once I was on a a342 heavy T.O from LEMD . ATIS reported wind calm. In our company we do our own calculation for T.O. speeds. As we where number 2 on sequence the tower reported to the number 1, wind 5 kts tail . when we roll into position wind calm. where using TOGA. 36L Its a long runway, but also long T.O. roll and wind might have changed or not being accurately reported along the runway. As we saw the end of the runway aproaching pretty fast and we didn't have V1 yet, the PF (FO) initated rotation. Had we waited for V1, today I would be driving a cab.
Got airborne normally but none of us where happy about the situation the say the least.
after T.O. we found out that the T.O. performance charts for 10 kts tail wind indicated V1,Vr,V2 much lower than the one we have used (example: V1 140 instead of 154, that's 14 knots less!) and that our T.O. rotation was perfectly safe. The reason: T.O. charts normaly uses improved climb speeds. I found usefull to take a look at the 10kts tail wind speeds on heavy T.O.'s . Just in case.