Originally Posted by
tdracer
Sorry, but that is not only wrong, it's dangerously wrong. If actual OAT is 25, and ATM comes up with 75 for thrust setting, that simply means that the thrust (typically N1) for 75 can still be used safely.
If the actual OAT goes up, the aircraft performance goes down (hotter air = less lift at the same airspeed). IF you don't redo your ATM calcualation for the new OAT, you're thrust setting will be too low for that OAT. Plus, if you're talking N1 thrust setting, 100% N1 at 50 degrees creates significantly less thrust than the same physical N1 at 25 (that old "square root theta" thing - thrust is constant at corrected N1, not physical N1).
I probably didn’t express myself correctly, and I would definitely hesitate to question your expertise.
But I have always read it as: Assumed/FLEX will set the power for that temperature. If the actual OAT is lower, as it should be, you will have an additional margin. If the OAT rises, that margin will decrease, but up to OAT reaching FLEX your FLEX power setting should be safe to use. You say that there would be no extra margin because the calculation takes OAT into account, and if OAT rises you would have to do a recalculation. I obviously understand that you are correct if there is no extra margin, so will go back to my books.