May I offer the following thoughts. Reference to an earlier thread ("737 Wind increment to Vref" on Tech Log, around OCT01) may be useful.
. .(a) the empirical profile equation normally used for AFM and certification purposes is
wc1/wc2 = (h1/h2)^(1/7).
This equation more or less fits the typically observed lower level wind profile where there are no obstructions of any note, ie for a largish, flattish sort of area.
In particular, it is used to correct the reported tower (flight test) wind (which will relate to the height of the anemometer) back to whatever the AFM data is (to be) based on, typically 50 feet, so that the distance calculations from the AFM have some repeatable basis.
(b) the AFM scheduled distance data is based on
(i) maximum effort landing factored by 1.67
(ii) 0.5 (1.5) of the advertised headwind (tailwind). . . .so there is a degree of conservatism to start with.
It was long ago considered appropriate in turboprop operations that a maximum additive of 15-20 kt would provide a reasonable protection against overrunning the scheduled distance and this was stated as such in some manuals.
For approach speeds of around 100 kt an increase of 20 kt would result in a notional distance increase of around 45-50 percent. Probably the 15-20 kt wisdom figure could be increased sensibly to account for the higher approach speeds on jets (eg at 140 kt an increase of 30 kt results in a similar reduction in basic margins... however, that has not occurred and the earlier recommendation continues.
In addition, it is generally common to land with LDA exceeding AFM minimum scheduled distance (minimum distance required, if you like) so, for many landings there is plenty of fat, assuming that the pilot constrains problems associated with floating during the flare.
. .To get a feel for the variation in wind component, consider the following sample data. Although I have extrapolated to 1000 ft for the exercise, I suspect that this is not appropriate in the real world and is included only to emphasise that the main concern is in that final 100-200 feet or so of the landing approach.
Anemometer height 50 ft. .Anemometer H/W 40 kt . .Ht (ft) 5 10 20 30 40 50 75 100 200 500 1000. .W/C (kt) 29 32 35 37 39 40 42 44 49 56 61
(apologies for the formatting ... I have no idea how to get it to accept what I am trying to send .. )
So, for instance, the postulated wind reduction between 50 ft and 10 ft (40 ft change) is around 8 kt while on the high side an 8 kt change fits about 150 ft height change - hence the main concern is around the vicinity of coming over the fence.
For the pilotage point of view, the concern comes down to balancing
(a) imprecise knowledge of the actual wind profile for THIS particular line operation landing especially when one considers that the real world generally has lots of wind-affecting bumps and lumps.... against
(b) a need to guard against overrunning the available runway.
As to the Boeing advice (which is not startlingly different to that offered by other manufacturers), they needed to give some imprecise guidance for a very imprecise topic. Just where does one start the speed bleed ? ... I suggest wherever you think is appropriate considering the specific conditions pertaining to the particular landing .. the aim, typically, is to constrain the approach speed to remain above Vref plus a bit but not above around Vref + 15-20 kts as the aircraft transits the landing flare .. I don't get too fussed if I end up a little faster than I would have liked (ie aimed for) in the flare .. BUT .. for me
(a) I use auto and manual brake as seems appropriate for the specific conditions and LDA.
(b) I don't tolerate aiming long.
(c) I don't tolerate anything more than a sensibly minimal float .. if the float looks like presenting a problem .. either put it on the ground or execute a missed approach.
Especially in less than ideal conditions, I am far more concerned about running out of speed in the very late landing approach (is it obvious that I have embarrassed myself in the 727-200 from time to time ?) than overrunning the runway so I try to maintain the approach speed down to a low height, say, 50 feet.
[ 04 February 2002: Message edited by: john_tullamarine ]</p>