767 Pegasus Question

[crj705]

On the Pegasus FMC Cruise page, what are the inputs that are used to determine and the defination of the Recommended and Optimum Altitudes?

[Spooky 2]

I believe they are gross weight, head/tail winds and SAT along with programed Mach. I forgot to mention it 1st time around but yes...CI will factor in as well.

[Brian Fantana]

Unfortunetly Mr Boeing has taken out a lot of info from the manuals. Mine says:
OPT - displays the most economical altitude based on gross weight and the active cruise speed.
RECMD - displays the most economical altitude based on winds and airplane performance.

And from the flt crew trng manual:
Optimum alt is cruise alt for minimum cost when operating in the ECON mode, and for minimum fuel burn when in the LRC or pilot-selected speed modes. In ECON mode, optimum alt increases as either the airplane weight or cost index decreases. In LRC or selected speed modes, optimum altitude increases as either airplane weight or speed decreases. On each flight, optimum altitude continues to increase as weight decreases during the flight.

BF

[Jetstream Rider]

If you can't get any further, why not experiment? With the FMC's on the 757 and 767 in my airline (not Pegasus), there are a couple of differences. For instance, on the 757 the max altitude will change depending on the climb derate selected (although it will not re-calculate until you change something else in the FMC). The 767 does not. Max and Optimum alt will also change depending on selected cruise mach.

Cost index speeds will change depending on winds entered - slow in a tailwind and fast in a headwind. Its worth playing a bit, you will discover all sorts of odd things. Having said that, I wish they gave us better books to describe the operation properly.

[extreme P]

For a start...

. OPT ALT is based on gross wt, speed entry, and route distance.
. ECON is based on lowest operating cost per nm.
. LRC and SEL SPD, fuel used per nm is minimized.
. For short range flights, this altitude allows for cruise time requirements.
Ref: PERF FACTORS

Pegasus RECMD...

. Recommended altitude is most
economical altitude based on two
engines, gross weight, target speed.
forecast cruise wind and temperature.
If no forecast winds are entered the
FMC uses sensed winds
. May be above or below cruise altitude