Originally Posted by
OK4Wire
One problem with using zero wind (if say the headwind is 5kt, and you want to go conservative) is that you can't get 50% of zero.
If, as you line up, you notice the wind is now 2kt tail, in theory you should either recalculate or go TOGA.
Exactly. People usually pay attention when close to perf limited TOW but hot/humid/high minimum flex takeoffs with headwind turning to tailwind can surprise too as the additional performance margin due to smaller TAS will be not be very large.
These are rarely spectacular on twins, but saw some interesting ones when flying A340-300 in aforementioned conditions. Personally I went for TOGA if not close to Vr within alternating white/red centreline light zone. No-one ever objected.