Way back in the good old days the gang of friends got into flat bottom drag bosts and hod rodded around Lake Berryessa in California. At 100 MPH if you reached over the edge of the boat and put your hand in the water it felt like it was being smashed on Concrets. I couldn't imagine the forces on a plane landing in the water . What speed does the average airliner stop flying?
Depends on the altitude, temperature, gross weight, configuration, and load factor (bank angle or G required).
There isn’t just “one speed”.
But there is usually one angle of attack at which the wing stops flying.
So, to answer the question, let’s back the discussion up to angle of attack. The angle of attack is the angle between the mean chord line (a reference through the wing’s chord, or thickness) and the relative wind (the direction from which the airflow is hitting the wing). as you increase the angle of attack, you increase the lift right up until the critical angle of attack, at which the wings stalls. When a wing stalls, the airflow over the top of the wing becomes separated, and the lift is greatly reduced.
The faster the air moving over the wing, the more lift can be created, so if I need a certain amount of lift in order to hold say a 300,000 pound airliner in the air, I need a combination of angle of attack ( but it must be less than the critical angle of attack) and airspeed.
But it isn’t really airspeed that we are interested in, it is the amount of air flowing over the control surface. In other words, if the air is thinner, because of temperature or altitude, the speed with which the air must be moving to have the same effect has to increase, so, in the cockpit, we have a gauge for airspeed, but it is known as indicated airspeed. That doesn’t mean the speed with which we are moving through the air, rather, it means the amount of dynamic pressure caused by both the air density and its motion.
So, properly, lift is created by a combination of both indicated airspeed, and angle of attack.
The true airspeed is the speed with which each molecule of air is going by the airplane. If the air is thinner, then indicated will be much lower than true at relatively high altitude, 35,000 feet were airliners fly, the indicated airspeed, that is the measure of dynamic pressure, is about half of the airplane true airspeed, that is the airplane velocity with respect to the air through which it’s flying.
On a standard day, in which the air density is precisely equivalent to a reference value, at roughly 59°F, then the indicated airspeed and true airspeed are in fact, the same. But if it is colder or warmer, or you are at a higher altitude, the relationship between indicated speed and true airspeed can vary.
The next thing to consider is load factor. If I roll an airplane into an angle of bank, say 30°, then I need slightly more lift to keep it in the air. That’s because the lift generated by the wings is in fact, perpendicular to the wings, and as I roll the aircraft into an angular bank, that lift vector: direction as well as magnitude of the lifting force, is off vertical. So, only the portion of the lift that is opposite gravity is actually keeping the airplane in the air, the portion of the lift to the inside of the turn is the force that actually causes the aircraft to turn. This becomes a relatively straightforward, trigonometry problem, and at 30° angle bank, you need about 1.2G, and so, 1.2 times the airplane weight, for the airplane to stay in the air. at 45° angle bank you need 1.4 G for there to be enough vertical component of the lift vector to keep the airplane in the air.
Finally, we need to think about configuration. Ever notice how the wing on an airliner has articulating components that change shape depending on the aircraft speed? On the front, or leading edge, of the wing are devices known as slats. On the trailing edge of the wing, our flaps. Both serve the same purpose, to keep that layer of air moving across the top of the wing attached to the wing by changing the shape of the wing, we can keep that air attached at a much lower speed. So, airliners use some amount of flaps and slats for takeoff, and a bit more flaps and slats for landing. This allows the wing to operate at a lower angle of attack, and avoid stalling, even though the amount of indicated airspeed is lower.
So let’s go back to your question. Since we are talking about water impact, we are at sea level, and we can assume that indicated airspeed will be relatively close to true airspeed. This, of course, assumes that there is no headwind or tailwind, so will assume that it’s both a normal day, in terms of temperature, and that the winds themselves are calm. We can also assume that it was a wings-level touchdown, and so, no extra lift for load factor/angle of bank.
The next question to be answered, is the aircraft configuration, were they able to get the flaps and slats extended? Without hydraulic power (which generally requires an engine to be running) to operate them, they may not have been able to, and so we are talking about a “clean“ wing. A clean wing stalls at a much higher airspeed than one on which the flaps and slats are extended.
The final question to be answered, of course is the aircraft weight. A 747, at maximum weight, with a clean wing, will stall at about 235 knots of indicated airspeed. Its “clean maneuvering speed“ is 1.3 times the stall speed and that’s 285 knots, clean, at max weight.
So, if we assume a normal landing weight on a 777, and we buy the conspiracy theory that the pilot did this deliberately, so we assume that the engines were still running, then we can reasonably estimate that this ditching took place with landing flaps. With full landing flaps, and normal passenger load, a 777 with nominal fuel reserves, will have about 135 knot approach speed.
Somewhere around 160 miles an hour, again, no wind, normal day, and all of the other assumptions that we’ve placed in that scenario.
But if the airplane were considerably heavier, and the flaps were retracted, and a clean wing, the touchdown speed (only a bit above stall) would be much closer to about 250 knots. Close to 300 mph.
Sully proved that you can successfully land an airliner on the water and have it survive the landing, but the Hudson was smooth that day, and the aircraft was relatively light, so he managed to touchdown at about 125 knots. A heavy sea state, and large waves, would greatly reduce the likelihood of impact survival, even at such low speeds.
A landing as slow as possible, also improves the chances of survival, so burning nearly all of the fuel prior to landing, would be advantageous if your goal was to have the airplane survive the impact.
Wrapped up in all of this discussion, remains the question “why “? Why fly off that way? Why make the landing survivable, if your intent was to sink the airplane?
I can answer the technical part, but nobody can answer the motivation, part, except through speculation, and guessing in which this author is fundamentally engaged.
