john-tullamarine:. .slightly confused about your level of understanding here...or are you taking the mickey? you appear to have knowledge but... the basics of flex vs derate (without trying to write a thesis) are:. .flex defines a thrust level below max at which perf reqmts are satisfied.. .derate defines the max TO thrust rating LIMIT.. .broadly speaking,flex works for less than MTOW,on a half decent rwy and derate works for limiting rwys.. .If, for the same TOW, the max thrust is reduced (derated) the assoc vmcg is decreased therefore V1 is reduced; accel/stop distance is reduced and V2 can still be met.. .you knew all this already didn't you? your remark 'not much at all' was a wind-up wasn't it? <img src="tongue.gif" border="0">