Hello again all.
How does the FMS determine the OPT altitude on the VNAV Cruise page? What variables are used? Thanks, Dave.
Hi Dave,
there's a certain curve for optimum altitude in relation to gross weight. The FMC also takes airline specific performance data into account that are set on the FMC's PERF FACTORS page (rate of climb minima in CLB/CRZ, and minimum CRZ time).
|-|ardy
Thanks Hardy. Does airspeed get factored in? I was playing around yesterday changing winds aloft and seeing the effect on step advice and Econ speed. Complicated stuff a bit above my pay grade.
Command speed is used for MAX ALT.
However, MAX ALT may also have an influence on the OPT ALT, but I don't know exactly how the real FMC calculates this influence. Obviously, the OPT ALT never exceeds the MAX ALT. So, yes, one could say that the command speed indirectly influences the OPT ALT as well.
If you change the predicted tailwind components along the route (by wind data entries), not only the ECON speed values along the route will change but also the predicted groundspeeds will change, i.e. you will reach a certain gross weight earlier or later, and so you will also reach a certain gross weight related OPT ALT earlier or later. So the wind data affects two things: 1. ECON speed values (based on cost index and a specific tailwind factor), 2. ETA values (based on ECON speed and full tailwind component).
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The optimum altitude displayed is a function of your gross weight and your speed mode (ie the 6 possible modes - ECON, SEL Spd, LRC, EO LRC, EO SEL, EO Min Drag)