Originally Posted by
hans brinker
Do you know what assumed temperature is? When you run the performance it gives you a Flex/assumed temperature. That temperature is the highest temperature you can safely take off and meet all the required distances/gradients, and that number is used by the fadec instead of the OAT to set N1/EPR. So unless it is hotter outside than your flex number you can take off. The actual OAT doesn't matter, as long as it is below the Flex number.
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).