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Does the pressure change inside a plane when it is opened for paratroopers?

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For low deployment the cabin pressure is already equal to the outside pressure.

For High altitude jumps the pressure is reduced to the outside pressure before opening the door (and everyone puts on oxygen masks).

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It depends!

When people jump off an airplane with parachutes, there are two categories:

Skydiving

This is normally done at lower altitudes, somewhere from 1,000 meters (3,000 ft) - 4,000 meters (13,000 ft). The airplanes used typically for this purpose do not have a pressurized cabin. So when the door opens for paratroopers to jump, pressure outside is almost the same as inside. After the jump, the door is closed and everyone is happy.

This is a Cessna 208 configured for skydiving, with a door visible on the port side of the aircraft.

Cessna 208

High-altitude military parachuting

These jumps are done from 4,600 meters (15000 ft) and 11,000 meters (35,000 ft). At 35,000 ft, only 26% of the Oxygen is available when compared to the amount at sea level.

When the door is opened at such high altitudes, amount of available Oxygen is no longer sufficient for humans and everyone needs to wear the Oxygen mask to avoid sickness due to high altitude. More about re-pressurization of cabins is discussed here.

As far as the paratroopers are concerned, they wear special gear, which looks like this:

Soldier jumping from high altitude

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    $\begingroup$ I guess there's a discrepancy between your answer and ratchet freak's: for a high-altitude jump, would the cabin be depressurized by the opening of the door, or would it be depressurized by other means (valves, etc) beforehand? $\endgroup$ – Nate Eldredge Nov 5 '14 at 5:54
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    $\begingroup$ @NateEldredge that'd no doubt depend on altitude and aircraft design. I'd guess most aircraft with a pressurised cabin will have means to gradually depressurise, which is far nicer for both the aircraft and its occupants. $\endgroup$ – jwenting Nov 5 '14 at 8:00
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    $\begingroup$ The door can't be opened when there is a pressure differential and if it was possible, it would be likely to cause structural damage. The pressure must be relieved by opening the outflow valves before opening the door. $\endgroup$ – Jan Hudec Nov 5 '14 at 10:33
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    $\begingroup$ @JanHudec wouldn't that be for a door that opens inward, like on a typical boeing or airbus? for HALO operations, the jump plane is something like a C-130, which has an outward opening door, and doesn't have the same pressure against the door flanges $\endgroup$ – rbp Nov 5 '14 at 17:48
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    $\begingroup$ @rbp: Even for door that open outward (like typical Airbus, by the way) the pressure against the locking mechanism prevents opening them when there is significant pressure differential. And if it did open, they'd end up like UA-811. $\endgroup$ – Jan Hudec Nov 6 '14 at 5:42
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It does.

When you open the door, Bernoulli's principle guarantees the pressure in the cabin will drop (yes, lower than outside).

The actual pressure difference will depend on various factors like door placement and size, aircraft slip, but mostly and especially on aircraft speed. For a normal skydiving operating speed (say <100kts) this will raise the cabin altitude by no more than 100-200 feet.

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  • $\begingroup$ OP is asking specifically about paratroopers, not skydivers, so they presumably want to know what happens to a military transport before/after/when the door is opened. $\endgroup$ – egid Nov 12 '14 at 17:42
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    $\begingroup$ on second reading you are right. Though this basically drops the question to the level of "if I open a door on a submarine will water come rushing in" $\endgroup$ – Radu094 Nov 13 '14 at 16:58

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