Unidentified Noise from an Aircraft at the Gate

Is it the boarding/disembarking tube being offered up to the exit door and being adjusted to fit the shape of the aircraft's hull?

Seems to happen too soon - there's generally another 5 minutes or so before anyone actually disembarks.

[BTW - Hat's off to Heathrow/BA/Terminal 5. When we came in yesterday, the baggage was on the carousel by the time we got to reclaim. We'd bussed to the long stay carpark and were driving off the airport about 25 minutes after touchdown.]

There is a lot of things going on when an airliner pulls into a gate as you can imagine. I would venture to say they are resetting flaps or opening cargo doors. Or its possible they are switching to ground power. I have not flown an A319 so I am not sure of the systems on board, but would assume it is normal sounds of gate operations. Maybe someone else with flight operation experience on an A319 would chime in.

Remember the Twilight Zone episode with William Shatner? The one with the fuzzy 'creature' that he could see but no one else? .

Nightmare at 20,000 Feet

Others could hear it too - honest!

I know exactly the noise you are talking about, and I've wondered about it too. I also came in last night on an A319, but I don't recall hearing the noise (it was late, after a 3 hour delay!), so I don't know when it happens, or if it happens every time. It's definitely onboard, and it's definitely not the flaps being wound back in. I suspect it has something to do with maintaining or sharing power between the hydraulic circuits, but now that you've aroused my interest I'd really like to know.

I agree with Fixitorelses' post, resetting of the flaps and possibly thrust reversers on turbines. If you happen to be in a seat where you can see the trailing edge of the wing, try to have a look to see if you notice any motion that corresponds to the "sawing" sound.

I once had the misfortune to sit next to a hysterical first-time flyer. She was doing OK on departure, right up to the time the landing gear came up, and made a lot of noise as it tucked in and gear doors closed. Thank goodness the flight crew moved me, so the poor sap who admitted he was doctor, could sit next to this panicking gal and reassure her that she was not "going to die" while she repeatedly insisted she indeed was going to do just that! They actually had to get the med kit out and sedate her, something the airlines prefer not to do, since there's a lot of paperwork that goes with opening up that medical kit they keep on board.

Ah, the joys of flying!

Tom D.

I've looked at whatever control surfaces I could see - they appear to be moved back to neutral positions while taxiing to the terminal. The thrust reversers/diverters (or whatever they're called) usually flip back as soon as the 'plane reaches taxiing speed before turning off the runway.

This is more irritating on Airbus than Boeing. I think it's the cargo bay doors opening/closing.

I am only guessing that this is the right answer (cargo hold devices). You can definitively rule out the flaps and leading edges since you do know that they make relatively smooth sound when retracted in flight after take-off. As someone else suggested, it could also be cargo door latches.

Pretty sure they use cargo containers (always seem to be stuck in the wrong place to see the cargo actually leaving the aircraft, but there seem to be plenty of trucks towing trains of containers, and few if any with stacks of baggage) - so restraining latches being released would certainly be happening about then.

Just can't imagine what kind of mechanism would make that particular sound.

Here is what Wikipedia mentions for "Power Transfer Unit", in complement to your correct answer :

"The PTU on the Airbus A320 family of aircraft makes a distinctive noise resembling a barking dog, and is quite audible from inside the cabin. Passengers on board A320-family aircraft are therefore very familiar with the sound of the PTU, but few know what it is."

Thanks, that's interesting. I never even thought to look in Wikipedia (having only recently told my son "Yes, engineers use Wikipedia too").

Anyway, I guess there's a few more of us that know what the noise is now. The only problem is that answers have an annoying habit of creating more questions. So I'd like to know:

Why is the PTU on the A318/319/320 so much noisier than on other aircraft?

What does it look like, and how does it function?

Hi all.

I think I found your "Gremlin" in that Airbus airliner.....

It appears to me that is most likely the Auxiliary Power Unit (APU) that you're hearing spooling down after the plane has reached the gate and the main jet engines have been shut down. It's used to power up the aircraft; primarily to "spool-up" the jet turbines. Basically, it's very small onboard jet engine that provides electrical power (by attached generator), hydraulic pressure (by attached hydraulic pumps), and air bleed to the high bypass turbofan jet engines.

Years ago, before most jets had these APU's on board, there would be wheeled APU cart with high pressure air supply hoses sitting next to the aircraft to aid in starting it. Really just a very large air compressor unit. Going way back to the 1950's and 1960's, you'd see jet engines started in another manner, like those in military fighter aircraft, would utilize engine starter cartridges much like shotgun shells to aid in spooling-up the turbines.

I hope this solves the question.......just another long time jet enthusiast + brother is a former F-16C Fighting Falcon fighter jock (Lt. Col)......

I'm not saying it's definitely not that - but the sound is absolutely nothing like a rotating component spooling down. There is no change in pitch, or the duration of what I have called the "strokes".

It's the Captain Mate. The microphone was still on and he was sighing relief, again and again, that he had made it. When you hear the bang you have survived.

Maybe step by step decompression of the cargo space?

I like the heavy breathing better, Ky.

The noise that JohnDG describes is a very distinctive grating noise, and has been described variously as a 'sawing' sound or a 'barking' noise. Following is John's description again:

"It sounds like some inept madman is desperately trying to hacksaw through one of the wings. It is in groups of 4 or 5 back-and-forth strokes of the 'saw' (each double-stroke lasts about 1 second), with short pauses between groups. It goes on for about 20 seconds, generally with a short group (1 or 2 strokes) before he gives up at the end."

It is definitely not the flaps (although there is often some modulation to the frequency, it is still a continuous noise, and the flaps are usually retracted well before reaching the stand) or anything external to the aircraft.

I have never seen containerized cargo used on the A319s I've flown on, and it doesn't sound like the noise of containers or cargo doors being latched/unlatched.

The APU is relatively quiet, and is a continuous hum, as you would expect such a piece of rotating equipment. (I would be very concerned if the APU sounded anything like the noise in question).

I'm convinced it is the PTU, it fits with the type of noise and the time/location when the noise occurs, and the function of the PTU. If you google something like 'Airbus A319 noise' or 'A319 PTU' you will find lots of references to this noise. Apparently Airbus have considered changing the PTU to a quieter unit.

I think you're onto it. I've found a number of videos of Airbus startups with the PTU sound clearly audible - e.g. here. Still not quite right though, as this has only one "stroke of the sawblade". I haven't yet found a recording of the sounds on shutdown.

That's the one!!!!! Thanks.

(3:45 into the clip).

Well thanks for raising the question, John. That's one that has always puzzled me when I heard it, but I never got around to investigating it. I only ever got as far as ruling out lephrechauns trying to escape from the cargo bay, since I never heard it on Ryanair flights (although I guess they use mostly Boeings).

I think I know the noise you are talking about.. I fly quite a bit on A series planes, is it like the sound of someone trying to slide a dry rubber ring on or off a shaft that is tight? Rubber on metal type of sound that is intermittant? or maybe a squeeking rubber suspension unit? Sounds like whatever it is, it is a rubber item being pushed along a slight taper to a end stop???

I would love to know what this is too.

I've always guessed it's the cargo doors. I'm sure they fit snug because it's an airtight seal. I would agree that noise is concerning because it's sounds like something is binding, plus it's not consistent from plane to plane.

Also, aircraft being built from a thin aluminum structure carry a lot of noise resonation. Noise damping materials are heavy so I'd guess they're not used.

Once flying over the Pacific in a DC-10, I was woken up by several extremely loud cracking noises. I figured there is no point in worrying, so I went back to sleep.

It sounds like "IS" some inept madman is desperately trying to hacksaw through one of the wings.

i assume you already talk to the officials about this, regarding the safety of the people.

All threads should end so politely. Nice style John.

The noise is the hectic disembarking of the passengers that flew in cargo because they were not able to pay all the hidden fees.

Damn - there was a thread on this and I missed it!

Well done Holzfeller and a better researched answer than I gave.

_{And thanks for the mention JD}