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What are design considerations and usage procedures for these slides in case of some successful water landing?

For instance, are they meant to be detached from the fuselage at some point?

Or instead are they supposed to keep the whole or remaining airframe floating?

Are there any braces or ropes to grab them?

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    $\begingroup$ They detach from the fuselage and they have ropes that allow them to be tied off to each other or the airframe. They are not meant to keep the airframe floating. If you don't get an answer a little later I'll type one up. $\endgroup$
    – Ron Beyer
    Commented Jan 12, 2018 at 17:22

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The inflatable slides are indeed also life rafts and they are meant to be detached. Consider US Air 1549 (landed on the Hudson). Here you can see the slides inflated and floating

Here you can see people in the slide, which has been detached from the airplane

The Wikipedia article mentions it specifically

Sullenberger advised the ferry crews to rescue those on the wings first, as they were in more jeopardy than those on the slides, which detached to become life rafts.

It's impractical to keep an airplane afloat in the water. Assuming you successfully land intact, the plane is still a metal tube on the water. It may be airtight, but water will eventually begin to invade. Airbus has a water ditch button to help slow (but not prevent) flooding

Some aircraft are designed with the possibility of a water landing in mind. Airbus aircraft, for example, feature a "ditching button" which, if pressed, closes valves and openings underneath the aircraft, including the outflow valve, the air inlet for the emergency RAT, the avionics inlet, the extract valve, and the flow control valve. It is meant to slow flooding in a water landing.

You want to evacuate as soon as possible. Remember, the slides help keep some water away from the door, but the plane will eventually flood and sink.

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    $\begingroup$ As a side topic, try to count how many people did bring their life vest (the airplane was fitted with live vests in spite of not crossing a water area) before exiting the cabin. $\endgroup$
    – mins
    Commented Jan 12, 2018 at 19:06
  • $\begingroup$ @Machavity Are there aircrafts designed with a "ditching button" jettisoning engine nacelles just before water landing in order to avoid front flipping hydrodrag tendency when touching water? $\endgroup$
    – user21228
    Commented Jan 12, 2018 at 19:16
  • $\begingroup$ @qqjkztd No, but I can't think of any reason to drop them. Provided you land evenly, the drag of the nacelles touching the water would simply add significant drag to the aircraft, which is a moot point since you're already landing in water. $\endgroup$
    – Machavity
    Commented Jan 12, 2018 at 19:27
  • $\begingroup$ @qqjkztd No aircraft has that, however some aircraft, like the Boeing 747, the engines are designed to shear off. $\endgroup$
    – Ron Beyer
    Commented Jan 12, 2018 at 19:56

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