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When looking at the following Transavia B737 safety cards, I noticed that after a water landing, passengers who evacuate via the overwing exits should leave the wing at the front, while during evacuations on land they should leave via the rear side of the wing.

Picture of normal evacuation instructions

It is clear for me why normal evacuations need to go via the rear side of the wing, but why does a water evacuation need to happen via the front side? Picture of water evacuation instructions

Does it have to do with the center of gravity position, in an attempt to keep it is far forward as possible so that the tail doesn't sink? Because in the second picture the rear exits are prohibited from use too.

(images cut pictures at this source)

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    $\begingroup$ Its an A320 but take a look at how it is sitting in the water. You don't want to open the rear doors because chances are it is pretty heavy in the back and will be tail-down. $\endgroup$ – Ron Beyer Jul 19 '16 at 14:51
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    $\begingroup$ I don't think it's necessarily heavy in the back. On US Air 1549 they didn't have time to hit the "ditch switch" which closes the outflow valve. That allowed water to enter at the back of the plane. I'm pretty sure if the crew had time for the entire procedure it will float level $\endgroup$ – TomMcW Jul 19 '16 at 17:41
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    $\begingroup$ I'm very impressed that you read the safety card so closely. Not many people do. $\endgroup$ – PJNoes Jul 19 '16 at 18:41
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    $\begingroup$ @PJNoes Unlike many people, I fully intend to get out of the aircraft when the flight ends! $\endgroup$ – Michael Hampton Jul 19 '16 at 19:31
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    $\begingroup$ Many people get so bored during flight that they even read the safety card! $\endgroup$ – Czechnology Jul 20 '16 at 20:41
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When I worked as a flight attendant back in the 80's we were told that water landings frequently resulted in very damaged trailing edges to the wings. Flaps and spoilers will most likely have been deployed prior to landing and the velocity of the water impacting these extended surfaces would tear them up badly, along with the hinges and fairings that support them. To avoid serious injury to passengers from ragged sheet metal, we were to direct them off the leading edge of the wing.

This wouldn't be a problem with an evacuation on land so in that case we would direct them off the trailing edge. The upper wing surface makes a good slide and a more natural and controlled drop to the ground.

Edit: I forgot to mention that additionally, water landings mean passengers would be wearing life vests and life rafts would be deployed. Jagged sheet metal would make short work of these inflated devices as well.

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    $\begingroup$ Can't argue with an answer with insider knowledge. Thanks for coming by to help out! $\endgroup$ – FreeMan Jul 19 '16 at 18:54
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    $\begingroup$ I always thought that the reason to go off the back of the wing was to avoid getting sucked into an engine. So I learned something today. $\endgroup$ – Michael Seifert Jul 19 '16 at 20:42
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    $\begingroup$ @MichaelSeifert I find that whole scenario difficult to envision, but decided to ask the experts. So Is there any situation in which engines would not be shut down before emergency evacuation of an aircraft? $\endgroup$ – a CVn Jul 20 '16 at 11:22
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    $\begingroup$ @MichaelKjörling In addition to the QF32 situation mentioned in the answer to your question, if the engines were at a high power setting when the crash occurred (like in Asiana 214, where they were unsuccessfully trying to go around,) it would take them quite a while to spin down to non-dangerous levels, especially with the monsters they have on a plane like the 777. $\endgroup$ – reirab Jul 20 '16 at 18:32
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    $\begingroup$ @MichaelKjörling Honestly, I'm not really sure and it would depend on the engine and conditions. It would presumably spin down quite a bit faster in water (and possibly also detach.) From a quick search, I didn't find any videos showing a shutdown from a full power setting, but this one shows a PW2043 (from a 757-300) shutting down from idle after parking. From idle, the first 15-20 seconds are still around the "come close and I will eat you" speed, though it's still spinning fast enough to perform an amputation for around 1.5 minutes or so. $\endgroup$ – reirab Jul 21 '16 at 6:53
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An answer that I once received from a flight attendant (I don't remember the airline or plane model, it was long ago) was that in the event of a water landing passengers would be able to assist each other if they were all together. Thus, passengers leave by the front of the wings in the direction of the other usable exits.

Otherwise, the wings would separate the passengers into groups.

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It all depends on the design of the exits as well as how the aircraft floats in the water prior to succumbing. It may have been determined that it would be easier for people to deplane at the leading edge of wing it water as opposed to the trailing edge for that reason.

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Additionally to @PJnoes answer, I spot that the path to the front of the wing is dangerously close to the (possibly hot) engines. That's less likely an issue when they are immerged into cooling water and may explain that going towards the tail of the aircraft is preferred in case of a landing on the ground.

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    $\begingroup$ The main reason for going aft on ground landing is that it is still rather high (B737 does not have wing slides) and the extended flaps will provide a makeshift slide aft. While on water the lack of wing slide means the evacuees need to go towards the forward slide, which is the only available inflated flotation device around. $\endgroup$ – Jan Hudec Jul 25 '16 at 8:22
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For comparison, here's a picture I've taken of an emergency card onboard an Airbus A319. Slides from the wings are deployed after emergency landings both on land and on water. I guess the slides have to be specially protected against damaged trailing edge of the wings, as discussed above.

A319

(Image source: Own work)

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