Originally Posted by
PurpleToolBox
On the 777 yesterday, we had driving rain and crosswinds for takeoff. The software computed a 20% reduction in thrust (TO2) but did not allow for a reduction by the assumed temperature method. My mind started thinking about this thread since we had a significant split between V1 and VR with a stop margin of less than 500 feet. I am glad FDX calculates stop margins.
The stop margin of <500' is likely an example of Derated thrust (TO2) being the only viable solution for takeoff as the TO or TO1 VMCG V1 would result in an Accel-Stop beyond the runway available. Assumed Temperature Method is not allowed for contaminated runways, but is available if the runway is wet.
In my view, on dry or wet runways, TO should be used with an assumed temperature. There is diminishing returns on engine wear below approx 20% thrust reduction. This would allow for full thrust (firewall) if needed throughout the takeoff. Also, assumed temperature is inherently conservative. Due to the differences in TAS, the accel-stop and accel-go distances will be less than what is calculated. Further, the most fuel efficient takeoff profile is maximum thrust, full climb thrust to altitude. At some given thrust setting there is an ideal intersection of fuel economy and engine wear.
TO1 and TO2 should be reserved for contaminated runways for the reasons addressed previously.