Originally Posted by
Gysbreght
The impact forces are primarily a function of the rate of descent at touchdown. With unlimited runway length the flight speed is far less important. More important is the optimum pitch attitude, which will dictate the horizontal speed at impact.
Part of the problem is that manuals may provide a procedure for all engines out, and a procedure for ditching. The ditching procedure supposes that you can maintain a certain speed and control the vertical speed with engine thrust. With all engines out you cannot maintain VREF40 and 200 - 300 fpm. Your rate of descent will be more like 500 - 600 fpm. That means that the normal approach speed is too low for a successful flare to ideally zero fpm rate of descent. You need to carry some extra speed during the approach that you then bleed off at low height and minimal vertical speed.
Someone asked if model ditching test are done with airliners. Yes, they are required for certification of transport airplanes approved for long overwater flights.
A ditching is a ditching. The difference is with power or without power? Obviously with power should be easier. Without power your rate of descent at touchdown does not have to be 500-600 FPM. You can make it as smooth as a normal touchdown if the sea state is fairly calm. You just need to maintain excess speed and trade that off, in the flare, until you're approaching the desired touchdown speed. With waves the equation to do it becomes much more difficult.
The reason US 1549 hit so hard in the Hudson is because Sully had run out of airspeed at over 100' AGL and was AOA limited. From that point on the AOA limiter prevented a stall but the lack of available airspeed (AOA) prevented any attempt at flaring to reduce the rate of descent at water impact.
The US 1549 investigation mentions using excess airspeed and converting that into a flare to reduce impact forces. Guys have to figure this out beforehand because there's no ability to regain airspeed at low altitude once you squander your airspeed and you have no power available.