Sorry, not a very good explanation. The assumed temperature will be higher than the engines 'flat' rated temperature, there FMC [flight management computer] will use a lower N1 [fan speed] to give the rated thrust for that particular temperature, i.e. less than the full (or derated) thrust.
To put it another way, although the rated thrust of an engine may be 20k of thrust up to 30 deg C, at 55 deg C the engine might only produce 15k. By entering an assumed temperature, one is instructing the FMC to calculate an N1 speed for that temperature.
Last edited by notdavegorman; 22nd April 2006 at 17:49.