Stirred up some thought .. good to see.
adding just 50% of the headwind component makes you go actually slower over ground ( comparing to vref no wind ) and reduces the disctance to the tdz on a steady glide .
I think you might be missing the point a tad. By including some wind in the calculations (whatever factor) the result is that the landing distance required is reduced. Certainly, for a given airspeed, the groundspeed will reduce as well but the concern is that the scheduled distance is less and, for a limiting runway you have less seal to play with ..
and neither hit hard nor stop descend fully , maybe climb again one feet, trying to correct for a smooth touchdown and so floating over the runway.
If your technique and finesse is of such a high order, you might be one of these exceptional folk who can flare onto the ground every time. As I indicated, as an example, I have managed only two landings where I didn't know I was on the ground .. others, I acknowledge, are rather more skilled than I.
However you might like to style it, the aim is to get the aircraft onto the ground as soon as is feasibly practical without shaking out everyone's false teeth. Clearly, on a runway much longer than required, one can play about a little and aim for a smoother touchdown. However, the problem here is that one develops undesirable habits which might bite one on the occasional limiting runway ?
Philosophically, for a dry runway, one endeavours to flare onto the ground or, at the least, flare and then put the aircraft onto the ground without worrying too much about finesse in the touchdown. I was fortunate in that I flew little aeroplanes (737/727) and, generally, had plenty of runway.
Of course we all like to get a nice, smooth touchdown. Just be careful that you don't fall into bad habits on the usual long runways and then get burnt when you have a short one ...
Landing distance calculations should be adjusted to account for the higher Vref.
.. consistent with whatever guidance the AFM provides .. Keep in mind that the landing distance factor is there to cover a host of minor errors and problems .. including a modest overspeed into the flare.
in line operations (and not a prescribed factor) we should use some distance that is between the 60% and the runway end?
No, I don't suggest that at all.
(a) in normal operations, one should ALWAYS have the scheduled distance available - preflight planning or inflight management. The QRH provides emergency data .. it doesn't mandate that you put yourself into a situation where you commit to landing with only such emergency distances available.
(b) in an emergency, the commander may exercise the relevant regulatory authority to vary the rules to suit the needs of the occasion.
However, the commander ought to consider very carefully the wisdom or otherwise of reducing available factors below what might reasonably be available to him/her.
For instance, inflight one has some emergency necessitating an urgent landing. If one has NO alternative, one will take one's chances with whatever only runway one has, as a general rule.
If, on the other hand, one has several runways available then, as PART of the emergency planning management, one should consider the various runways - strengths and weaknesses - as part of the overall strategy with the intention to end up with the best compromise practicable to achieve the most favourable outcome.
do we as pilots have to cross the runway threshold at Vref? Is that mandatory?
No, not necessarily. One should be aiming to operate in accordance with the AFM protocols and company SOP. Generally, this will prescribe a modest margin above Vref.
The speed correction should be
The actual recommendation will vary a little from one OEM to another. That cited follows the Boeing line
should we reduce from Vap from Vref somewhere before crossing the threshold
We should aim to follow whatever AFM and SOP requirements are prescribed in line operations.
question if the certification is just so that the manufacturer is demonstrating that the airplane can perform adequately
Performance landing trials are for the purpose of getting numbers for the certification process. The average line pilot will NOT reproduce the TP's efforts.
I was assuming that all pilots should perform exactly, or better, as the requirement dictates.
The TP provides the absolute brick wall data - it is foolish, in the extreme, for the line pilot to presume that he/she will be able to achieve raw test data results derived from the factored AFM data on any given occasion.
The certification process puts a factored pad in for the rest of us average pilot folk. Our responsibility is to do our best to fly in a manner which is compatible with AFM and SOP requirements and recommendations.
The 60% safety margin should be considered a normal minimum,
concur - my bolding. Note that, depending on the source, AFM landing data may be factored or unfactored and the use needs to account for that.
particularly on wet runways where there can be many more unknowns and greater variability in the conditions.
Generally, and traditionally, wet runways attracted an additional 15% on top of the normal factor. This has been superseded in many cases by contaminated surface protocols