RE: Landing on water
Obviously there are some airline employees here, which is good news especially if we have ‘front line’ airliner pilots as it were.
Water Landings:
How do submarines float? they displace less water than is surounding them, unless they want to take on more…..and they sink.
so if you have an airliner which is basically less dense than the water it displaces it will float. You can make rocks float if they have a very low density! however small stone chips work by being lighter(and thus having less force if you like) than the force available to the surface tension of the water.
There are a number of factors that need to be taken into account to achieve successful water landing. I would say the most important factor is having the aircrafts wings level.
You will be aware that if one wing is flying slower than the other, the faster wing will generate more lift, and raise itself, whilst also moving forwards in relation to the centre of gravity, whilst the slower wing will move backwards in relation to the centre of gravity. So if one wing hits the water before the other it is going to retard itself much quicker, and effectively ‘water loop’ generally snapping the wing off a the weakest point.
it is therefore essential that the pilot attempts to hold the wings as level as possible, thus leaving the swell in the lap of the gods to not rise up and grab one wing.
The second most important factor in my mind is the speed at which you enter the water. The faster you go(and the more dense the medium) the greater the deceleration force when you enter a medium denser than the one your currently in, ie water/ground so you can see that speed is proportional to the deceleration force. Which means that the slower you go the lower the deceleration force, there are other factors, but it is well known that if you hit the water from high up it is very easy to break all the bones in your body, whereas if you jump off the side of a pool, the worst you might expect is red ‘belly flop’ marks! lol
A major contributory factor to the survivability of a water landing has to be the location of the engines. Just about all airliner types now have underslung engine pods for one aerodynamic reason or another(the weight of the engines balances the twisting forces encountered by the wings in some cases- a WW2 german finding from the JU287 forward swept aircraft. We can relate a water landing to a vector sum, and hence a solution which sees the engine hitting the water first.
This then provides a turning moment not experienced by the rest of the aircraft. This turning moment 99.99999999999%(!) of the time moves so as to provide a nose down pitching moment( i.e. the water is decelerating the engines only, these are below the CofG and thus, from a side view, nose pointing forward, the turning moment is anti clockwise). So for the short time that the engine mountings(two or three loose bolts in most cases)can hold out against the decelerating forces( again proportional to cross sectional area of the vehicle or engine pod, and density of the medium you are entering, and the speed you are entering that medium)the engines provide a prohibitive turning moment- which is a bit of a bugger really. Ideally for water landings you want the engines above the CofG , on top of the wings, embedded in the wing/fuselage join or around the tail. Again, if an engine seperates the main structure before the other, this can provide a big yawing moment, presenting a larger cross sectional area for the water to act upon, should the fuselage still be still in the air. If an engine digs in, no amount of opposite yaw is going to help you out. Incedentally, this is a reason why you never see water landings with the undercarriage down, apart from the fact that very quickly they will be removed anyway!
So ideally you want to hit the water as slow as you can possibly go, with as high an angle of attack as you can(possibly so that the tail can act as a kind of rudder initially to help out with cancelling turning moments, and decelerating the aircraft slowly, before it begins to fully enter the water. This high AOA will also be productive in that the immediate decelerative forces generated by podded engines is a gradual buildup, instead of a sudden load, structurally this is a good technique to ensure a longer life of any part- shock loading is VERY bad! The wings also need to be level and rock steady, with no flap, with which to enter the water first.
This is of course with the fact the water is a flat calm, and that there is a good headwind, and that some complete and utter bastards havent tried to take over the aircraft.
The chances of a successful water landing in an underslung podded engine aircraft -like the majority of todays fleets-is very very low.
however I know of at least one succesful landing at sea, where the aircraft has in fact been able to float for some time, with some minor injuries only.
This is with regard to a Nimrod R1 which had to make a landing at sea due to a serious failure at low level(it is a well practiced procedure to fly nimrods on one engine for normal patrols at sea.) The aircraft remained structurally complete, and afloat for a number of hours. The crew had ample time to destroy any sensitive equipment and information, before exiting the aircraft.
No doubt the deciding factor in the ability to survive such a landing has to go to the fact that the engines arent underslung. the flat surface is excellent for a short period of planing on the water whilst decelerating slowly.
I have read the RAF inspectorate of flight safety bulletin booklet they published for this accident, I believe it took place in may 95, but I read the publication a few years ago, and this is also where I have seen the only picture of the aircraft, taken from a seaking, as the aircraft floats partially submerged in the ocean.
Looking through the net it appears that the aircraft suffered a massive engine fire on a test flight after a major servicing.
coanda
sorry for the long post!
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