Airplanes are supposed to be air-tight and pressurized. However, when USAirways Flight 1549 landed on the Hudson river; water started to come into the airplane.
How is this possible if the airplane is sealed? Was there a crack in the fuselage?
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2$\begingroup$ I once flew in a Metroliner where the hissing from leaks in the seal of the aft door was louder than the engines. Pressurized fuselages are like sieves. Also, many seals only work one-sided: When internal pressure is higher they are tight, for external pressure they open (think of Space Shuttle SRB seals). $\endgroup$ – Peter Kämpf May 31 '20 at 20:14
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$\begingroup$ this answer specify they didn't have time to perform all checklist, which may contain items about closing some openings. One comment of this answer specify that the ditch switch was never activated. $\endgroup$ – Manu H May 31 '20 at 20:22
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$\begingroup$ Pressurization goes away when both engines were lost. Having the engines submerged certainly doesn't help matters any. $\endgroup$ – Ralph J Jun 1 '20 at 0:40
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Two reasons:
Airplanes are not completely, perfectly hermetically sealed. They are pressurized using positive air pressure from the engine bleed air and packs. If there is a failure in the system, the aircraft internal pressure will return to the outside ambient pressure. Air will escape the aircraft or enter the aircraft if the outside ambient pressure is greater. Water will react the same way. Especially when you have the combined greater pressure of the water and the atmosphere above it.
Combine this with the structural damage caused by the dynamic pressure of the aircraft impacting the water, the aircraft will have ways of water entering the aircraft through burst seams. The structural and hydrodynamic load alone, created by the rapid deceleration of the aircraft will be enough to tear apart the airframe, open sealed doors and bulkheads, and blow out windows.
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$\begingroup$ Interesting, I always thought these things were sealed like Microwavable vegetables in plastic bags, Thanks for the explanation. $\endgroup$ – Asynchronous May 31 '20 at 17:09
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$\begingroup$ Also, at least some planes use an inflatable seal that would deflate when bleed air pressure is lost. $\endgroup$ – Michael Hall May 31 '20 at 18:33
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$\begingroup$ @Asynchronous - That would make things rather hot and sweaty 😜😁. Even at high altitudes, the concentration of oxygen is fairly constant, even when the density of molecules is low. We need access to the outside air to facilitate oxygen-carbon dioxide exchange. Lower density just means there are fewer oxygen molecules present to exchange. Also, a certain amount of pressure must be present to allow the lungs to diffuse the oxygen into the blood regardless of how much oxygen is present. $\endgroup$ – Dean F. May 31 '20 at 19:31
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$\begingroup$ @DeanF.Fascinating analyses. Now I feel smarter. :) $\endgroup$ – Asynchronous May 31 '20 at 20:04
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USAir 1549 flooded because the doors opened. Passengers attempted to open the rear doors, worsening the problem.
Airplanes are not sealed at all. Not remotely close. They're not air-tight. Pressurization is maintained by varying the air outlet, called the outflow valve. A large volume of air is constantly being pushed into the cabin via the engine bleed air valves and air conditioning packs. A large volume of air is constantly being released from the cabin, meaning the cabin air is completely changed every few minutes.
The aircraft doors use seals which are inflated or held sealed by air pressure. The metal door doesn't actually seal; flexible seals around the door do that, and those seals work based on air pressure; when the aircraft touches down, it is no longer pressurized; most aircraft must be depressurized before landing, or depressurized to a minimum differential value, typically no more than .5 psid.
In the case of USAir 1549, insufficient time remained to complete the ditching checklist, which included shutting the ditching valve, or avionics exhaust valve, which is done prior to ditching to prevent water from entering.
Flotation of the airframe is largely due to air in the tanks, rather than air in the fuselage. The airplane isn't designed to float like a boat, and in many cases, a ditching results in a breakup on contact with the water, as water enters pack inlets and other openings and hydraulically tears the airplane apart like a hollowpoint bullet. The Some aircraft have managed to float for an extended period, but the hope is that it will remain intact and afloat long enough to get out and get away.
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There was a hole in the fuselage, and cargo doors had come open. And someone had opened a rear door that couldn't be closed.
