High cabin altitude incidents?

[HappyPass]

Hi all,
has an incident in which an unusually high cabin altitude pressure was selected ever happened?
Is it even possible to have a cabin pressure higher than sea level?
(I know the usual cabin alt is around 8000 ft)
Thanks in advance.

Edit: typo

[DaveReidUK]

has an incident in which an unusually high cabin altitude was selected ever happened?
Is it even possible to have a cabin pressure higher than sea level?

Which do you mean: high cabin pressure or high cabin altitude ?

[HappyPass]

Which do you mean: high cabin pressure or high cabin altitude ?

Sorry, high pressure, negative altitude.
Like, 4 bar cabin pressure, enough to cause damage to the inner ear.

[exeng]

Yes it is possible to have a higher cabin pressure than at sea level (i.e. a cabin altitude that is below sea level)

It certainly used to be carried out by maintenance on purpose for leak checking the pressure hull on the ground.

There was an accident that occurred to I think a USAF aircraft that was overpressurised on the ground - it resulted in severe rupturing of the pressure hull at the rear of the aircraft. I'll try and search for relevant pics and post them.


Kind regards
Exeng

[evansb]

Lockheed Super 'G' Constellation c/n 4648 L1049G N7121C to TWA. Connie was damaged beyond repair during a pressurization leak test in a hangar at Idlewild airport, New York at 11.45 hrs (local time) on 25 June 1959. Severe damage to front fuselage before wing. TTAF: 10,833 hrs.

[HappyPass]

I see... thanks all!

[pilotmike]

HappyPass:Sorry, high pressure, negative altitude.
Like, 4 bar cabin pressure, enough to cause damage to the inner ear.
Wow! That would cause more than some damage to the ear! It would 'blow the bloody doors off!'

Max. diff pressure is usually circa. 7psi - around 500mB - about 0.5 Bar, in very round numbers.

[tonytales]

Regarding over-pressurizing L-1049 Constellation. At LASI New York, then KIDL and later KJFK) we came close to experiencing what happened to the TWA aircraft. It was a military WV-2 Warning Star radar Constellation. On the post-check runup we did a pressurization test. Aircraft had a max differential of 11.7 "/Hg with a relief valve setting of 12.4 "/Hg.
Pressurization was controlled on the "260" panel aft of the F/E desk. The differential pressure gauge and the controller got outside ambient air pressure through a static port located under the sliding cockpit entrance door. It was of the "salt shaker" type, that is there were a group of small holes drilled in a disk set into the skin.
So we did the runup and then set off on pressurization using the manual "star valve" control. It is fairly slow to get up there and we found we could not get above about 5 "/Hg. The problem was quickly found. The pressure relief valve mounted on the rear pressure bulkhead was wide open preventing an increase in cabin pressure. It was adjusted using an allen wrench but we couldn't get above about 7 "/Hg.
I went up and down the cabin looking for leaks - nothing bad at all. I returned to the cockpit and reported. I was standing next to the radio rack and I heard a loud hissing sound. A small instrument type flex hose was hanging loose. The PB-10A autopilot amplifier was missing but not needed for the runup. I put my finger over the hose fitting opening and joked that maybe it was all going out here. The Lead Mechanic at the F/E desk let out a loud cry. When I capped the little line the differential pressure gauge needle snapped over to the maximum reading, way above allowable.
The loose line was a static pressure hose feeding the autopilot for the "altitude hold" function. Same static port was used as for cabin pressure indication and control. Those small "salt shaker" holes caused a back pressure leading to false reading on the diff gauge. I know what a rapid depressurization looks like, dense vapor in the cabin and also how it feels like - badly hurting ears and noses. We reported it and they came up with an absolute pressure gauge we carried up on every pressurization runup. Later, an SB changed the "salt shaker" static port to a single large hole.
I saw the TWA Connie. It split open from the right aft corner of the nose wheel wll back to the wing. Inverters were dangling from electric cables, floor collapsed, awful. I believe they encountered the situation but in a commercial airliner with lots more hours and cycles than our WV-2. Also I believe that the cabin superchargers just couldn't raise it any more for the relief valve was closed. We must have been right on the edge though. The WV-2 received a special inspection but no damage was found.

[PDR1]

Aircraft have been overpressurised when being tested (due to maintainer error/neglect), like this KC 135 (https://theaviationgeekclub.com/time-kc-135-stratotanker-aircraft-exploded-failed-pressure-test/):

https://theaviationgeekclub.com/wp-content/uploads/2017/12/KC-135-explosion.jpg

But I've only ever seen this kind of damage when the pressure came from an external source during testing. Whilst never suggesting it's completely impossible, I'd be surprised to discover any significant over-pressurisation from the internal cabin pressurisation system. You'd have to line up a heck of a lot of very dumb holes in the cheese slices for that to happen.

Like, 4 bar cabin pressure, enough to cause damage to the inner ear.

On a point of order - being in a pressurised chamber at 4bar won't necessarily damage the inner ear. Any scuba diver who goes down to the 120ft level exposes his/her ears to that sort of pressure. Sudden changes inb pressure would be a different matter, but to be honest if you're in a chamber at 150psi when someone opens the door your inner ear is probably one of your more mninor concerns...

PDR

[TCU]

Incident: Kuwait A320 at Lahore on Jan 11th 2018, cabin overpressure (http://avherald.com/h?article=4b383737&opt=0)

[tonytales]

Aircraft have been overpressurised when being tested (due to maintainer error/neglect), like this KC 135:
But I've only ever seen this kind of damage when the pressure came from an external source during testing. Whilst never suggesting it's completely impossible, I'd be surprised to discover any significant over-pressurisation from the internal cabin pressurisation system. You'd have to line up a heck of a lot of very dumb holes in the cheese slices for that to happen.
We did use an external air blower when testing a fuselage integrity after major structural alterations or repair. On the Connies there was a fitting in the nose wheel well to attach it to. Had the hell scared out of me one day when they were testing an RC-121 that had been converted to the ALRI standard. It was two bays away from where I was. The was a very loud near explosive sound and then horrendous whooshing as a whole fuselage full of compressed air gushed out. It was a cabin window that failed though, one of the round monolithic type and not one of the rectangular ones used on pax types. No one was on board during those checks.
Regarding our over-pressurization, it did take some holes in the Swiss cheese to line up. The "salt shaker" static port was used to prevent insects from getting in and blocking it. But who would have thought that a missing autopilot amplifier could cause an over-pressurization, at least indirectly? Checking the cabin pressure relief valve was part of the post-check runup and adjusting it was common. Not having an independent pressure gauge now definitely seems daft but there was no requirement for it prior to our incident.
We could not use a ground blower on a post-maintenance check test because we were testing the entire cabin pressure system including the cabin compressors, cooling turbines, etc. not proof testing the integrity of the hull. That was where the external unit was used. I remember doing a lot of Connie pressurization runs but rarely had to check DC-6/7 aircraft. After the TWA incident several structural SB's/AD's were issued adding some straps to the fuselage top and reinforcing the aft corners of the nose wheel well.

[HappyPass]

*snip*
On a point of order - being in a pressurised chamber at 4bar won't necessarily damage the inner ear. Any scuba diver who goes down to the 120ft level exposes his/her ears to that sort of pressure. Sudden changes inb pressure would be a different matter, but to be honest if you're in a chamber at 150psi when someone opens the door your inner ear is probably one of your more mninor concerns...

PDR

Yeah, we are talking about the usual timeframe for a climb/descent, so quite fast I'd say.

Thanks again :ok: