Hi all,
All varyingly interesting explanations, I see some don't understand this subject as well as others.
I thought I would add a couple of points:
The difference between 'assumed temp' and 'derated' thrust is that derated is a fixed amount of power decrease, usually 10% and 20% reductions, and assumed temp is a varying amount, dependent on the maximum temp the aircraft is capable of legally using the runway.
Of course, the two may be used together if the conditions are right.
A very important point overlooked here so far is that:
The power set during an assumed temperature takeoff can be exceeded at any time the Captain so decides, up to the maximum t/o power available.
The power set during a derated power takeoff cannot be exceeded, it is considered the maximum power available.
The reason for the difference is Vmcg.
For an assumed temp thrust takeoff, the Vmcg is calculated at the maximum thrust level.
For a derated thrust takeoff, the Vmcg is calculated at the derated thrust setting.
This difference can allow, in some cases, a greater takeoff weight for a derated takeoff the a maximum thrust takeoff, look at the figures for light weight takeoffs and shorter runways!
Cheers, FD
