Let's suppose a catastrophic EFATO has occurred while climbing out of JFK from Runway 31L, the aircraft is at its MTOW and the best option is to set it down in Jamaica Bay. How long would the aeroplane float and what are the chances of surviving?
Aircraft are designed with ditching in mind, but the outcome depends on many circumstances, most importantly on how calm the water is and how well the aircraft can be controlled.
The Wikipedia article on Ditching lists emergency water landings and other accidents where aircraft ended in water.
There is no ditching by 747, so we don't have any specific reference. However:
- B747 is not that different from other types of aircraft. Controlled ditchings on calm water were successful in different types; it is likely that one would be possible with B747 too.
- How long the aircraft would remain afloat depends on the damage. But if it did not break up, it would likely manage to keep afloat for the 90 seconds needed for evacuation. And the slides can be used as life rafts.
- There was one case where B747 ended up in water after overrunning runway (so at lower speed than landing), China Airlines flight 605, and the aircraft did remain afloat long enough to evacuate it.
- Also any aircraft that will be used for flying across large bodies of water, which obviously includes all variants of B747, must be certified for ditching. This involves designing it so that, at least according to model, a controlled ditching will be smooth enough not to cause serious injuries to occupants and will leave it afloat long enough for successful evacuation.
Concerning floating: planes tend to float for a long time, potentially indefinitely. The reason for this is that they are much lighter than water. For example, jet fuel is about 80% of the density of fresh water, so even if the wings were fully loaded with fuel, they would be lighter than water. Some of the space in the wing tanks is air, which of course is even lighter. There are vents in the tanks, but it would tank a long time like many hours or probably days before a wing could even partially fill up with water.
Concerning survivability: there is no way to predict the outcome. It would depend completely on the reactions and skill of the pilot and exactly when the engines lost power. In general, landing on water is not a problem, as was seen in the accident of US Air 1549. A big plane can (and has) actually run over a small house and stay relatively intact. So, even if it went up one of the beaches on the spit it might be OK. The biggest threats are running at high speed into an object or terrain feature, or stalling; both of those not happening are largely dependent on the skill and cool nerve of the pilot.
When aircraft take off, they usually have a steep rate of ascent which would be a stall configuration in unpowered flight, so if the engines fail the pilot has to immediately react to avoid a stall. When this happens, the pilot has to react within a fraction of a second or so. If the pilot panics and pulls back on the stick, it's game over.
If the pilot is good, they will have an "emergency plan" and will mentally know ahead of time exactly what they will do if they lose power at any given point on any given runway. That plan includes a landing spot. For example, when I fly to a new airport I look at Google Earth and decide where I will land in the case of an emergency during take off. The plan sometimes has to have height thresholds, too. For example, if I lose power at 100 feet I do this, but if lose power at 400 feet I do that. The plan always has a "minimum turnaround altitude". This is the height where you are safe to do a 180-degree turn and go back to the runway. Note that the plan has to recalculated every time you fly because of wind. So, if there is a crosswind coming from one direction or another that could affect your choice of emergency landing location.