B737 Gust correction

[PEI_3721]

I suspect that there have been several threads on gust additives; however, it might be worthwhile going back to basics.
Here are some that I recall:-
The aircraft certification will have demonstrated a landing at Vref-5 (at the threshold).
Most manufacturers accept that a normal approach speed (e.g. Vref+5) is an average speed datum and not an absolute minimum. Vref should provide aprox 20% speed margin above stall warning (stick shake) and 30% above the stall.
Aircraft stall, and thus stall margins are related to AoA; not all gusts will result in a AoA change proportionate to the indicated speed change, i.e. the gust may not be as severe as indicated by speed.
The performance certification normally assumes a speed loss of 7 kts in the flare.
The 'operational' margins in landing distance assume a maximum deviation of +15kts at the threshold. This margin also considers other aspects such as height above 50ft, reported wind error, etc, but these errors are not considered in combination, i.e. not high and fast together. Consider runway length required, add a margin if advised by the manufacturer e.g. increase distance by 2% for every kt above vref+5. The assumptions about performance on a wet runway provide lesser margins and there is no margin on a contaminated runway. Check runway length required / available very carefully.
The reported wind gusts are normally associated with 50ft near the runway. Thus the conditions earlier in an approach may not indicate the landing conditions – generally the lower you get the better – the less the gust (but not always). This variability may depend on the type and nature of the turbulence, i.e. mechanical turbulence from trees buildings. Thus the gust characteristics may be airfield specific.
Gusts greater than 15 kt are deemed to be windshear; use special procedures in windshear.
Be aware of how close the gust is to the crosswind limit; give yourself an additional margin. Take particular care with crosswinds on wet runways; check any revised limitations and give yourself a margin.
Approach and landing with less flap may provide a quicker aircraft control response or improved gust response, but do not forget the changes in landing distance.

These are just some of the issues to be considered in a landing decision in gusty conditions. If these are considered collectively they ‘box in’ an operation; the limits of this box should be reflected in SOPs, but these may not include the runway conditions and any changes to landing distance required. Thus crews have to assess the variables on the day, if these require an operation near the edge of the box then reconsiders the alternatives; all conditions requiring operations outside of the box should be diversions.
Any other ‘basics’ or considerations?

Airbus aircraft may have specific differences; see FCOM; also see the briefing notes in these sections - Approach / Landing techniques.

Wing Files- several good references here; in particular see the ‘Performance’ section.

Other information here:- ALAR Additional Resources. (including Spanish and Russian versions).