The original post:
So it is decided to use the longer runway. The only thing is that with the longer runway a derate and ATM thrust reduction is used for the takeoff along with a different flap setting.
Without including a bunch of variables such as the longer runway covers weather changes, payload adjustments, etc.......How much advantage have you gained by using the longer runway. Is the reduced thrust takeoff negating most or all of the advantage that the longer runway offers?
Because of the derate/Flex, the TORR/TODR on the longer runway will be longer than that off TOGA take off on the shorter runway but it will meet all regulatory requirements of take off. As GR stated using flex produces better performance than that of TOGA at that OAT because although the engine power is same the TAS at lower actual OAT is less due to higher air density. Since there is limit of 25% to 40% on Flex the advantage of using longer runway is not totally negated. Off course lower EGT results in reduced maintenance costs. That is the very purpose of flex take off. Flex take off uses more fuel than TOGA. All companies want Flex and lower thrust reduction altitudes for reducing fuel cost and only weather, wind shear is valid reason to use TOGA when Flex is possible.