I come across this video for engine failures bellow 50ft in DA42NG, it’s impressive how the aircraft float on rejected takeoff vs landing flare, anyone has an explanation?
POH data show 250m “float during landing flare” (delta between 50ft landing distance and ground roll), however on “rejected takeoff float from 50ft” the aircraft goes as far as 600m !
I have noticed the same in gliders during winch & aerotow failure bellow 50ft, they will just float forever compared to the float on landing without air-breaks
I guess ground effect, takeoff/landing config drag and maybe small induced drag during low G flying comes into the mix?
On landing flare, the wings are positicely loaded at 50ft, so they may have more induced drag?
Maybe unrelated but one can make similar observation during bounces when stick is held where it is, aircraft can eat lot of runway at slow speeds, maybe it’s just psychological…
More flaps in a landing flare?
Probably it’s obvious drag from flaps, looking at DA40NG POH you would get max 400m delta between 50ft landing distance and ground roll without flaps
The trajectories between dead engine on takeoff at 50ft and landing from 50ft are not the same? even on same IAS one is less draggy than the other, I think the former is less draggy (if one unloads the wings) and maybe has more energy (positive ROC or 0VSI) but I doubt this explains everything…
These I am guessing are supposed to proper certification tests, so the pilot can’t use unusual skill, i.e. there could be a considerable delay before the power on the non failed engine is reduced, there needs to a reasonable amount of time to identify the problem beyond the basics of just flying the plane. FAR 25 is something like 2 secs, I am not sure if there is time it laid out for this category of aircraft but it can’t require unusual skill.
I’m not sure what they were testing there. However, when I got my PPL in 92 we did so called touch and go with cut. Procedure is stick fwd to maintain speed, then full flaps. This hasn’t changed, we still do this, several times until it is second nature. I wonder why they don’t extend flaps, is this some particular twin procedure, or just the way the tests are supposed to be?
I think this is a really worthwhile topic for discussion. The Diamond video is very useful for thinking, though being a twin, is not entirely the same as a single would be. My first, and most basic thought to the question is that if you’ve taken off properly (as the Diamond test pilot certainly did) you achieved Vy very early, and certainly before the engine cut. Vyse for the DA-42 should be 82KIAS, so expect that was the airspeed at the cut. Vyse is a little faster than the final approach speed of 76 KIAS (though noting there’s a flap setting difference in there too). And, the Diamond test pilot will have done an awesome job of setting it down neatly, rather than just dumping it on with little care for speed at touchdown – just get it down. The other observation I’ll make is that after an EFATO, and while establishing a good glide return, I would be retaining my approach speed longer, and accepting a float in the flare if that were the result, rather than slowing to approach speed earlier in the descent from 50 feet.
I’ve done the engine cut at 50 feet testing on several singles, and it ranges from “hmmm, I did it!” to terrifying, depending on what speed you’re testing. I agree with Ibra’s observation that things should be more or less the same, if you’re landing power off from 50 feet, rather than from an EFATO, or an intended approach. This is a skill which comes only with practice, and it somewhat type dependent, but a really important topic for occasional review.
Pilot_DAR wrote:
My first, and most basic thought to the question is that if you’ve taken off properly (as the Diamond test pilot certainly did) you achieved Vy very early, and certainly before the engine cut
That is a good principal in pilot life: always get Vy as soon as possible 
highly applicable in Diamonds, problem solved: they simply don’t publish Vx for their aircraft:
- They do not publish Vx for the twin DA42 as it already has a blue line Vyse to catch
- They do not publish Vx for the single DA40, it’s 2kts-2kts above stall VS1 and judged “unsafe for climbs” (stall horns all the time and tiny pull, gust, yaw, cut, error…and it goes to the rats), they do publish a light version" V50, sort of Vx to be used in initial climb up to runway threshold & bellow 50ft height
PS: hitting obstacles at Vy is still a risk that is largely exaggerated, in the other hand you have better speed and better “total energy”, besides one can view it as personal pride: at least someone who died while flying his aircraft efficiently rather than dragging it
Pilot_DAR wrote:
The other observation I’ll make is that after an EFATO, and while establishing a good glide return, I would be retaining my approach speed longer, and accepting a float in the flare if that were the result, rather than slowing to approach speed earlier in the descent from 50 feet.
What is the logic? increase wing load in the recovery and having more energy to flare?
Simply to retain a reserve of energy to account for a misjudgement or wing change. I can always slip it off at the last second if I’m too fast. A very important principle is that if you’re going to misjudge a forced landing, misjudge too fast, and go off the far end slowly, rather than misjudging, and stall and crash short.
So two factors in addition to landing configuration (flaps or airbrakes):
1) As Pilot_DAR mentions, if you’re going faster you will have more energy. Also, a plane with an upwards trajectory has more energy to lose than a plane at the same altitude that is already descending.
2) If you have an engine failure, your priority will be to push the nose down and avoid stalling, probably resulting in a more aggressive descent than if you maintain a landing configuration just a little above the stall speed. This means that you are likely to end up in ground effect sooner than if you were in a stable descent, and you will burn off energy more slowly as a result.
kwlf wrote:
Also, a plane with an upwards trajectory has more energy to lose than a plane at the same altitude that is already descending.
Energy is simply the sum of potential and kinetic energy. It’s a scalar and independent of direction. Consider a loop with equal, but opposite velocity in both verticals, lets say 120 kts. The energy is exactly the same at those points when the alt is the same. You could also fly horizontal circles at that alt at 120 kts, and the energy would be the same, or simply straight and level.
An aircraft above the runway at 50 ft. IAS is constant at 80 kts, VSI is constant at -500 ft/min, has exactly the same amount of energy as an aircraft at 50 ft, IAS of 80 kts and VSI of +500 ft/min. The difference is in the power (rate of change of energy). The one landing has negative power (losing energy), the one taking off has positive power (gaining energy). This is why losing the engine right after take off is dangerous. Suddenly the power is gone (rate of change of energy is zero, when disregarding drag), but you are in an upward motion. Only one thing can happen: you gain potential energy and lose kinetic energy, and it happens fast, in particular in a light and draggy aircraft. You have to unload the aircraft immediately to stop induced drag from eating all your energy and to prevent stalling (you won’t stall, even at zero IAS as long as the AOA is zero). Doing this correctly (unloading the wings), and the aircraft will go farther than the other aircraft. The reason is much less drag in the unloading phase.
