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The 737's rear exit doors cannot be used to evacuate the aircraft in the event of a water landing, as shown, for example, in this safety card:

737 safety card

(Image from flight-report, via Jordy here at AvSE.)

In contrast, the rear doors on (for instance) the A320 series can be used for a water evacuation:

A319 safety card

(Image by Czechnology here at AvSE.)

Why can't the 737's rear doors be used during a water landing?

Vikki
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    I like the optimism of these "safety cards" showing a pristine aircraft floating nicely on the water after a ditching. Statistically unlikely but makes for a pleasant looking card. – Ron Beyer Apr 04 '19 at 02:55
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    There was that time a guy with a lot of glider experience landed an airliner in the Hudson River a few years back, after the engines died right after he took off from the airport. – nick012000 Apr 04 '19 at 03:55
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    Yeah, and they couldn't use the rear doors. Know why? They were underwater... This was an A320. – Harper - Reinstate Monica Apr 04 '19 at 04:00
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    @nick012000 and they call it "The Miracle on the Hudson" for a reason. – Arcanist Lupus Apr 04 '19 at 06:02
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    Looks like Airbus tell you to check, but Boeing reckon it's not even worth checking. In the one photo I can find of a successful Airbus landing on water, you wouldn't open the rear doors anyway... https://home.bt.com/images/passenger-plane-landed-in-hudson-river-136395463010202601-150114153840.jpg – Jason Apr 04 '19 at 07:13

2 Answers2

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The bottom of the door opening sits too close to, or below, the water line when the airplane is floating.

John K
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It all goes back to how the aircraft is designed; the ways different planes float vary. when the 737 ditches on water the tail-section of the plane is deeper in the water than front of it, because the wings are a huge floating point and support most of the weight of the aircraft when afloat, and the bigger front of the airplane contains more air so when floating it will be lighter hence pitching the nose up, causing the tail and the rear doors to be below or very close to the water. this is why these doors remain shut in the event of evacuating after an emergency water landing so that water doesn't get in any faster, giving the plane and its passengers and the crew more time to evacuate and stay afloat longer until help arrives.

AndroidSmoker74
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    "[T]he bigger front of the airplane contains more air so when floating it will be lighter" and, by the same argument, a big empty box weighs less than a small empty box because it contains more air! You've forgotten that the bigger container also has bigger walls, which weigh more. – David Richerby Apr 04 '19 at 10:23
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    @DavidRicherby You're forgetting the square-cube law. – Katastic Voyage Apr 04 '19 at 12:03
  • @DavidRicherby when comes to objects with the same shape bigger objects have bigger volume to surface area ratio. So the bigger object has more trapped air to mass ratio causing it to be lighter when floating. – AndroidSmoker74 Apr 04 '19 at 14:48
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    @DavidRicherby - Yes, bigger empty boxes weight less than smaller empty boxes when both are submerged in water. – Pere Apr 04 '19 at 15:18
  • @Pere That's not what weight means. – David Richerby Apr 04 '19 at 15:48
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    @DavidRicherby You are the one introducing the interpretation of "lighter" in the answer as meaning less weight rather than more buoyancy. The answer did not say the former, and clearly meant the latter. If you really want to nitpick, it is the wording, not the concept. – Backgammon Apr 04 '19 at 17:12
  • @Backgammon I'm not introducing anything. "Less weight" is the meaning of the word "lighter". – David Richerby Apr 04 '19 at 20:42
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    @DavidRicherby, the tail is a pointy cone with a bunch of thin bits of metal sticking off of it. It's got a lot of metal and only a little air. The nose is a rounded object containing a whole lot of air and only a little metal. It's not surprising that the nose floats higher than the tail. – Mark Apr 04 '19 at 22:17
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    @DavidRicherby A larger empty box of the same strength has "more mass" everywhere and "less weight" when in the water. By your words, a larger empty box is "lighter" in the water. The answer specifies "when floating". – Backgammon Apr 05 '19 at 18:57
  • @Backgammon No. Weight is the gravitational force on an object due to its mass. Weight does not change if you're floating on water. – David Richerby Apr 05 '19 at 19:03
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    I see what the contention is now. Is "apparent weight in water" satisfactory? It should not be necessary to specify this in every single discussion to avoid drawing corrections when everyone understands what the shorthand means. – Backgammon Apr 05 '19 at 19:27