Well my qusetion was different folks .According to A.F. it seem that it is a certification problem and J.F. think it is cause of the flare problem and also due to the extra stopping distance (which i beleive is when we are R/W limited ),however I just dont understand, what is the effect on the performance of an aircraft(say b-737 or any airliner) if the pilot does T/O orland, at the max tail wind limit and also how come that there is same tail wind limit for 747 and 737 and also for small turboprop.