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I was looking for a checklist for the Cessna 182 and I stumbled upon this one. When I looked at the speed-table, I found an interesting entry. It says "Ditching" with a speed of 65 KIAS:

Ditching Speed

Everything I read and watched about ditching so far told me to ditch at the lowest speed possible.

Why should I ditch with 13 knots more than I could (Vso is 52 KIAS)? I can think of no reason I would like to do that.

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    $\begingroup$ I dunno, seems like it's probably so you don't stall-spin it into the water at 50 feet? $\endgroup$ – egid May 12 '17 at 22:49
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    $\begingroup$ How will you flare if you're flying an approach at Vso? That seems like a rather bad idea for any sort of approach. $\endgroup$ – reirab May 12 '17 at 22:50
  • $\begingroup$ If you have to ditch a plane with fixed undercarriage at any speed, it's very probable that the plane will tumble over on contact, and you'll drown... In my opinion, with a light plane, it is better to approach at the best glide speed, and then skim the surface in ground effect, gradually lowering flaps and pulling the stick till it stalls, hitting the water with the minimum possible horizontal speed. Perhaps you are lucky enough to survive... $\endgroup$ – xxavier May 13 '17 at 7:57
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    $\begingroup$ Because stalling speed is the speed at which the airplane stops flying and starts falling. When ditching the objective is to land the airplane int the water in a controlled manner, rather than having it fall into the water in an uncontrolled fashion. Thus, ditching speed must be greater than stalling speed. Same as with touching down on a runway - approach speed must be greater than stalling speed or else you're not "landing" - you're "crashing". A ditching is not supposed to be a crash. $\endgroup$ – Bob Jarvis - Reinstate Monica May 14 '17 at 12:27
  • $\begingroup$ The dynamics of landing on water are very different from landing on a runway, because water isn't "solid". Instead of using the dampers in the landing gear to kill the vertical velocity and then using the brakes to stop, on water you want the plane to "hydrofoil" on the fuselage and then slow down and float. (Of course landing a seaplane normally is different again, because the landing gear has floats). $\endgroup$ – alephzero May 14 '17 at 14:34
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This is somewhat restated and overlaps the other postings. Here are my thoughts on this:

  1. The ditching speed is to provide a safe approach speed. Depending upon the seas, you may not have the ability to judge your altitude, and this avoids stall/spin issues when ditching. The reality is that this type of complication to an unfortunate situation happens frequently. There are Youtube videos of people stall/spinning in as they are doing a ditching.

  2. Frequently over smooth water, and even rough water, the height above the water is difficult to judge. Even if you don't have a seaplane rating, find an instructor to give you some simulated glassy water landing practice. A good time to do it is when you have night ILS approaches. You can even do it under the hood, and simulate a 0/0 approach.

  3. Many ditchings are done with loss of power. If you are faced with performing one, try to have some fuel left, because your A/C is more controllable with a little power. From those who have experienced it, I have heard that they see the gear hit the water out of the corner of their eye (C182) or they feel the prop picking up spray (Arrow). Back elevator is a real good thing, as it may avoid flipping over, which can hurt you and be very disorienting. Also, I wouldn't compromise your ability to fly, but you might hold the yoke/stick differently, as the force of hitting the water has broken a few hands/wrists or caused dislocations. That only hampers your ability to manage everything else.

Also for preparedness, consider the FAA underwater egress course. After you've been dunked in a high school swimming pool a couple of times, it is easier to keep your wits about you when you plunk into a great lake with a 42F offshore water temperature mid-summer. Someone who has been there swears by the underwater egress course. (not me)

Also read up on what you can about ditching orientation. Like where the swells are and how you are oriented to them. It softens the bang.

A few other things: for great lakes crossings, the preflight briefing includes having passengers identify what bags, sleeping bags, or whatever that they are going to put in front of them if there is a ditching. It includes what to do if the plane gets inverted. Most GA aircraft do not have 5 point, or shoulder harnesses in all seat positions, so protecting faces is important. Also when doing a lake crossing I recommend that everyone have their life preserver on before feet wet. Because SAR at night on the water is problematic, I normally do not fly across large bodies at water at night in SE piston aircraft. Just too many factors multiplying and stacking against me. Flashlights and strobes are nice, but having looked for people with strobes over open water at night, while raining, I can say that they were very very difficult to spot under only mildly adverse conditions.

Finally, let me address then shedding of 13k airspeed. At 52k Vso, the v**2 component is 2704, and at 65k it is 4225, so you are right, in that you can reduce substantially your kinetic energy of impact. But given the likely lack of horizon, the difficulty and inexperience you are likely to have ditching, the handling characteristics of the aircraft at Vso, the downside of a stall/spin, and the possibly increased kinetic energy, the impact disorientation, etc. it is easy to see why the higher speed and better controlability of a more "normal" approach speed is beneficial.

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As far as I'm aware - and maybe a seaplane pilot could give a better answer - one of the big issues with ditching is judging your height above the water. If you stall it in, you might get it wrong and lose control completely.

I checked the POH for a 182RG, which has a Vso of 39 KIAS and ditching speed of 60 KIAS. The ditching checklist gives descent rate as the primary consideration, not airspeed:

  1. Power -- ESTABLISH 300FT/MIN DESCENT at 60 KIAS

In the amplified procedures section, it says:

Avoid a landing flare because of difficulty in judging height over a water surface

In other words, better to descend slowly under control until impact than stall it at an unknown height above the water. As Bob Hoover famously said, "If you're faced with a forced landing, fly the thing as far into the crash as possible".

I did a bit of Googling and found similar checklists for other aircraft, e.g. a Piper Arrow:

  1. Power--ESTABLISH 300 FPM DESCENT AT 90 MPH
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I tried to put this in as a comment to @Pondlife's answer, but ran out of room, thus this answer is an amplification of what he said. He is correct. Judging height above the water is a challenge, especially if you've not had experience doing that, which typically a ditching pilot will not have had. Wave action helps, but the problem there is that you may misjudge how big the waves are and thus will misjudge your height. Landing alongside a shoreline can be a big help.

When people get their seaplane rating, they're taught how to do "glassy water landings". In other words, landings on water that has no wind or current action to disturb the surface. That makes judging height difficult even for an experienced seaplane pilot if the water surface is all there is to look at. The procedure for a glassy water landing is the same as for ditching: maintain a constant rate of descent carrying power until the aircraft contacts the water, then cut the power and up elevator.

The same procedure is used for night landings on water, although in general one should try to avoid having to make such landings.

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    $\begingroup$ BTW: the sprinklers/bubblers below the diving tower in a pool used for competitive diving are there for pretty much the same reason: you just can't tell your distance from the water surface easily. $\endgroup$ – Jörg W Mittag May 13 '17 at 0:29

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