Help Needed:Stabilty of Helicopters when Ditched.

[bondu]

Hi Rotorheads,

I am trying to find some information on the stability of helicopters when they have ditched. Specifically, at what angle the aircraft is likely to capsize. I know this will vary with load and wind loading on the side but there must be a minimum and maximum angle for each type.
Why am I wanting this info? Having spoken to a senior marine forecaster who knows about waves, I believe it could be possible to determine the steepness of waves in an operating area when sea states are high and thus produce a form of ‘steepness index’. The combination of this ‘steepness index’ and the stability performance of the helicopter, would provide a better data set for offshore pilots to base their decisions on. At the moment there is a tendency to fixate on the wave height alone, whether that be ‘significant wave height’ or ‘maximum wave height’.
Also, does anyone have any info on where the manufacturers do their environmental tank testing of floatation systems? As I understand it at the moment, a scale model of the type being tested is placed in the tank and subjected to simulated waves up to the equivalent of 4m. The results are the extrapolated to give the ‘demonstrated’ limit of the floatation system.
So any information/help would be greatly appreciated. PM if you wish.

bondu :)

[Thomas coupling]

Bondu - how long is a piece of string? Presumably you are suggesting the helo has survived the landing and that the rotor blades have stopped - yes? Because if the rotors are still turning, it won't be long before one of them strikes the next swell and all bets are then off.
So assuming the cab is intact and not leaking like a seive and remains upright and not leaning due to Cof G shift/poor irregular inflation of the flot gear etc, are you asking about the steepness gradient from then on in?
Do modern day pilots calculate a steepness gradient for contacting the sea?
Do they care - as the decision to ditch will not be affected by the sea state. The decsion to ditch is one taken by a pilot because they have no choice. They will simply have to accept the steepness of the swell, no???

What are you getting at?

[bondu]

TC,

I agree with all your points as to blades hitting the next wave etc. My point is giving the pilot all the relevant info before he/she gets airborne.
Steep waves will increase the possibility of the blades hitting the next wave, but lets assume a minor miracle has taken place and the rotors have been stopped. It seems that we offshore pilots only concern ourselves with the area immediately round an installation (500m in the UK PFEER regs). But we spend the majority of the time airborne over the open sea en-route to and from. The data now available from direct live real-time readings and satellite observations means we can now have a more than reasonably accurate assessment of the sea states en-route.
After the two ditchings this year in the North Sea, shouldn't we now have a look at getting better data for decision making. What would have happened if REDK and/or CHCN had their problem when the sea states were around the 5m mark? Or higher? UK Oil and Gas are happy for offshore flights to operate up to the 'limit' of the DACON scoop, which they believe to be around 7m, which just happens to be at least 1m in excess of the 'demonstrated limit' of our floation gear!
Any clearer?

bondu :)

[Hummingfrog]

TC

I think he is after data for " don't fly there its too dangerous". Before I retired there was a lot of discussion ref DACON nets used by the standby boats to pick up survivors in the water. I don't know what the state of play is with them now but there was discussion that in fairly moderate sea states they would do more damage to the survivor than was good for them!! The idea was put forward that if that was the only form of rescue available then flying should cease if the sea state got too high.

In todays risk averse society there is a tendency for risk to be assesed too highly and therefore thou shall not put yourself, or others, into that area of risk.

I remember flying back into Aberdeen, on an offshore based aircraft, to hear on the company frequency aircraft being recalled after taxing from the stand. I landed and wandered up to the ops room to see if our urgent freight had arrived. All the pilots and management were crowded round a computer screen looking at the weather info showing "lightening strikes". They had been recalled because of a threat of lightening! I had flown 120miles in CAVOK conditions with not a sight of a thunderhead yet they were all grounded because of a perceived threat - risk management gone mad.:ugh:

HF

[Thomas coupling]

Thanks - clear now. So you are looking for data to make a captaincy Go/No Go decision? Presumably, once airborne the decsion making is made for you, because if you have to land (LAND immediately) SS has no say in it - you are going down whether you like it or not.

I used to teach waterbird in Nova Scotia with the CAF. These guys have maritime ops down to a "Tee".

I recall ground school covering the following:

Position the aircraft prior to landing on top of the swell at 30 degrees to the swell. This will reduce the risk of tip path and/or tail rotor contact.

Always motor fwd at about 3kts to maintain directional control.

In the event the rotors are stopped, deploy the drag chute to avoid the cab braodsiding and keeling over.

IF contact with the rotating blades is imminent during the landing remember that the advancing blade striking the swell first will eject the gearbox backwards out of its housing. Allowing the retreating blade to strike first will cause the gearbox to eject fwd and dwon into the cockpit!

Anything more than 3 degrees nose up during touch down will almost certainly cause the tail rotor to contact the sea surface.

ANY prolonged delay on the surface will inevitably result in being unable to take off again due to ingress of water and overload. Cof G shifts perilously
fwd.
Russian Mi-8 Take off from water - YouTube

The bottom line - if you land on the sea - stay put! Hence no more sea hull shaped helos.

[Ascend Charlie]

Once he had done an OEI splash in the water, that Mi-8 pilot did the right thing by lowering his gear to get the cg lower to reduce the risk of capsize.

But when he decided to try a single-engine takeoff, he left the gear down causing the nose to dip under. Goose.

[Thomas coupling]

AC: gear up or down makes absolutely no difference whatsoever - trust me.
The sole reason this guy stoofed was because he probably critically fractured the airframe on landing so heavily and the water poured in, predominently into the electronics bay in the nose causing a complete loss of Cof G limit fwd.

Gea down does assist stabilisation in low sea states, but doesn't affect takeoff or landing.

[Ascend Charlie]

Well, there you go!

[212man]

Some research here:

www.caa.co.uk/docs/33/2005_06.PDF

[bondu]

212man,

Thanks for this: I had some of the integrated reports already, but it's great to get the whole thing.
A quick look through shows that while some research has been done on the subject, it was quite some time ago. And the CAA paper is 7 years old. Which means most of the current types flying over the North Sea would have been certified long before the paper came out (the AW139 may be the exception). Also, it appears that very few of the recommendations have been acted on.
The Met Office now has a very comprehensive data set on waves around the UK, far better than was used in all the previous studies. And all those studies of wave dynamics agree that wave steepness is the crucial factor.
It is also very interesting to see the different definitions of 'ditching floats' and 'emergency floats'.

I found the following quote from Appendix A page 23 very 'illuminating'!

These ditching incidents are summarised in Annex D, and it can be seen that about half the total ditching incidents involved the S61 type. If one ignores the capsize of a Chinook which was leaking through the stern ramp, all the capsizes were S61’s. However, none of the other types ditched experienced weather worse than seastate 2!

Admittedly, this is old data, but the old S61 was one of the types you would expect to behave very well when in the water.

Anyway, thanks again for the link. I hope many of the readers of this thread will have a look at the report.

bondu :ok:

[Agaricus bisporus]

Specifically, at what angle the aircraft is likely to capsize. I know this will vary with load and wind loading on the side

(my bold)

Sounds like someone needs to revisit their basic mechanics. Very basic mechanics indeed.

[bigglesbutler]

Specifically, at what angle the aircraft is likely to capsize. I know this will vary with load and wind loading on the side

(my bold)

Sounds like someone needs to revisit their basic mechanics. Very basic mechanics indeed.

Not really, he means the first load is internal (pax and bags acting as ballast) and the second is the windage on the airframe acting like a sail to cause instability.

Si

[Helinut]

Once the hele is in the water (rotors stopped) it is a (not very good) boat, so you are into naval architecture. I suggest the best way to look at it is in stages:

Static Stability
Dynamic Stability due to wave motion
Other wave effects (breaking waves)
Wind Loading

Although you may just choose to look at lateral stability, the complete picture would need to consider longitudinal too. However, the numbers would probably work so that the hele would capsize laterally before longitudinally. What would be sensible to consider might be affected by whether and how likely it would be that a drogue got deployed, and how it was deployed. This could be type and even operation specific, depending upon whether and where a drogue was deployed.

For static stability, you need to consider the position of the C of G and the centre of flotation. Statically, the hele will capsize once the C of G acts vertically outside the centre of flotation, giving a positive upsetting force/arm. The basics are against you because typically the C of G is high and the centre of flotation low, which is (one of the reasons) why heles are not good boats.

Dynamic stability is related to how the wave moves and changes the slope of the water under the hele "hull". It could act with or against static stability. The worst case is, of course, when the two upsetting forces/arms act in unison.

Wind loading is a rather different animal. It need not act in the same direction as forces generated due to the sea surface.

You also need to consider the effect of breaking waves as well, if you want to get a complete picture. They could provide a very significant extra upsetting force, as a transient addition to the other forces.

My suggestion would be to look at static stability first. If you can get a forecast of sea surface gradient (which in principle is possible), you would get a first order estimate. There are lots of variables though, and you need to understand how they all might change.

Although there are other ways of dealing with it, the static stability problem is capable of being calculated using some basic naval architecture and readily extractable physical data from the aircraft.

[Agaricus bisporus]

Biggles, as I understand ship's stability it is concerned with three parameters only. Centre of Bouyancy, Centre of Mass and Metacentre.

A vessel capsises at a certain angle of heel determined by the relationship of those parameters and no others. It is a simple system of mechanical moments. Wind cannot change any of those as they are all physical characteristics if the vessel - so as load is part of the vessel's mass it certainly features.

Wind may well cause the heel, but it can't affect the angle at which the system becomes unstable and falls over which is what stability is concerned with.

[bigglesbutler]

AB, sorry am not a mariner just yet, hopefully getting my day skipper and on later. I understand what you are saying and it was misunderstanding in that I thought you had misunderstood Bondu's post. No harm no foul :D.

Si

[Agaricus bisporus]

Biggles, I doubt you'll learn much about stability on a Dayskipper course (or anything else taught by the RYA) but you'll certainly have a great time - and the fun that comes after is hard to beat. Best of luck!