VRef speeds and half the HW comp.
This subject was discussed widely some months earlier under Tech Log and finally faded into the sunset unresolved as far I was concerned. On reflection this would have been a performance engineer's area. Here goes:
Since time immemorial, Boeing have recommended a speed additive of half the steady headwind component plus all the gust factor (up to a combined total of 20 knots) to be added to the Vref speed for landing. The HW additive to be bled back approaching touch-down, while the all the gust factor is retained.
Forget the gust factor for the time being - my question is what is the origin of half the steady headwind component policy?
It is obviously (?) associated with wind gradient of sorts. Normally the free stream wind starts around 2000 feet above the ground and below this the wind eases off due to ground friction.
If for example the full headwind is 30 knots, then following the Boeing rule, 15 knots is added to Vref.
At some point or other this additive is deliberately bled off such that the aircraft arrives over the threshold at Vref.
You cannot bleed off 15 knots solely by the actual flare manoeuvre - so at what altitude should you deliberately aim to start bleeding back this 15 knots additive, yet still maintain the requirements of a stable approach below 500 feet? In other words maintaining the correct speed for the conditions.
I understand that for purposes of certification landing length performance, only half the full headwind component is assumed and twice the tailwind component.
That said, is there any relationship therefore between this certification conservatism of half the steady headwind - and the Boeing policy of adding half the HW component to Vref? If so, why?
If the landing is made using autothrottle, Boeing require no additives to Ref apart from the usual five knots to the approach speed - it too being bled off in theory approaching touchdown.
And how is "approaching touchdown" defined? Is it somewhere on late final (say) 500 ft agl - or is it the period between actual flare manoeuvre and actual touchdown (around 2-3 seconds?).
Sure, autothrottle response is fast, but from what I have seen, the human hand moves twice as fast - so why no additives with autothrottle and possibly significant speed additives required up to 20 knots simply because the pilot is manually using the throttles?
Why is half the steady headwind component added to Vref and not (say) one third - or even two thirds the steady HW component.