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After watching many documentaries about air plane crashes, it seems that aviation industry learned many of its lessons from disasters. For example, from the 2 mid-air explosion of de Havilland DH 106 Comet in 1950s they learned a lot about metal fatigue that can cause Explosive decompression. So, I'm curious to know how did the industry come to know about the drastic difference in air pressure that occurs in high altitude? Was the knowledge already there before an airplane was even invented?

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    $\begingroup$ It wasn't the fact that it was pressurised. It was the fact that the windows were square which is a bad design (we know now thanks to the extensive investigation) which concentrates the fatigue forces of repeated pressurisation/de-pressurisation cycles into the corners which eventually cracked and caused catastrophic fuselage failure. $\endgroup$ – Simon Aug 3 '15 at 13:05
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    $\begingroup$ @Simon Well I did say "metal fatigue that can cause Explosive decompression". The metal fatigue itself happens because the pressurized air continuously pushes the metal structure of the plane. The air is pressurized in the first place because air is thinner in high altitude. If the air was not thin, the accident wouldn't occur at all. $\endgroup$ – the_naive Aug 3 '15 at 13:31
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    $\begingroup$ Errr.... AFAIK the change in air pressure is gradual, not "drastic". The difference could perhaps be called "drastic", but not the change. $\endgroup$ – DevSolar Aug 3 '15 at 13:39
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    $\begingroup$ Nobody's really answered the question of whether there were any accidents caused by pilot oxygen deprivation from high altitude flight, or even the first designs of high-flying planes took this into account. Hillary didn't summit Everest until 1953, and I thought hypoxia wasn't well understood until after high-altitude flight was possible. $\endgroup$ – Peter Cordes Aug 3 '15 at 17:56
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    $\begingroup$ @PeterCordes Because no-one has asked the question. $\endgroup$ – Simon Aug 3 '15 at 18:46
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The entire reason why the Comet broke up in midair is because they pressurized the plane in the first place.

It was known for a long time that air at high altitude was thinner. This can be experimentally verified by taking a barometer up to a mountain. This was first done in 1648 (and subject to hot debate regarding the nature of air) more than 250 years before the Wright brothers first flew.

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    $\begingroup$ Fun related fact: most smartphones these days come with a barometer, which is sensitive enough to notice the difference between air pressure at your head and your feet (no need to climb up a mountain these days). $\endgroup$ – Sanchises Aug 4 '15 at 11:48
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    $\begingroup$ @sanchises Interesting. I had forgotten about those being in phones. In a search just now I saw a couple of articles from last year saying that the iPhone was going to use the barometer to tell you your altitude... I'm curious how that's working out for them. :) Sounds like it would work great if you happen to have a standard atmosphere at your current location... and not so great otherwise. "Hmm... It's hot today... why did our house just move up 150 feet?" $\endgroup$ – reirab Aug 4 '15 at 14:09
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    $\begingroup$ @reirab I actually have an Android app (Sensitive Altimeter) that reads METAR data of an airport of choice and displays your altitude relative to that (with an aviation-style indicator if you like, although I guess "mom called" is not a good reason for entering restricted airspace). $\endgroup$ – Sanchises Aug 4 '15 at 19:00
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Aside from mountaineering investigations, the service ceiling of WWII propeller-driven aircraft like the Spitfire was pushed up from about 34,000 feet to 45,000 feet, so the general effect of reduced air pressure on aircraft and engine performance was already known. Of course both civil and military aircraft were unpressurised before the start of jet-powered civil aviation (though there were some experiments with full-body pressurised suits, to avoid damaging the aircrew more than the plane at those altitudes).

I think the significant novelty was discovering the effect of cyclic stresses (a.k.a. "fatigue") in a new situation, namely of the order of 100 to 1000 loading cycles to failure. Before that, the most common design consideration was one loading cycle, where over-stressing caused immediate failure. With a suitably large safety factor and fortuitous choice of materials, that design criterion would also give an "infinite life" for high-cycle fatigue situations like rotating machinery (e.g. piston engines). New materials, plus lower safety factors driven by the need for minimum weight, discovered a new "cliff edge" for designers to "fall off" - and the history of engineering contains many examples of cliff edges being discovered by falling off them.

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    $\begingroup$ Most German high performance military planes of WWII had a pressurized cockpit. For example, all Me-109s with an even number behind the subtype letter had one (e.g. 109 G-6, 109 K-4). $\endgroup$ – Peter Kämpf Aug 3 '15 at 14:50
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    $\begingroup$ Not to mention the Lockheed Constellation, the Boeing B-29, or the Boeing 307. The latter was a pressurized passenger plane in service (in limited numbers) prior to WWII. $\endgroup$ – Eric Hauenstein Aug 3 '15 at 19:07
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The range of air pressure from sea level to the maximum ceiling of modern aircraft was known even before the airplane was invented. The difference between the troposphere and stratosphere was discovered by balloon measurements by Teisserenc de Bort in 1902. Most passenger airlines fly in lower levels of the stratosphere. No planes can fly above the stratosphere.

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