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I watched a documentary yesterday about the Hudson River ditching of US Airways Flight 1549. In the documentary, one of the investigators explained that the reason the water was entering so quickly was because the FO did not have time to hit the 'Ditching' switch the A320 is equipped with. This switch closes all vents etc. to prevent water from entering the cabin.

If an A320 or similar is ditched and the switch is activated in time, does that totally prevent water from filling the cabin (in a best-case scenario)? I'm thinking if one ditched in the Atlantic and had to wait for hours for help, would it be possible to stay afloat for so long?

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    $\begingroup$ Actual boats won't stay afloat forever without additional help! But "long enough for rescue" is a good criterion for this scenario. $\endgroup$ – Toby Speight Feb 25 at 16:26
  • $\begingroup$ There was significant damage to the rear lower fuselage, so it is not obvious how much water intrusion the ditch switch would have prevented anyway: flyian.net/aircraft/museum/ncam/1549/1549.htm $\endgroup$ – Adam Feb 25 at 17:42
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    $\begingroup$ US-1549 was floating for two days after the landing into water even without the switch. $\endgroup$ – h22 Feb 25 at 19:41
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    $\begingroup$ Using the word indefinitely seems to trigger a lot of besserwissers, maybe you need to change the title a bit. $\endgroup$ – pipe Feb 26 at 10:16
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The answer is no, not totally, but it would really slow things down. I don't think anybody knows the precise answer because only flat water ditchings seem to result in the airplane stopping in the water in one piece (such as 1549 and a similar one in Malaysia) and flat water incidents (like Malaysia and some airport overruns) are usually in shallows where the ditching valve is moot.

But assuming an A320 was able to ditch on open ocean swells without breaking somewhere, certainly it would float for a much longer time, but not forever. Pressure hulls are never absolutely fluid tight; door/hatch seals leak a little bit, and there may be tiny leaks through various entry points below the water line like bleed air shutoff valves, fay-sealed skin laps, rivets, control cable pressure bulkhead fairleads, etc (the older the airframe the leakier).

Wing dry bays may be sealed off with a tape made for the purpose but they are not usually totally water tight. Water can migrate into the fuel tanks through the vents. Full fuel or nearly full fuel would be a bonus, fuel being buoyant.

You certainly would have a lot more time to get onto the rafts (perhaps Airbus has estimated the theoretical sink time by calculation), but it would sink eventually (it would take quite a few hours, maybe even a day or two), and you aren't going to be staying on board any longer than necessary regardless.

(Now, if you ditch in a composite foam sandwich sailplane, well you've got yourself an unsinkable boat right there, and if you took a paddle along, you could actually go places. If I'm in a glider that gets low over rough forested terrain, and there are lakes around, I'm putting it in the water and paddling it to shore with whatever I have at hand).

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  • $\begingroup$ Unsinkable, huh? Is your sailplane named Titanic? :) $\endgroup$ – T.J.L. Oct 3 at 13:38
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In theory, yes. In practice, no.

FAA regulations (specifically, 14 CFR 25.801(d)) require that, under reasonable water-landing conditions, an airplane must remain afloat long enough for the occupants to evacuate. Most airplane manufacturers don't actually test the ditching performance of their aircraft, but instead rely on the clause of paragraph (c) to let them compare their airplane to a similar one that has been tested.

The A320 is certified based on ditching tests of a scale model of an Airbus A300 done in 1974. Under the landing conditions specified in the certification, the A320's fuselage is supposed to remain intact in the event of a water landing, and with the "ditch switch" activated, water entry is supposed to be minimal. (Flotation won't be truly indefinite, but when the main source of water entry is things like leakage around door seals, it's quite possible to stay afloat for days or longer.)

If you read the investigation report from US Airways flight 1549, the specified landing conditions were found to be nearly impossible to attain in the event of a dual engine failure. Flight 1549 actually hit the water with the nose lower than specified, producing a higher vertical speed and a greater glideslope angle, causing significant structural damage. Specifically, a number of lower aft fuselage panels were buckled or torn, letting water freely enter the airplane. In this situation, the use or non-use of the "ditch switch" is irrelevant.

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NO. Indefinitely is a very, very, long time. In fact it's so long that by the time it's over there's no more water for the aircraft to float in. Heck, there's no more planet for the water to exist on.

But even for a shorter period, say hours, it wouldn't work. The switch may close some holes, but the doors would still be opened to allow egress into life rafts. Wave action may break windows and erode door seals pretty quickly.

And there has to be some holes still open to allow air circulation into the cabin to prevent everyone inside suffocating. Water can enter through those as well.

The switch is there so slow down water ingress to allow more time for evacuation, not completely prevent it. That's all it is designed to do, and that's all it does. It's not designed as nor does it work to turn the aircraft itself into a life boat for the crew and passengers.

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    $\begingroup$ I find it hard to believe that wave action would break aircraft windows -- are you sure about that? $\endgroup$ – David Richerby Feb 26 at 17:32
  • $\begingroup$ The A320 floats nose-high, with the front and overwing doors well clear of the water. It would take pretty significant wave action to get water to flow in through them. $\endgroup$ – Mark Feb 26 at 21:10
  • $\begingroup$ If the fuselage is intact, it would slow down the ingress of water due to the inability of air to be forced out. Open the exits and all bets are off. $\endgroup$ – Mike Brass Mar 2 at 1:02

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