flaps are only extended after both engines have started.

Many threads on the PTU on a.net, many people have posted the sound of the PTU on YouTube.

Like this right? http://www.youtube.com/watch?v=nX1GcxbXAiM. That dog is loud! 

Here's what the unit actually looks like: http://www.youtube.com/watch?v=9ffJgk1-tCA

[Edited 2015-02-01 20:12:29]

I've heard this noise on most A32X flights, if I am sitting over the wing, or forward of the wing. It's perfectly normal.

Some other strange noises in aircraft I've noticed:

- B737NG has a weird whistling sound once the engines start (AC ducts I think?)
- Bae146/Avro - when the flaps are retracted and extended, they make this very weird sound.

Quoting zeke (Reply 1): What you heard is known as the PTU, called a power transfer unit. Some people think it also sounds like a barking dog. The initial sounds you would have heard would be the the PTU operating as the hydraulic pressure increased in the engine that was starting.

Dear Zeke, I understand this but to have this noise continous for over 15 minutes of taxi is an experience I have never encountered, can you explain why? or probable reason? Yes indeed, flaps and slots are released once engines had started.

Quoting Starlionblue (Reply 2): Like this right? http://www.youtube.com/watch?v=nX1GcxbXAiM. That dog is loud! Here's what the unit actually looks like: http://www.youtube.com/watch?v=9ffJg...1-tCA

Dear Starlionblue, yes, that is the noise. I heard this everytime when the plane is being pushed back or is releasing the slots or flaps. What is troubling is the noise continues for well over the entire duration of our taxing. I counted, it must have been over 15 minutes (well, we kind of taxi for that long as well) That continous noise for that extended period of time, despite sitting right in front of the wing, seeing the slots had been locked in place, was very much disturbing and worrying to me at that time.

Like I said, I heard such noise all the time but never for that extended period of time, let alone, we are taxing towards the runway for take off. Is that normal? Why the sound can go on unabated? Is that due to the cold freezing weather - it was 0 degree C outside.

Quoting celestar (Reply 4): slots

sl A ts ... slats

Sounds like you had an APU inop and they single engine taxiied after starting the right engine only.

Every time the parking brake is released, the PTU runs until the second engine is started.

Quoting celestar (Reply 4): Why the sound can go on unabated?

While the PTU sounds as if a piece of machinery has seized up, this is just its normal operating noise. My guess is that the pilots would worry if they can't hear it.

Quoting zeke (Reply 1): Slats and flaps are only extended after both engines have started.

On Narrow body aircraft it is becoming more common to taxy out on one engine if there is a queue for take off.
We still extend flaps to T/O before taxy (unless deiced, and it is snowing). So the PTU will bark on taxy out as well as taxy in.

Quoting Starlionblue (Reply 7): My guess is that the pilots would worry if they can't hear it.

Luckily the PTU is so far away from the cockpit we can't hear it  

Unluckily we can't hear it either, if the PTU is running when it should not. According our procedures (and I assume we are in line with standard Airbus procedures in this point) we pressurize the yellow HYD system via the Y ELEC PUMP for one engine taxi, thus avoiding the PTU to kick in whenever there is a drop in the yellow system pressure.
So if one forgets to start the ELEC PUMP, the PTU will run during the whole taxi again and again. On the other hand if one forgets to stop the ELEC PUMP after ENG shutdown, the PTU will run at the gate, as the yellow system will then continuosly pressurize the green system.

Quoting Tristarsteve (Reply 8): On Narrow body aircraft it is becoming more common to taxy out on one engine if there is a queue for take off. We still extend flaps to T/O before taxy (unless deiced, and it is snowing). So the PTU will bark on taxy out as well as taxy in.

Not the current KA procedure, I assume this was MEL related like the automatic start feature was inop so the PTU was running all the time.

Thanks all.
From reading all the feedback, am I right to gather that the PTU was left 'barking' due to possibility single engine taxing? or some sort of auto start feature failed (as stated by Zeke, but I don't quite understand)
I don't think it was due to single engine taxing because right at the end of the runway where we begin to proceed with take off thrust, that barking noise disappear and almost instantly, we line up and apply take off thrust. I did not hear the second engine spooling up or maybe, it started spooling up as we are near the take off point but the noise was covered by the 'barking' - what a wild dog this is?

Quoting celestar (Reply 11):

The PTU normally starts automatically, in that it turns on when there is a difference in hydraulic pressure between the green and yellow systems. It is permissible to fly with the automatic start inoperative, in that case they have the PTU on all the time. It is just one of many explanations.

I was wondering why I could hear a PTU in the last row of a Delta 777. It turned out to be a dog loaded in the cargo hold keepings us all awake while we waited 1½ hour for deicing in AMS.

/Lars

Quoting celestar (Reply 4): What is troubling is the noise continues for well over the entire duration of our taxing.

Same here with A319, but not as long.

http://www.youtube.com/watch?feature...detailpage&v=MTIeJUcQoDs#t=185

[Edited 2015-02-02 22:40:33]

[Edited 2015-02-02 22:42:24]

We're doing single engine taxi at United as well. It does take some getting used to and after a while the cabin crews are used to it. The Saber flight plan and the Thales MCDU has a fuel predict function that requires both engines to be started to display fuel used, CG and GWT. and this has taken he pilots a while to sort out for the single engine taxi as well. Now? It seems to be old hat to themand it is saving money on long taxi distances like at DIA where it's rumored to be half way to Nebraska to get to the runway

Dear all,
I have been reading this left and right and still confused.
This PTU - Power Transfer Unit - what exactly is its function?
Does this noise happen if the aircraft is doing a single engine run on taxing?

Quoting zeke (Reply 12): The PTU normally starts automatically, in that it turns on when there is a difference in hydraulic pressure between the green and yellow systems. It is permissible to fly with the automatic start inoperative, in that case they have the PTU on all the time. It is just one of many explanations. Dear Zeke, what do you mean by from switching from gren and yellow systems? Appreciate your answer Any other situation where we will hear this noise during taxing? Aside from single engine taxing? Is this something Boeing or other make also exhibit? Will an Airbus A330 exhibit such noise under similiar condition. Sorry guys, I am really confused..... We are addicted to our thoughts. We cannot change anything

Quoting celestar (Reply 16): I have been reading this left and right and still confused. This PTU - Power Transfer Unit - what exactly is its function? Does this noise happen if the aircraft is doing a single engine run on taxing?

As I understand it: The A320 has two main hydraulic systems, and each one is powered by a pump in one engine. So if one of the engines is off, that system's pump is also off. Thus the system would be unpowered, but the PTU kicks in and uses power from the system that is on, transferring to the other system. That way both systems can be powered with only one engine on. The PTU is reversible, so it can work both ways.



[Edited 2015-02-03 20:28:26]

[Edited 2015-02-03 20:47:13]

The A320 series has three hydraulic circuits, two have hydraulic pressure generated by engine driven pumps (green and yellow) and the blue system is powered by a electric pump in the fuselage, the circuits are not linked so a fluid leak from one system does not drain the other.

To transfer hydraulic pressure from one engine driven hydraulic circuit to the other, there is a hydraulic clutch installed which will pressurise the fluid on the other side of the clutch, this is what is referred to as the power transfer unit (PTU). It works similar to the way a transformer would work in an electric circuit, power is transferred however each side is also isolated.

During single engine operations, the PTU automatically operates to power the other side to meet domaines of brakes, steering, flight controls, setting flaps etc, if this automatic feature is not working, it is manually on all the time.

Dear Zeke, thanks again for the detailed explanation, I appreciated.
One last thing. While we were turning around right before lining up the runway for take off. The PTU noise just disappeared, appears to be someone just turned off the switch instantly. Am I right to say that, somehow during the taxing, the pilot must have switched on the second engine and then manually turn the PTU off because it has sufficient hydrallic pressue generated? Thanks in advance.

I do not have any specific information on the aircraft that day, all that suggests to me is the hydraulic demands in each system were similar.

Quoting celestar (Reply 19): Am I right to say that, somehow during the taxing, the pilot must have switched on the second engine and then manually turn the PTU off because it has sufficient hydrallic pressue generated?

As the PTU is an automatic function the pilots don't have to switch it off. As soon as the second engine is running, both (the green and the yellow) hydraulic systems are pressurized. At that moment the PTU stops working automatically.
As mentioned in reply 9 the kicking in of the PTU during one engine taxi can be avoided by pressurizing the yellow system via the electric pump (AC PUMP in the layout provided by starlionblue). By the way, the elec. pump is sufficient to provide full system pressure of 3000psi on ground, however it can not provide enough flow of hydraulic fluid to operate the flight controls in flight.

Big thank you to Glen and Zeke!
People like you makes this forum so much more rewarding and fun!

http://www.youtube.com/watch?v=IFYTGdlJPn8

Heres one where the PTU was still running during the first few seconds of climb!

The source of the noise is the vibrations coming from the electric pump in the PTU. It is said that, in the newer A320 neo, this noise is considerably reduced by adding two dampers near the engines and at the wing root.

zeke wrote:

The A320 series has three hydraulic circuits, two have hydraulic pressure generated by engine driven pumps (green and yellow) and the blue system is powered by a electric pump in the fuselage, the circuits are not linked so a fluid leak from one system does not drain the other.

To transfer hydraulic pressure from one engine driven hydraulic circuit to the other, there is a hydraulic clutch installed which will pressurise the fluid on the other side of the clutch, this is what is referred to as the power transfer unit (PTU). It works similar to the way a transformer would work in an electric circuit, power is transferred however each side is also isolated.

During single engine operations, the PTU automatically operates to power the other side to meet domaines of brakes, steering, flight controls, setting flaps etc, if this automatic feature is not working, it is manually on all the time.

It is not, primarily, a clutch - although maybe there is a clutch in it.
And the transformer analogy is not the best electrical analogy.
A better electrical system analogy would be a motor-generator set.
Now - I don't know the specifics of an A320 PTU - but I do know about PTU's
A PTU is made up of a hydraulic motor tied to a hydraulic pump.
- The operational side - the hydraulic system with pressure - runs a hydraulic motor.
- That motor turns a shaft which turns a hydraulic pump.
- That hydraulic pump pressurizes the non-operational side - the hydraulic system that does not have pressure.
One way to design such a system would be to have the hydraulic motor running all the time and a clutch between it and the pump. The motor would freewheel until pressure is needed, then the clutch would engage to turn the pump. I've never see one like that, but I've never seen aircraft ones. I'd worry about the continuous wear and tear on the motor....

Most PTU's I've seen only work one way - the motor side can only be driven by hydraulic and the pump side can only pump hydraulic.
But, I think you could design one to work both ways - and it seems that is what the airbus one is. It can transfer power either way.

PRNJ wrote:

The source of the noise is the vibrations coming from the electric pump in the PTU. It is said that, in the newer A320 neo, this noise is considerably reduced by adding two dampers near the engines and at the wing root.

There is no electric pump in the PTU. That would make no sense - you would have to have a hydraulic motor spin a generator to generate electricity to run an electric pump to run a hydraulic pump. Much more efficient to just have the hydraulic motor run the hydraulic pump.

The noise is the pump/motor set operating.