Boeing OPT and FMC discrepancies

[Broomstick Flier]

Hi gents,

This one has been keeping me busy reading the 767 manuals, but no luck there. Lets say you make a take-off calculation with Boeing's OPT, ATOW already inserted and equal to load sheet weight, and leave the ATM temperature set as MAX, ensuing result shows an ATM of 58 degrees. When setting this temperature into the FMC or dialling it in through the TMC it automatically limits it to, lets say, 40 degrees.

So why does this happens? I had the impression the OPT software had the same algorithms/performance database as the FMC, and essentially we suply the program with the same data the FMC is able to abtain through many input sources (OAT, QNH, altitude, etc).

Is there any way to predict the max value the FMC will accept?

Thanks in advance!

[zlin77]

Possibly to ensure that CLB Thrust is not more than T/OFF Thrust..

[Wizofoz]

1dot-yes

zlin77-no.

Depending on type and customer preference, some 777 engines are allowed up to 40% derate, while some are limited to 25%

The OPT may calculate an ATM that would meet performance criteria, but exceed the allowable thrust reduction for the particular engine.

[ANCPER]

Have a question re Zlin77's comment. Is there any engine wear/economic advantage having a Reduced Thrust T/O (N1) less than Clb Pwr and not just limiting T/O N1 to the Clb Pwr N1?

[zerozero]

The aircraft in original question is 767.

Not 777.

And my understanding agrees with zlin77.

[misd-agin]

Ensuring that CLB thrust is not more than TO thrust is not a restriction on many Boeing a/c that I've flown. At thrust reduction altitude the throttles advance to a higher N1 under certain conditions(light a/c with a long runway)

[Skyjob]

Certification settings of engine not resembled by OPT most likely the cause for discrepancy in OPT.

Essentially, OPT is as good as the Administrator makes it.
The Administrator can select certain selections for the configuration of OPT to ensure conformity for calculations.

If any discrepancies are found, these should always be highlighted to your OPS department or even better your EFB Administrator who can liaise with the OPT Administrator the findings (could be the same person in your company).

[Broomstick Flier]

Thanks for the very informative answers, T/O setting x CLB setting makes sense, and most o the time, we experience a thrust reduction, have yet to see the opposite.

Will check with the performance gurus at the office what they can do, or not, to modify it. On day by day operations it is not a problem at all, but the discrepancies took my attention

Cheers!