Now I know there is an obvious answer to this question but it evades me at present but here goes:
Let's assume an engine bleeds on take off with OAT 14 C. For the 20K engine the FMC gives 90.7 N1. Now enter an assumed temperature (ATM) of 45C and the FMC drops back to 87.2 N1.
However let's forget any ATM for a moment and assume the actual OAT is 45C then the FMC shows 91.8 which ties in with the QRH figure for 45C.
Question: If the assumed temperature method of 45C gives an N1 of 87.2 N1, then how come the actual OAT at 45C gives a much higher N1 figure of 91.8 N1?
After all, the whole purpose of the ATM is increasing engine life hence plugging in 45C to the ATM box gives the lower figure of 87.2 N1 and subsequent reduction in EGT.
I just know the above is a stupid question but I am having one of those senior moments when my brain simply fails to see the obvious.
