CF6-80 N1 mismatch in ATM calculations

[PITUFITTI]

Hi there,

My question is why the values of N1 in the GE engines when using ATM do not match between the FMC and the FPPM (you have to go to the AFM to get the correct values), while on the PW's, EPR calculations match perfectly in the FPPM with no need to refer to the AFM. Why is that? Can somebody shed some light here...

Many thanks.

[bigduke6]

I have never noticed a mis-match, but then I normally don't looks at the FFPM for my N1 setting.

So are you saying you calculate and then use a different N1 from the FFPM, instead of using the N1 the FMC is using? Then you manually set the N1 you want?

Are you using OAT and airport elevation or sensed TAT and p-alt (what the FMC is using), as they/that might account for the difference. If so, is your TAT reading accurate before the engines are running?

[PITUFITTI]

Thanks for the suggestion, I guess i use the correct data...:)

The calculations on takeoff N1 look rather more complicated on the GE case (AFM, appendix 1, section 4.2, page 10) than on the PW EPR case...

More suggestions welcome.

[c100driver]

Just a guess but.

FMC N1 uses the sensed TAT to calculate the N1 which is rotor speed setting. The OPT/AFM you use the airport temp to calculate the N1. The temperatures can be different if the stand is tucked into the terminal compared to out on the taxiway.

The Pratt and Roller use EPR which is just a ratio

If you checked the N1 on the Pratt or Roller I suspect you will see the same type of difference.

[PITUFITTI]

Fully agree. There must be something more...

Thanks.