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I noticed something from investigating my other answer to a not-really-related question.

A wiki article here says:

The "water bird" emergency landing is a technique developed by the Canadian Forces to safely land the Sikorsky CH-124 Sea King helicopter when one engine fails while flying over water. The emergency landing technique allows the boat-hull equipped aircraft to land on the water in a controlled fashion.[37]

Which leads me to the obvious question: Is landing any different for water landings? If so, how?

EDIT: the question was poorly worded before. I'll try to explain better here:

I don't know much about helicopters. For a normal landing, I assume they just hover down and I thought the same thing would be done for a water landing. But the wiki article I read implies that something is different.

For auto-rotation without any engine power, apparently there is a flare at the end. Is this flare done any differently when landing on water?

Basically, the spirit of the question is, how is landing different (if at all) on the water, whether powered or not?

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  • $\begingroup$ "or emergency descent in general" what do you mean by this? A controlled, but rapid descent with power? The quickest way of getting a helicopter down with power is to autorotate to about 20 feet then bring the power back in to land normally. $\endgroup$ – Simon Sep 15 '16 at 12:30
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    $\begingroup$ The Sea King is a multi-engine helicopter, this procedure you posted is for a single engine water landing, so its not really an auto-rotation situation, the helicopter still has (reduced) power available, enough probably to slow the descent enough to make water landing without damaging the airframe. $\endgroup$ – Ron Beyer Sep 15 '16 at 14:18
  • $\begingroup$ @Simon "emergency descent in general", any landing that needs to take place immediately. Could be powered or not depending on failures. The article seems to distinguish descents over land from descents over water, which I found really odd. $\endgroup$ – DrZ214 Sep 15 '16 at 20:08
  • $\begingroup$ @RonBeyer I also included "emergency descent" in my question. The way I read the article, it seems to distinguish descents over land from descents over water, which I found really odd. Maybe I am not reading it right? $\endgroup$ – DrZ214 Sep 15 '16 at 20:09
  • $\begingroup$ I think you need another edit, perhaps to remove the link to the "water bird" landing altogether. A water bird landing is not an auto-rotation or an "emergency descent". It is a single engine landing on water from forward flight in a particular type, the Sea King. What exactly are you asking? I've voted to close as unclear but will happily retract that if you can edit the question to be precise. If the question is simply "what is the difference between landing on water and landing on the ground?", then you should ask just that. $\endgroup$ – Simon Sep 16 '16 at 6:53
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As a helicopter pilot, I've never made a water based landing, so I'm not speaking directly from experience, only from my training, but there are a couple things that are missed in the discussion here that I would certainly worry about.

If I had a real engine failure, and was forced to perform an auto-rotation, in all likelihood, I would not end the auto-rotation with zero forward velocity, I would probably perform a running landing (where you slide on your skids). This, of course, is assuming I'm landing on a flat surface where I'm not likely to roll over (such as a street, parking lot, or runway). Holding the flare to bring the helicopter to a complete standstill has a few risks for me, the biggest one being that if I flare and stop my descent while I still have forward velocity, holding the flare will start to increase my altitude again. Higher inertia (heavier) rotors in small helicopters will suffer from this problem, for example, in training, a Robinson R-22 will generally not have this issue, but an R-44 has much heavier rotors, and we get trained to anticipate a flare at the end of the auto-rotation. Depending on the helicopter, weather, and how I've performed my auto-rotation, if I'm in a real emergency, I would rather put the helicopter down on the ground and slide a little bit than suffer a rotor stall at 50 feet, but a running landing is clearly not an option in a water based landing.

Once I've performed an auto, and the helicopter is on the ground, I'd apply the rotor brake to get the rotors to stop moving before I exited the aircraft, and generally speaking, won't have to worry about the rotors any longer at that point.

If I am performing a water based auto-rotation, there is quite a bit more to worry about. The first thing I have to worry about is that I won't be able to perform a running landing. This means that I'll have less margin for error in my auto-rotation. The second problem is that as soon as I set the helicopter down into the water, it's going to start sinking, and I'll still be strapped in. If I jump out, I'll have to worry about hitting the rotors (or the rotors hitting me) as the helicopter sinks. This means that as soon as I hit the water, I have to remain in the aircraft, and I'll pitch the cyclic over in the opposite direction from the side I'm sitting on, so that I can stop the rotors and still exit the aircraft on the surface side rather than underneath the sinking helicopter. This gets more complicated in the case of passengers, I would encourage them to jump out before the helicopter hits the water, but after my flare, so that they can clear the aircraft and I don't have to worry about them. I only mention it as an additional complication I'd potentially have to deal with.

All of this complexity is a large reason why most of the helicopter pilots I know really do not like flying very far over water. When I'm flying over the water, I tend to stick to an area near the coastline specifically for this reason.

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  • $\begingroup$ When I'm flying over the water, I tend to stick to an area near the coastline specifically for this reason. With only one engine, that's a good way to look at things. $\endgroup$ – KorvinStarmast Oct 6 '16 at 2:44
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Let's separate the terns you've used in your question and comments.

Emergency descent

This is a very ambiguous phrase and doesn't really describe a flight phase or operation whereas auto-rotation and water landing do. The pilots operational manual will simply say something like "land as soon as possible".

An emergency descent means that you have a problem which requires you to get on the ground as fast as possible, not including all engine failure since that is an auto-rotation.

The fastest way of getting any helicopter down is to enter auto-rotation and if I really had to get down fast, that's what I would do. In the last few feet though, I'm going to get the engine(s) driving the rotor again so I can make a normal landing.

Auto-rotation

A true auto-rotation does not have an option of a powered landing, since by definition you have no engine(s). If you have power, then it's training, practice, testing or precautionary. Everything you do until the last fifty feet or so is making sure that you manage the rate of descent and airspeed to keep the rotor in the operating range. The end goal is to arrive at fifty foot with enough airspeed to be able to flare so that you can trade your kinetic energy from airspeed to kinetic energy in rotor rotation so that you can increase pitch and therefore lift and therefore drag in the flare, whilst feeding in kinetic energy to stop the drag from stalling the rotor.

It's all about energy conservation and conversion. You start off with some potential energy (height and mass) and some kinetic energy (airspeed and rotor RPM). An auto-rotation instantly starts converting potential energy into kinetic energy - you start going down.

Take two identical helicopters flying in identical conditions except one is at minimum weight and one at maximum weight. The one at maximum weight has more potential energy since:

PE=MxGxH

Where M is mass, G is the acceleration due to gravity and H is height.

Both helicopters enter auto-rotation. Quite soon, both will stop accelerating downwards and become stabilised with a constant rate of descent. The heavier helicopter though needs more lift to support it's weight. The airflow in auto-rotation always comes up through the disc from below. You can gain the extra lift by allowing the rotor RPM to increase or you can increase the angle of attack of the blades by raising the collective. The rotor RPM always have an upper fixed limit so in practice, the only way to gain the lift, and support the extra weight, is to raise the collective to prevent the rotor from over-speeding. You also have a limit to how much pitch you can use as the angle of attack must remain positive to the relative airflow from below. If you try to reduce rate of descent by continuing to raise the collective, you will lose lift as the angle of attack moves towards the critical angle so eventually, you reach a steady rate of descent, greater than that of the lighter helicopter.

Since the heavier helicopter is now generating more lift, it is also now generating more drag which can only be overcome by converting more potential energy into kinetic energy. Therefore, the heavier helicopter will use up it's potential energy faster and, since the rotor cannot be 100% efficient, this means that the rate of descent will be higher than in the lighter helicopter.

(It's a very complex topic beyond either the scope of this answer or, to be honest, my complete understanding but some helicopters, in some weight ranges in some flight conditions can descend more slowly when heavier due to rotor efficiency and the size and location of the "auto-rotative driven region" vs the stalled region of the disc).

So, when you arrive at the bottom of the auto and it's time to flare, you have a high rate of descent in a heavy helicopter and the only way to arrest this is to increase collective pitch, to increase lift and increase drag. The only power available to you is in your kinetic energy so you must trade airspeed for rotor RPM in the flare. So a higher rate of descent means you need a higher airspeed to make a controlled power-off landing.

All of that explanation was only to explain that a heavy helicopter arrives at the bottom of an auto-rotation with a high rate of descent and high airspeed.

I couldn't find a video of a Sea King auto-rotating, probably because heavy helicopter pilots don't practice auto-rotations like private and sports pilots do. But here is a heavy helicopter coming in to land. Notice how fast it's going when the flare starts, how high it is and for how the long the flare is held. Even after that, it still lands with a significant rate of descent and airspeed.

S-92 helicopter autorotation (power off landing)

In a light helicopter, especially with some wind on the nose, a good pilot can land with zero airspeed and just enough rate of descent to gently touch down. You could not do that in a heavy without a lot of wind.

Water Landing

Now for the unclear bit.

If you mean landing on water at the end of an auto-rotation from normal flight, then clearly, a heavy is not going to fare well with that arrival so unless you have no choice, you're going to go for firm ground.

If you mean a hovering auto-rotation, assuming that you are within the limits of the height-velocity curve, then you have no choice and it's not going to end well unless you have flotation devices which are deployed and you make a really good touchdown.

If you mean the "water bird", then this is neither an emergency descent or an auto-rotation but is a single engine landing, therefore powered, where you use the boat hull shape of the fuselage to arrive at a precise speed and pitch angle to allow the hull to plane and cushion the landing to enable the helicopter to settle into a float.

What a long way of saying that emergency descents, auto-rotations and water bird landings are very different things and therefore not analogous.

Phew.

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  • $\begingroup$ Sorry for the confusion. I made an edit to the title and added paragraphs trying to explain better. Basically what I want to know, is landing on the water (either powered or unpowered) a different procedure than on land? AFAIK you just hover down, but maybe the flares are supposed to go differently on the water for some reason? $\endgroup$ – DrZ214 Sep 15 '16 at 22:07
  • $\begingroup$ I really don't think you need that much detail on autorotation, it's good but not relevant and too long. $\endgroup$ – Notts90 Sep 16 '16 at 8:26
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    $\begingroup$ @Robert I beg to differ, it's entirely relevant. The size and weight of the helicopter has a very direct bearing on what kind of landing is carried out and where. I added the detail since, through experience, I know that if I don't the answer is less complete and will prompt further questions in the comments and, smart a** comments, such as "a heavier helicopter does not always have a higher ROD". Many lay people and non-helicopter drivers also fail to understand the basics of auto-rotation. $\endgroup$ – Simon Sep 18 '16 at 20:05
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    $\begingroup$ A further point is that the Sea King is uniquely suited for a water landing due to its boat hull fuselage shape. The Sea Hawk, Lynx, or Sea Sprite, for example, do not have the same hydrodynamic shape and thus will have significantly more trouble trying to do the same thing a Sea King can do. $\endgroup$ – KorvinStarmast Oct 3 '16 at 12:39
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Incomplete answer as your question is mostly answered by the other posters.

Emergency landing in a light helicopter equipped for water landings - normal ish landing, if it is glassy with no good ground reference, a slow forward speed with continues descent until you touch the water

Emergency landing in a light helicopter NOT equipped for water landings don't fly away from shore simple. - same as above, coming to a stable hover with skids in water, setting it slowly down [with doors open etc.] when it is sort of floating, hard right or left cyclic and roll it, when rotors stop [they quickly will] unbuckle seat belt and get out.

Auto - water with good reference, ie, beside the shore, normal flare and landing, when down, roll sideways, and get out.

Auto - Water with bad reference, ie, whitecaps, floating log etc, flare higher than normal and plan to hit harder, when down, roll sideways, and get out.

Auto - Water with no reference, ie glassy water far from shore, set up for no flare auto 30kts [check your POH] when you KNOW your low enough pull collective, do not flare, better to hit really hard than to pull at 150 feet. if your still alive, get out. Some POH's may have detailed instructions for this scenario

No experience with helicopters planning on landing on water, or muti-engine.

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