# Why do high-speed water impacts / ditching usually result in structural loss of the airframe?

Plywood sheets can penetrate tree trunks unscathed - when pushed by hurricane or tornado winds. Why wouldn't an airplane fuselage, wings and tail behave the same way impacting ocean water at a steep angle at 300-500 mph? That is, penetrate and remain intact. I have seen a lot of supposition written about this which seems based on common sense, but common sense might be misleading. Has the outcome of such an event ever been demonstrated?

• The penetration succeeds because there is a void behind the object being penetrated, that's not the case with the ocean. – ratchet freak Dec 30 '14 at 0:00
• possible duplicate of Has a commercial airliner ever crash-landed in an ocean? – Danny Beckett Dec 30 '14 at 1:12
• @ratchetfreak - I can just picture an airline busting all the way through the entire planet and shooting out the other side... at the same speed it was going! ROFL. – Keegan Dec 30 '14 at 3:37
• And airplanes are designed to crumble, similar to an automobile, in an effort to absorb energy that would otherwise be transferred to the passenger(s). – Keegan Dec 30 '14 at 3:38
• However you look at it, a piece of plywood and and an airplane (no matter what type) are just not comparable in terms of structure or structural performance. Plywood may withstand several 100 g´s, while an airplane can only take a few without (gloabally) crumpling like tin-foil. – yankeekilo Jan 1 '15 at 10:51

## 4 Answers

Plywood sheets can penetrate tree trunks unscathed

The plywood isn't unscathed by the process

Why wouldn't an airplane [be unscathed by] impacting ocean water at a steep angle at 300-500 mph?

The aircraft will not travel at that same speed underwater. That means that a considerable force is applied to the aircraft to decelerate it in a short distance. That force greatly exceeds what the structure was designed for.

• The aircraft will not travel at that same speed underwater. Not for very long, at least... – reirab Dec 31 '14 at 20:41

The plywood was like a cleaver blade hitting the tree. Lots of relatively dense mass and momentum concentrating on a very thin edge.

An airplane wing (let alone fuselage) is not a thin edge. It is blunt, rounded, and thick. Behind that edge is a low mass and low density object. For the size, aircraft are very light weight and are a carrying a tiny fraction of the momentum a solid (non hollow) aluminum object of the same shape and volume would carry.

If that plywood wasn't a solid material but rather was hollow like an airplane (thus reducing it's mass, momentum, and consequentially the force acting on the tree), we'd probably see the tree intact and shattered remains of the plywood on the ground.

• "An airplane wing (let alone fuselage) is not a thin edge" - Some wings are. – Sean May 1 at 4:58

Has the outcome of such an event ever been demonstrated?

The outcome of an aircraft flying nose-first into water has been demonstrated an unfortunately-large number of times. And it doesn't end well for the aircraft.

The situations you're describing are very different. The physics of a plywood sheet penetrating the trunk of a tree happen because of the extremely high winds. When the wind is 150 mph (and parallel to the board,) the path of least resistance may very well be straight through the tree. Any deflection of the board to one side or the other would increase its cross-sectional area relative to the wind and, thus, would be countered by the force of the wind itself. If you were to otherwise attempt to fire a plywood board into a tree (in the absence of wind,) I think you'd find quite different results (though not quite as different as the aircraft impacting water scenario.)

On the other hand, the physics of flying an aircraft nose-first into water at 300-500 mph differ only slightly from those of flying an aircraft nose-first into the ground at 300-500 mph. In the first few seconds, the water will displace somewhat more than the ground would have, but not much. It takes a lot of force to accelerate the water to the sides quickly enough for it to be displaced by the aircraft at 300-500 mph and, since the force is applied equally (but opposite in direction) to both the aircraft and the water, the forces applied to the aircraft will be plenty to destroy it very quickly. The pieces of the aircraft will continue to lose speed until either they're floating on top of the water (for pieces that are less dense than water) or are sinking at their terminal velocity into the water (which will be quite slow in water.) Meanwhile, all of the kinetic energy of the aircraft is being dissipated very quickly into doing things like crushing the airframe and/or ripping it apart, accelerating water out of the path of the aircraft, heating up the water and (more so) the aircraft, etc.

As far as why the forces would be enough to crush the aircraft (as opposed to what would happen if you fired a long, narrow, cylindrical chunk of solid aluminum into the water at the same angle and speed,) the aircraft is relatively hollow. The cross-sectional area impacting the water is much greater for an aircraft than for a cylinder of solid aluminum of equal mass and length, which means that a much higher total force will be applied to it (similar pressure, but integrated over a much larger surface area.) Furthermore, since a solid chunk of aluminum is a bit more dense than the air that fills most of the cabin of an aircraft, the aircraft would be more apt to crush. The portion of the hull impacting the water would be accelerated into the space behind it, since the forward force being applied to it by the contents of the cabin and the more aft portions of the hull would be much smaller than the backward force being applied to it by the water it's impacting. That is to say, the hull would be crushed very much like an aluminum can. When you stomp on an aluminum can with your shoe, the can crushes rather than penetrating your foot (at least you hope so...) This situation is much more analogous to an aircraft ramming into water than the case of plywood in a hurricane encountering a tree that you mention.

Finally, as others have mentioned, the reason that the plywood can actually exit the tree is just that it doesn't get slowed down completely to a stop by the collision and friction with the tree as it's entering the tree. For a sufficiently-narrow tree, this allows it to exit the other side before it's completely stopped. However, for a sufficiently-thick tree, the situation would be largely unchanged except that the leading edge of the board would stop somewhere inside the tree.

• Good observations in these answers. At high speed impact, a lot of water would have to be moved out of the way by the nose, producing large forces. The plywood, to survive, only needs to split apart the tree, must be undergoing only compressive force, no bending, and plywood is pretty strong in compression. So is a cylinder, although as you point out, there is a limit. Thanks. – Larry Jan 1 '15 at 2:38
• Plywood into tree super slo mo might show the plywood impact shock splitting the tree, plywood bends slightly but goes through, little resistance to split as there is air on either side. Water is (unfortunately) non compressible. As has been stated, at high speed there simply isn't time to push it out of the way. The impact into water destroys the plane in a fraction of a second. – Robert DiGiovanni Dec 20 '18 at 16:37

In the case of plywood penetrating a tree trunk, consider the conservation of energy. The plywood might be flying at a certain speed before impact, some kinetic energy is used to break the tree trunk, and then the plywood (presumably) keeps flying at a reduced speed.

Still considering conservation of energy, an aircraft impacting the ocean obviously cannot "continue" flying at a similar speed, so all that kinetic energy (and there is a lot of it) must be used to do some work. That work will be crushing the (relatively delicate) airframe.

It is possible to make a missile or something that can penetrate water (eg. Aerial torpedo). However, such things are designed to do so and passenger aircraft are definitely not.

• You seem to be claiming that the kinetic energy of the plane is converted into heat (by deforming the plane's structure). Yes, a significant amount of the energy will go into deforming the structure but a large amount will also go into launching water into the air. You're not considering enough of the system to be able to use conservation of energy. – David Richerby Jan 1 '15 at 2:14
• Could be conceptualized by treating water as an elastic solid. There is very little "give" or cushion. Water can only be displaced. The plane probably crushes 10x faster than the water can be moved away. High divers know to lock their hands and go "straight in", minimizing area of impact. But at 300 mph, water is essentially no less forgiving than stone. – Robert DiGiovanni Dec 21 '18 at 0:08
• @RobertDiGiovanni: I think you meant to say "no more forgiving". – Sean May 1 at 4:59