"Sawing noise" inside Airbus cabin

[gatman]

I've flown on a 318,319 and 320 and each time when taxiing there is a "sawing" sound mid cabin. It goes on for a minute or so and then stops. Yesterday it started again on final approach.
Never heard it on other aircraft types but was curious what it could be.

[FE Hoppy]

Hydraulic PTU

[Shaman]

Power transfer unit - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Power_transfer_unit)

[procede]

Its the hydraulic system transfer power unit, transferring power between hydraulic systems without transferring fluids.
Moe info here: http://renewedpilot.com/2012/01/25/whats-the-loud-barking-noise-on-the-airbus-a320/

[gatman]

Thanks everyone, I'm booked on a 320 to Heathrow again tomorrow. I'll be able to impress the wife with my newly acquired knowledge:)

[underfire]

or scare the :mad: out of her!

[captplaystation]

The worst seems to be after parking / engine shutdown. . . of course at that stage it is no longer "worrying", but sounds grotesque ( to anyone with any degree of mechanical sympathy ) . . .something (mechanical) sounds in extreme pain. :}

[compressor stall]

Also known as the barking Dog.

[DADDY-OH!]

Does it coincide with the opening of the cargo doors after you've arrived on stand?

[underfire]

compressor,

The 320 is known as the howler monkey, on final when you go to config 1..havent heard it called the barking dog...

recently on a flight, that sound was a bit loud on DEP, I was thinking WTF is that noise...the older woman next to me saw me looking around ..she said everything is okay, dont worry, the pilot has this under control.

after landing, when walking off, the FO recognized me and told the crew to block the flightdeck! Dont let that guy in!
I told the crew, you let that guy drive, and you worry about me?!?!?!

The older woman was just behind me, and the look on her face was priceless!

[compressor stall]

I'm sure it has many names.

My MIL asked me about the barking dog noise the other day when she came to visit. And she knows nothing about flying!


Interesting how the A319 it's not heard in flight (well I haven;t anyway) but it is on the A320. I didn't think the hydraulics were any that different?

[dream747]

I do seem to notice the sound only occurs on the older A320s. Don't seem to hear any of it during engine start or 1 engine shutdown during single engine taxi on the newer MSNs.

[compressor stall]

That is a possible explanation, given the age of the various 319s and 320s I occasionally find myself in the back of.

[Uplinker]

I think the barking dog/sawing noise of the PTU probably gets worse as the hydraulic accumulators get worn, leading to more work being required from the PTU to keep the HYD pressures topped up.

[ruddman]

Sitting in the cabin of an A320 is like sitting at a building site. All kinds of whining, whirring, crunching, sawing noises can be heard.


My dads '79 Acco made less noise then an A320. :p

[compressor stall]

Maybe so on the ground, but it's much quieter in the cruise than it's main opposition.

[TSIO540]

The PTU is scheduled to operate after the first engine start until all hydraulics are pressurized (mid way through the second engine start) - approx 1 minute.

During this time it will cycle on (until pressure = 3000psi) and then off until pressure drops to about 2500psi then back on again.

It is a small unit, not much bigger than a football with high speed rotating parts that we can thank for the noise.

During single engine taxi, it is our procedure to shut down engine 2 and run the electric yellow hydraulic pump (in lieu of the number 2 engine yellow mechanical pump). This keeps the yellow system pressurized and stops the PTU from triggering.

[flyboyike]

Ah yes, the question everyone who's ever commuted on an Airbus while in uniform has gotten many times. I've used answers like "no worries, it's just a French midget with a buzzsaw" and "it's just a beaver chewing on some pipes".

[ruddman]

compressor stall :

Maybe so on the ground, but it's much quieter in the cruise than it's main opposition.


Very true. Flew only a few weeks ago on a 737. Few day after that on the A320. Noticeably quieter.


Which made all the screaming babies that were on the flight stand out more so.:{

[onetrack]

It is a small unit, not much bigger than a football with high speed rotating parts that we can thank for the noise.

With regard to hydraulics, the largest noise component comes from turbulence in the piping, and in particular, in elbow fittings.
When a large-radius elbow with a relatively smooth bore is used in piping, fluid turbulence is minimised.
However, modern manufacturing methods produces 90° elbow designs that are easier to produce on factory production lines.

These elbows are made from a solid block of L-shaped steel - the outside threads are rolled into the metal, and the fluid passageway is rapidly drilled from two directions, at 90° to each other.

Where the drilling ends, right at the inside corner of the elbow, there is a mismatch in internal surfaces, as regards smooth finish.
This mismatched rough internal passageway surface produces much turbulence in fluids flowing at high speeds and high pressures through the elbow and piping.
Thus you have loud noises being produced, as that turbulence is being created.

The engineers probably didn't even consider the effect on anxious listeners in the back, as they would have disregarded any of this noise as being of no consequence.
The design engineers are more focussed on keeping engine noise levels down, particularly at cruise settings.

[Uplinker]

As an aside; When doing DIY plumbing at home, I try to avoid using any 90 degree pipe bends to keep the noise of water in the pipes down. I use a pipe bender to produce gentle radii turns, (and I carefully de-burr and smooth the pipe ends too before joining them), and the systems I have put in so far have been very quiet.

[Ozlander1]

Thanks everyone, I'm booked on a 320 to Heathrow again tomorrow. I'll be able to impress the wife with my newly acquired knowledge:)

As if she'll give a flip. :8