Engine choice vs. OPT ALT

[United744]

Hi,

Does the choice of equipped engine affect OPT ALT prediction of FMC? I tried changing the engine type in flight, and found that all 3 engine choices had about the same fuel flow value, though significantly different thrust settings at cruise.

From the specs, PW4062 is the most powerful at 62500 lbs thrust. Would this result in higher OPT ALT due to the additional thrust available? It appears it should be able to fly higher due to the additional thrust, and take advantage of reduced fuel burn.

[Hardy Heinlin]

Hi,

the opt alt is based on gross weight and on the maneouver margin set on the FMC's performance factors page.

The indicated EPR values vary between engine models while maintaining the same airspeed, but the actual thrust is the same when the drag and airspeed are the same. It's just the way EPR is calculated: core EPR versus integrated EPR.

See page 511, 512, lower right box.


Cheers,

|-|ardy


Perhaps you confuse OPT ALT with MAX ALT. Max alt is also a function of the airspeed selected. But your higher thrust from the PW doesn't help when you're limited by the Mach buffet. You may reach your altitude faster though.

[United744]

That's what I wondered - ultimately it's about the wing and its characteristics, but couldn't figure out if the more powerful engines would affect cruise performance.

Thanks for the info.