Does the pilot refer only to VR to rotate or should he/she have an approximate landmark for the rotation point? (I guess Yes)
Actually, No. I'm not aware of any civil operator using physical length of runway to measure if the acceleration is normal, this would need marker boards on every runway, and take-off calculations would have to relate speed to distance for every runway used - possible but not really practical, tho' I believe the RAF did this at some of their airfields - don't quote or shoot me, don't know for sure.
The critical speed is V1, after that there is insufficient length to stop in the remaining length of concrete, and it would be very difficult to judge - if the acceleration felt sluggish - whether to accept the overrun or haul it into the air at the last minute and hope for the best.
My reading of this situation is that there was apparently plenty of runway left after rotation, which would indicate that the crew thought that they had reached VR - rotation speed - and had attemped to get airborne, otherwise they would have held it down as long as possible, or even run off straight ahead, which doesn't gel with the suggestion that they might have been airborne too slowly and stalled - why would they pull it off if there was a lot of runway left, unless they thought they were at the right VR speed ? Which then begs the question - why didn't it accelerate to V2 - if that in fact happened ?
Still a long way to go, we will just have to be patient.
Whilst writing this I missed the post of OnePostOnly, and can accept that his hypothesis has some merit, except that I still feel that they must have reached VR, and started to fly, the question then is why didn't the acceleration to V2 continue, or when leaving ground effect, with some problem that they weren't aware of, even a mistake in computing V speeds maybe, OnePostOnly's theory then took over ?
