A320 Noise Due to Air Leak?

Old Feb 24, 2019, 12:01 pm
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Question A320 Noise Due to Air Leak?

I was on an A320 the other day. About 1 hour after takeoff, a loud humming sound started. It was so loud, I needed to dig up an earplug.

The FAs came. One first said she did not hear anything. Another said "these Airbus make a lot of sounds". When a few of us objected, the contacted the pilots. The purser came back said it was probably an air leak.

The noise continued for about 1.5 hours. It started when we had some turbulence and probably changed altitude. It stopped as we were descending. So, maybe it was altitude related. It was loud in the middle section of the plane (by the exit rows) and fine a few rows back.

My questions:

1 - what was it?
2 - if it was an air leak, was that OK to ignore it? We continued and the next flight also seemed to have departed without a MX

Thanks.
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Old Feb 24, 2019, 12:13 pm
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So weird - I was going to post this too but about the 737-900s that do the SFO/LAX/SNA routes. Sounds like a seriously loud hissing noise coming from the LAV like the pressure valve that pulls the air in to flush the toilet was stuck open.

Last edited by Rumples; Feb 24, 2019 at 4:37 pm
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Old Feb 24, 2019, 1:39 pm
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Sounds like a small leak in the seal around a door. Just a small nick or foreign object can let enough air through the cause vibration and a very loud sound. This is like letting air out of a balloon in such a way that it makes a buzzing sound.

The leak itself is very small and is not a problem itself. The only problem is when the noise is so loud that it becomes an issue.

The fix can only be applied after landing and that is to clean or repair the seal.
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Old Feb 24, 2019, 2:34 pm
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Before I ask this question, please keep in mind that I'm genuinely curious and not trying to be a fear monger. Here goes:

Even though this is probably cause by a tiny, tiny leak, could this lead to a catastrophic decompression ala Aloha 243?
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Old Feb 24, 2019, 2:42 pm
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What @LarryJ said -- when OP said around the "middle of the aircraft" I was thinking a poor exit window seal.

Air pressure wants to equalize so when you have a (relatively) highly pressurized cylinder at an altitude with a much lower relative air pressure the cabin air will naturally seek equilibrium and the greater the difference the louder the noise -- especially if you have a seal resonating in addition to just having air moving -- you're basically sitting in a poorly tuned oboe.

As you reach a pressure equilibrium (either by descending to/below ~10k feet or by depressurizing the cabin [not ideal and remember to secure your masks before assisting those around you]) the volume of air moving will decrease and the noise will follow.

The door seals on the ERJ 145 seem particularly prone to getting unhappy with age -- surely all of the nicks, dirt and dust, but I've been next to some whistling 737 and Airbus exits in my time.
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Old Feb 24, 2019, 2:49 pm
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Thanks. The noise was pretty bad. Even with my earplugs, it was not comfortable. How do they determine if the noise is too loud and divert the plane?

The other thing is really you wonder if they took it seriously - was it really an air leak, did they really write it up, etc - since the FAs were trying to write off the problem initially.

So, you spend the flight wondering if this was something worse that could lead to something bad...

Last edited by username; Feb 24, 2019 at 2:56 pm
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Old Feb 24, 2019, 3:01 pm
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Originally Posted by username
Thanks. The noise was pretty bad. Even with my earplugs, it was not comfortable. How do they determine if the noise is too loud and divert the plane?

The other thing is really you wonder if they took it seriously - did they really write it up, etc - since the FAs were trying to write off the problem initially.
I can't answer the first question (I imagine it would take a significant number of passengers or crew complaining or an actual problem maintaining pressurization), but for the latter -- the maybe a dozen times I've explicitly asked the FAs to write something up in the past year (the 737 F seat tray tables refusing completely stow being my #1 offender, reading lights and seat recline locks being close behind) they've been quite pleasant about it, and apparently it can now be done from their iPhone app (one of my trips one of the FAs was very excited about showing her colleague "how easy it is now")
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Old Feb 24, 2019, 3:13 pm
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Originally Posted by n198ua
Before I ask this question, please keep in mind that I'm genuinely curious and not trying to be a fear monger. Here goes:

Even though this is probably cause by a tiny, tiny leak, could this lead to a catastrophic decompression ala Aloha 243?
Anything can happen, but there is bunch of 30 years old A320 chugging through cycles like:

Air Canada A320 C-FDQV - 87,011 Flight hours
Lufthansa A320 D-AIPD - 53,997 Flight cycles

I'm not aware of any decompression similar to Aloha. It's mostly localized leaks or windows gone. United Airbuses are 20 years old at most.

Edit: I just checked and Aloha flight had 90k flight cycles and wonky maintennance. That's massive. I don't think you can go that high with US3. Westjet or similar, maybe.

Last edited by danielSuper; Feb 24, 2019 at 3:30 pm
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Old Feb 24, 2019, 3:22 pm
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Originally Posted by username
Thanks. The noise was pretty bad. Even with my earplugs, it was not comfortable. How do they determine if the noise is too loud and divert the plane?
When it annoys the pilots.
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Old Feb 24, 2019, 3:23 pm
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The way cabin pressure works is by using air from the engines to pressurize the cabin and essentially over pressurize it. Then there are 2 (I think 2) "Over pressure valves" that essentially bleed off excess pressure in the cabin to maintain the relative air pressure. As long as any leak is less then the ability of the engines to provide make up for, then you won't depressurize.

As far as a rapid depressurization, it would have to take a pretty big hit to do that, I mean it happens, but just a hiss from a dirty seal isn't going to cause problems.
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Old Feb 24, 2019, 3:24 pm
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Are you sure it was an air leak? I was on a 764 today that had loud cabin air conditioning.
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Old Feb 24, 2019, 3:30 pm
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Originally Posted by n198ua
Even though this is probably cause by a tiny, tiny leak, could this lead to a catastrophic decompression ala Aloha 243?
No.

You could completely remove the seal around a door and the airplane could likely still maintain pressurization. Think of the airplane as one of those bouncy-castles that you see at fairs or birthday parties. You pressurize it by pumping air in faster than it is leaking out. The airplane works the same way. The cabin altitude is controlled by modulating a rather large outflow valve near the tail of the airplane. As the amount of leaking throughout the airplane increases the outflow valve moves toward "closed" to compensate.

Originally Posted by lincolnjkc
it can now be done from their iPhone app (one of my trips one of the FAs was very excited about showing her colleague "how easy it is now")
They have a list of predetermined cabin discrepancies that they can send in from roughly 15 minutes after takeoff until about 30 minutes prior to landing. That streamlines the old process which was for them to pass the information up to use and have us entered it through the ACARS. When an F/A creates one of these write ups we receive notification via ACARS and a summary report at the end of the flight.
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Old Feb 24, 2019, 3:54 pm
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Originally Posted by lincolnjkc
The door seals on the ERJ 145 seem particularly prone to getting unhappy with age -- surely all of the nicks, dirt and dust, but I've been next to some whistling 737 and Airbus exits in my time.
The way the ERJ145 doors squeak & creak as the plane reaches altitude was terrifying to me the first time I was on one. It was so loud, and so I just kept looking at the FA & started to relax when they were completely unconcerned & going about their business.
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Old Feb 24, 2019, 8:09 pm
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Originally Posted by LarryJ
You could completely remove the seal around a door and the airplane could likely still maintain pressurization. Think of the airplane as one of those bouncy-castles that you see at fairs or birthday parties. You pressurize it by pumping air in faster than it is leaking out. The airplane works the same way. The cabin altitude is controlled by modulating a rather large outflow valve near the tail of the airplane. As the amount of leaking throughout the airplane increases the outflow valve moves toward "closed" to compensate.
Is it stress on the airframe that limits the cabin altitude above a certain level (and why the 787s can go lower)?
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Old Feb 24, 2019, 8:12 pm
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Originally Posted by findark
Is it stress on the airframe that limits the cabin altitude above a certain level (and why the 787s can go lower)?
Yes. The cabin needs to be able to support the pressurization differential within certification limits.
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