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I'm wondering if the pressurization of the cabin has a part to play in the structural integrity of the fuselage, the same way an unopened soda bottle is stiffer compared to one that's open.

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    $\begingroup$ Airliners can still fly with holes in them, so even if pressurization plays a part in structural integrity, it's not a necessary part. $\endgroup$ Feb 9 at 4:08
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    $\begingroup$ @GregHewgill en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 is an even better example. $\endgroup$
    – ceejayoz
    Feb 9 at 15:33
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    $\begingroup$ DC-3s were airliners, and didn't even have the capability for cabin pressure. $\endgroup$
    – Zeiss Ikon
    Feb 9 at 16:48
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    $\begingroup$ If leaving the cabin unpressurized is an issue, the airliner just sitting on the ground is a problem. $\endgroup$
    – Jon Custer
    Feb 9 at 18:08
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    $\begingroup$ You are aware that near sea level, where the air plane experiences high loads during take-off and landing, there is no pressure difference to the surrounding air, right? That kindof answers your question, if you think about it. (The soda bottle has a much higher internal pressure than the surrounding air at all times.) $\endgroup$ Feb 10 at 15:13

5 Answers 5

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There is no aircraft limitation on the 737 that requires pressurization to fly. Realistically, even on O2 the whole time, going very high could be unpleasant. I think the Air Force limit was FL 250 for routine unpressurized flight (recalling the T-37); when we depressurized the C-130 for high altitude air drops, we had some amount of pre-breathing that we did, and I never dropped anything near, at, or above FL 250.

But all of those are policy limitations, based on the human physiology involved, not the aircraft itself.

In airline life, I've done a maintenance ferry flight at 10,000' unpressurized, which is the highest we can have the cabin without needing to put on O2 masks. There are probably ways to ferry higher than that if you really had to (crossing the Rockies, for example), but it wouldn't be preferable. Again, though, that's looking at the human element rather than the jet.

I suspect that the reason that there aren't any limitations as postulated in the OP is that the aircraft itself is sufficiently strong that the extra rigidity of pressurization doesn't add anything necessary. After all, the aircraft structure has to be able to withstand a sudden depressurization at cruise altitude, along with the ensuing emergency descent down to 14,000'. When the structure is strong enough to handle all of that, I expect that what's gained by pressurization is just icing on the cake.

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    $\begingroup$ Additionally, aircraft take off and land unpressurized, so the fuselage needs to be strong enough for that anyway. $\endgroup$
    – Bianfable
    Feb 9 at 8:37
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    $\begingroup$ Actually, they takeoff and land very slightly pressurized, a few hundred feet worth, to allow the pressure to be tapered on and off gently so that passengers don't sense the "bump" in pressure. $\endgroup$
    – John K
    Feb 9 at 14:12
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    $\begingroup$ And of course most people riding in a C-130 are at their peak - young, physically fit, with military training specifically for that sort of thing. On a 737, you've potentially got people who are children, elderly, or with any number of physical ailments that would be a problem in an unpressurized flight. $\endgroup$ Feb 9 at 14:42
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    $\begingroup$ As as aside, sometimes the cause of no pressurization can impose an altitude limitation, say, no pressurization due to two malfunctioning packs might cause an avionics cooling issue at too high an altitude, but that a secondary effect. $\endgroup$
    – nexus_2006
    Feb 9 at 23:38
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    $\begingroup$ @nexus_2006 Good point - all packs inop may well ground the aircraft. But that's a separate (tho not unrelated) situation. The ferry flight I did was due to a suspect pressurization controller; we at least had good packs, thus adequate cooling. $\endgroup$
    – Ralph J
    Feb 10 at 4:05
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Yes airliners can fly unpressurised, because pressurisation is not part of the critical load calculations.

The aeroplane fuselage is dimensioned using the critical G-loads from gusts. The largest gust impacts are at:

  • low altitude (higher air density, ground effect)
  • low airspeed (gust load is introduced more rapidly into the structure)

Both conditions are at take-off and landing, where the pressure differential is much lower than in cruise.

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Any airliner, or any other airplane for that matter, that falls apart if made to fly unpressurized, does not pass certification. The 'Soda Bottle Effect' does add to the structural rigidity of the fuselage, but this is in no way considered part of the required structural integrity of the airframe. On the contrary, if considered at all, it is to see if it jeopardizes it. Don't beat me up over what specific rule, section or paragraph says so, but I recall the use of pressurized gas as a component of structural integrity in aviation is for obvious reasons categorically prohibited. It would be widely in use if that wasn't so.

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    $\begingroup$ Nobody here will beat you up, but adding a reference would be a nice touch. $\endgroup$ Feb 9 at 19:45
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    $\begingroup$ @PeterKämpf I'm not even sure if there is any one specific rule that directly forbids it. I didn't find any satisfactory one and gave up. Either that or I just got lazy. I think it is more like it just never passed because it didn't survive tests. It would be great if it did. Pressurization as a component of structural integrity is what keeps most plants from falling over, so it does work. Filaments filled with compressed gas for building airplanes are being used successfully in RC planes, so maybe one day,... who knows? $\endgroup$
    – user55607
    Feb 10 at 16:29
  • $\begingroup$ "the use of pressurized gas as a component of structural integrity in aviation is for obvious reasons categorically prohibited." Isn't that part of the definition of a blimp? $\endgroup$
    – TLW
    Feb 11 at 2:11
  • $\begingroup$ @TWL Sort of, but not really. Blimps and helium balloons rely on the lighter than air characteristic of helium, rather than on pressurized gas of any kind. Moreover, they may tend to return in the original shape after being pushed in, but that hardly counts as structural integrity. $\endgroup$
    – user55607
    Feb 11 at 2:19
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This actually did happen with a Boeing 737 on Helios Airways 522. Due to a bad switch setting and lack of communication between ground and flight crew, the pressurization was disabled. The plane sounded an alarm, but it used the same sound as an alarm that meant something completely different on the ground. The flight crew mistook that alarm for the one that couldn't happen in flight, and therefore ignored it. The plane never pressurized so the internal and external pressures were equal for the whole flight. The flight crew became incapacitated, the passenger oxygen masks dropped, and the plane flew its entire flight plan unpressurized on autopilot, up to entering a holding pattern over its destination. One interesting thing that did happen is that the alarm light for avionics cooling came on. The avionics are air-cooled, and without pressurization there might not be enough air to cool them. Even with the alarm, the avionics including autopilot continued to function.

Unfortunately the crew never woke up and never landed the plane. It eventually ran out of fuel and crashed, killing all aboard, but the plane itself functioned perfectly until it ran out of fuel.

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Because of the cost of flying an empty plane, commercial flights are sometimes undertaken unpressurised, according to a British Airways maintenance engineer I knew (so it’s an unverified third-party claim). They stay below 10,000ft - normal pressure altitude in an airliner is 8000ft based on my own measurements.

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