The reason modern jets have rounded windows

[slo town]

Is the outflow valve closed, or open?

The outflow valve on a widebody is BIG.

It will leak slowly - very, very slowly - if the valve is closed.

So slowly in fact, that you can blow up the airplane by adding interior pressure. It's not "gushing" out through door seals, windows, or anything (and no, I don't think rivets leak).

https://theaviationgeekclub.com/time-kc-135-stratotanker-aircraft-exploded-failed-pressure-test/

If course the outflow valve would be closed.

It just seems to me that something as large as an airliner fuselage - it would be impossible to be 100% airtight. It sounds like they're "only 99.9%" (or whatever) hermetically sealed. How could it be otherwise?

Scott

 

[mk378]

The intentional outflow is much larger than any leaks would be. Since people are breathing the air inside it is necessary to exchange it during the flight to remove CO2 and bring in oxygen.

A leak big enough to affect the system would be apparent in other ways than a pressure test. Something would be clearly cracked or broken on a visual inspection.

 

[slo town]

The intentional outflow is much larger than any leaks would be.

Yes, of course.

...the airplane is pressurized by air from the engines (compressors in the case of a 787), and the air is exchanged, and excess air is dumped overboard via a “outflow“ valve.

...The outgoing air is offset by the continuous flow of fresh air from the engines or compressors.

So as you say, any "slow leaks" would be more than compensated by the compressor's input volume. Makes perfect sense.

Of course I understand the need to introduce fresh air into the cabin during flight. My question is about whether or not a fuselage is truly 100% airtight when it's sealed up and empty of passengers. My question is kind of a "philosophical" one.

Scott

 

[Astro14]

If course the outflow valve would be closed.

It just seems to me that something as large as an airliner fuselage - it would be impossible to be 100% airtight. It sounds like they're "only 99.9%" (or whatever) hermetically sealed. How could it be otherwise?

Scott

No, they're not hermetically sealed, but even the doors seal under pressure - if they don't, you get a loud squeal that lasts the entire flight, and there are places where air can leak out, but they're very, very small amounts of air even under max PSI.

Avionics cooling can dump air overboard (depending on design and if differential pressure is required to move air in the event of a fan failure) and that, too, is small compared with the outflow valve.

On a 777, for example, each outflow valve is 90 square inches. So, 180 square inches total outflow on a widebody. 180 square inches at 9 PSI is moving a LOT of air.

Every other air flow source is tiny, really, tiny, by comparison.

The airplane will hold pressure for several minutes if the valve is closed and the bleeds (inflow) are off. Not exactly leaking like a sieve. It's pretty tight.

 

[slo town]

The airplane will hold pressure for several minutes if the valve is closed and the bleeds (inflow) are off. Not exactly leaking like a sieve. It's pretty tight.

That's what I presumed and is the answer I was looking for.

Scott

 

[oilboy123]

Is the outflow valve closed, or open?

The outflow valve on a widebody is BIG.

It will leak slowly - very, very slowly - if the valve is closed.

So slowly in fact, that you can blow up the airplane by adding interior pressure. It's not "gushing" out through door seals, windows, or anything (and no, I don't think rivets leak).

https://theaviationgeekclub.com/time-kc-135-stratotanker-aircraft-exploded-failed-pressure-test/

Loved the "Always done it that way" I wonder how his hearing was afterwards?

 

[MolaKule]

Here is what I believe to have been a potential flow control valve failure in the ECS of a Lear Jet:

https://en.wikipedia.org/wiki/1999_South_Dakota_Learjet_crash