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barkerlut
6th Sep 2023, 10:37
Hi folks, as our airline is receiving more and more NEOs with the P&W GTF there is something which I can't wrap my head around. Maybe some of you are more knowledgeable than I am: We recently did ferry one of the NEOs and during taxi out for departure there was a constant high pitched humming noise coming from the back of the cabin. Certainly not normal, but system indications where all normal. We stopped and I went back to have a look where it came from. It was audible throughout the whole cabin, but became far more noticeable towards the rear. Sounded like the Y elec pump constantly running (which wasn't the case), but with a bit of a lower frequency. In the rear part of the cabin that humming noise induced a slight resonance of the cabin floor, which wasn't the case further up front. In the rear galley it was actually really loud, like prop blades resonating, but without the frequency shifting. It's also not some sort of fan blade resonance because nothing changed after increasing thrust on one and then both of the engines. It's also not the Y elec pump because I know that sound. Pretty please also don't suggest the PTU. Was thinking the gearboxes slowly starting to dismember itself, but oil was 18qt and no entries in the tech log regarding any of this. And as I said - indications where normal. What do you guys think? Maybe some of the more knowledgeable folks could hint me in the right direction. Also I noticed that even though they pay me to push buttons, all manuals keep you totally clueless on what motors / systems etc are placed below the cabin floor between wingbox and rear galley. Is there something about that in the AMM?

Cheers 🫡

PEI_3721
6th Sep 2023, 11:57
Does the noise vary with speed / altitude ?

Check cabin door seals.

Check toilet / galley sink drains.

HOVIS
6th Sep 2023, 12:38
Is it only noticeable with engine(S) running?
What was the action taken by maintenance when you reported it?

DaveReidUK
6th Sep 2023, 14:31
Am I correct in inferring that the issue only affects one airframe from your Neo fleet ?

barkerlut
6th Sep 2023, 22:30
Hey, thanks all for your input. Highly appreciated!

*Does the noise vary with speed / altitude ?
-> No, it didn't. Wind noises during flight where masking/covering it, but it remained audible and steady. That's why I was particularly interested in what kind of generator is placed under the cabin floor. Airpumps of some hydraulic reservoir or so.


*Check cabin door seals. Check toilet / galley sink drains.-> As per my uneducated guess it was not pressurisation related, more (quite surely) a mechanical induced sound. In case you've driven a manual transmission car in reverse gear, think about the sound emitted when going backwards at a high speed.


*Is it only noticeable with engine(S) running?What was the action taken by maintenance when you reported it? -> Yes. We're not sure when exactly it started, but with only GPU/APU running it was not there. Maintenance was of the opinion it's more noticeable on the NEO compared to the CEO because of the otherwise reduced noise pollution but I kind of disagree. Even if so, the tech nerd inside of me would still be interested in knowing what it is. 3 days later the ATL neither shows any rectifying actions taken nor any additional writeup by some other crew.


*Am I correct in inferring that the issue only affects one airframe from your Neo fleet ?
-> Yes and no. None of the CEOs have it. The one NEO has it, and it's persisting as to our WhatsApp group. Since it's the only NEO in the base so far I unfortunately can't elaborate further.

nr1chris
6th Sep 2023, 23:29
I would think about a ventilation or extract fan with a worn ball bearing.

HOVIS
7th Sep 2023, 00:24
If its only happening with engines running could it be a resonance thing or even bleed air?

First.officer
7th Sep 2023, 09:08
Maybe.....?

https://www.flightglobal.com/engines/a320neo-operators-affected-by-pw1100g-vibration-issue/129485.article

Manual Pitch Trim
11th Sep 2023, 19:15
It may or not be related butÖThe Pratt and Whitneys are having serious issues and the industry is keeping quiet about it.

I would suspect the engines as we have the same problem and theres not enough a vibration for engine vibration report. Once written in the TLB hard to troubleshoot

Your maintenance department will definitely have to take it up with Pratt and Whitney, as the main suspect.

Separate or not..a recall is in the midst.

https://www.reuters.com/business/aerospace-defense/airlines-brace-hit-pratt-whitneys-new-engine-problem-2023-07-26/

Longtimer
11th Sep 2023, 21:03
https://www.flightglobal.com/engines/a320neo-operators-affected-by-pw1100g-vibration-issue/129485.article
Pratt & Whitney has confirmed that a number of PW1100G engines powering Airbus A320neos have been affected by a vibration issue.

AerocatS2A
11th Sep 2023, 21:32
I’ve flown a PW neo with the vibration and it doesn’t fit the OP’s description. Vibration is transient, occurs during the climb through around FL150-FL200, briefly exceeds 5.0 to generate an ECAM, then subsides to normal levels. It’s not detectable as either a sound or by feel. Totally different to what the OP describes.

tdracer
11th Sep 2023, 22:15
Iíve flown a PW neo with the vibration and it doesnít fit the OPís description. Vibration is transient, occurs during the climb through around FL150-FL200, briefly exceeds 5.0 to generate an ECAM, then subsides to normal levels. Itís not detectable as either a sound or by feel. Totally different to what the OP describes.
That sounds suspiciously like the fan gearbox - and I seriously doubt it would be considered normal or healthy for the engine.

barkerlut
11th Sep 2023, 22:37
There's been a few interesting suggestions which might be fitting - thanks for that! I'm continuing to check the eTLB about updates, but nothing specific in the workorders so far. Ventilation or extract fan with a worn ball bearing, as nr1chris (javascript:void(0)) mentioned, and by the feel (not just the sound) of it, some worn-out or out-of-balance bearing sounds plausible. Albeit I somehow doubt an extract fan has such an inertia to create a resonance in the cabin floor.

Since I've been sitting in row 0 during that particular flight I can tell you all indications where normal. If that wouldn't have been the case it'd be drinking coffee in some hotel lobby because we returned to stand :cool:

Regarding the P&W engine vibration issue it is as AerocatS2A (javascript:void(0)) described - only transient for a few seconds at (or near) TOC, slowly creeping up over the course of about 1 minute up to the warning threshold and then comes through as amber ECAM. If you select V/S during climb to force the engines out of max climb thrust the vibration indication on the ECAM immediately jumps back to normal values. We received a CONOTAM about it - apparently it's some sort of mismatched O-seal which creates the issue.

MechEngr
12th Sep 2023, 02:18
The reason reverse gear sounds different for manual transmissions is they are usually straight tooth gears vs the helical gears for all other forward speeds. This kind of gear is what I would expect on an aircraft hydraulic gear pumps or hydraulic gear motors. The reduction drives in the engines are most likely helical as that allows lower collision forces as the teeth come into contact, lower contact stresses, and longer life. Straight tooth gears see higher contact loads but lower bending loads, but used as reverse speed they are cheaper, run at low speeds, and very limited use, so an OK application.

In pumps and motors straight teeth are submersed and cushioned in hydraulic fluid.

I can't say this is the cause of the concerning noise, but as you noted sounds from the Y elec pump, that would be the likely cause there.

Failing bearings tend to squeal or growl rather than hum.

AerocatS2A
12th Sep 2023, 07:39
That sounds suspiciously like the fan gearbox - and I seriously doubt it would be considered normal or healthy for the engine.
I donít know what the cause is. All I know is that itís a known issue and we have a tech bulletin about it along with an amendment to the QRH procedure for engine vibration.

threep
12th Sep 2023, 14:11
The symptoms described sound like there is an engine/ancillary vibration frequency at taxi speeds which is exciting some part of the airframe.
--> noise does not manifest at engine speeds in flight
--> only observed when engine is running
--> only observed on NEO

I would have thought that would have been noticed on ground/flight test by Airbus.

Do all of your NEOs display this behaviour?

Uplinker
12th Sep 2023, 14:48
Cabin air recirculation fan? Not sure where these are on the Neo, but maybe one has got damaged fan blade(s) causing the sound?

Something in the galley - have you tried switching off the galley(s)?

Aircon pack? Have you tried turning each pack off one at a time?

Door seals can be extremely noisy too and can make what sounds like a mechanical noise.

Cabin over or under pressure valve?

Could even be a faulty electrical power supply for a light fitting resonating against something.

Just ideas - obviously hard to assess without hearing the actual sound.

punkalouver
12th Sep 2023, 15:14
I flew on a 321 NEO the other day. As we taxied out, it seemed to me that there was a 'constant high pitched humming noise'. I was located at mid-cabin. It specifically reminded me of the system B hydraulic pump on the 727 although at a different pitch. I remember that 727 sound well because I heard it a lot in situations where it was left on during a long stopover where the APU was shutdown(and sometimes they were multi-hour stopovers where we would stay in the aircraft). Those 727 days were when I learned that a hydraulic pump could make such a noise and it was a surprise to me to discover that a hydraulic pump could make such an annoying sound. It should be noted that the hydraulic B-pump noise on the 727 was just as annoying outside the aircraft as in it(was so nice and quiet when it was turned off).

So here I was the other day on the taxi-out in the back of the A321 thinking that maybe this new Airbus sound was a hydraulic pump and that perhaps the final result of what Airbus had re-designed to stop that horrible hacksaw noise that happens with single-engine taxi(usually on shutdown) on the older versions. But shortly after, I heard some of that hacksaw sound for a few seconds and also heard a second engine starting while we continued taxiing. Didn't notice it much after that and certainly not in the air(perhaps covered over by other noise).

Uplinker
12th Sep 2023, 17:45
I presume you, (punkalouver), know the sound of the Airbus FBW yellow hyd pump, which runs during single engine taxi?

The 'hacksaw' noise is the hydraulic PTU transferring power between the Green and Yellow hydraulic systems, as they pressurise or depressurise when the engines are started or shut down.
The sound, also known as a 'barking dog in the hold' is worse when the hydraulic accumulators are not fully gas charged. (The PTU is also automatically tested during engine start).


PS to the OP; the cabin recirc fans can be switched off to see if they are the cause, (as can the avionics fans, but I doubt it is those).

punkalouver
12th Sep 2023, 21:30
I presume you, (punkalouver), know the sound of the Airbus FBW yellow hyd pump, which runs during single engine taxi?

The 'hacksaw' noise is the hydraulic PTU transferring power between the Green and Yellow hydraulic systems, as they pressurise or depressurise when the engines are started or shut down.
The sound, also known as a 'barking dog in the hold' is worse when the hydraulic accumulators are not fully gas charged. (The PTU is also automatically tested during engine start).

PS to the OP; the cabin recirc fans can be switched off to see if they are the cause, (as can the avionics fans, but I doubt it is those).


Thanks,

All I will claim to know for sure is that the sound I heard the other day reminded me of a hydraulic pump sound I heard on a different aircraft type, and therefore concluded that it could be a hydraulic pump on this aircraft type. But, it is only a guess.

I seem to remember reading that Airbus has made a modification at some point to try and end the hacksaw noise.

I would suggest the OP look into seeing if the noise comes from a hydraulic pump.

autoflight
25th Sep 2023, 05:29
Tech log entry for this type of event could be applicable. Typical result might be "ground checked, no fault found, Further report please". The writer of the original report will need to respond again if the problem remains. Other crews will see the continuing fault(?) but no need to write it up as the originator seems to be persistent. After a few months originator starts to think its his imagination as no-one else is reporting.
Unreported matters and those that are difficult to diagnose, can still be significant, and there is an example of loss of aircraft from non reporting. In a case that I know of, a persistent lone voice prevented an almost certain loss.