Hudson, I fear that you are asking manufacturers to disclose the black art of their judgement. Having devilled with the dark side on more than one occasion I offer the following view that is by no means definitive, and I doubt that I will answer your questions.
The manufacturer is boxed in by several hard certification constraints that within them hide assumptions on what statistically the crew will do in order to achieve a safe landing i.e.
- Landing (stopping) performance is based a range of threshold speeds (Vat) at 50 ft between +15 and -5 (some -zero), and factored landing distance.
- The aircraft has sufficient control for a safe landing over a similar range of speeds including a decelerating landing from Vat-5 (tail scrape); also for some aircraft Vat +15 to touchdown (nose wheel first).
Then variability cuts in;
- For aircraft with good gust response characteristics - they do not loose lift quickly or change pitch / trim, then some manufacturers will only consider gusting winds. Thus, the additive is one half or one third of the gust, the choice of magnitude depending on manufacturer and aircraft type – the engine response, lift characteristics from prop wash, jet recirculation, ground effect, etc.
- For aircraft with poorer gust response then it is more common to add a factored steady wind, but some manufacturers even add a further additional wind gust factor.
- For more fun then mix in icing limitations if applicable.
The change in speed additive with auto throttle depends on the design of the system and how it responds to both steady wind shear (with decreasing altitude) and wind gusts (turbulence). Some modern systems mix airspeed and angle of attack to improve performance; others also consider ground speed, thus ‘sense’ the wind. During the flare auto throttle thrust is normally reduced by a schedule with altitude, but again this can be modified by GS; whereas the pilot can modulate the point of retardation, the rate, and even reapply thrust. The certification for auto systems only considers the simple rule.
Numerical and statistical wind, windshear, and gust profiles are given in JAR-AWO, but unless you are going to challenge an aircraft’s certification don’t even think of referring to them.
I would interpret ‘approaching touchdown’ as quite late during the approach i.e. from 100 ft start the speed bleed off just in time to meet the Vat constraints at 50 ft. Therefore, for an aircraft that normally looses 7 kt in the flare below 50 ft, it is reasonable to bleed the speed off from Vat +15 when below 100 ft.
Now this of course could make a mockery of the stabilised approach concept, but most manufacturers would argue that stability for a safe approach is required earlier in the approach in order to get the aircraft near Vat and 50 ft at the threshold.
The best advice for speed bleed off is to use judgement based on training, in conjunction with knowledge of the basis of landing performance and the aircraft limitations. Also, read the small print in the aircraft’s performance manual / AFM. Often there are additional restrictions for higher speed landings i.e add 2% landing distance for every 1 kt above Vat +15.
The accident statistics suggest that both judgement and knowledge is in short supply; thus, the key points are to aim to touchdown at the correct speed, in the correct position and then use all retarding devices immediately at their maximum setting. If the approach is unstabilised or the speed high at the threshold, then Go Around.
--------------------
Unless specifically authorized everything else is forbidden.