Cheers gentlemen.
As
Wino &
Techman allude to, you can find graphs of max altitude versus weight
or temperature, but I haven't seen many parametric graphs with both on...
None in our performance manuals that I've come across....(must look harder).
The FMC Guide doesn't indicate the variables that affect the calculation.
Its not that I wasn't aware of the fact that temperature affects maximum altitude capability but I wanted to know how the FMC works.
If
FlareArmed is correct, I wouldn't have expected MAX ALT to have changed
in the climb in the absence of changing any other variables.
In this case, I would expect the predictive ability for MAX ALT to behave as per the ECON TGT SPD (in the CLB and CRZ phases of flight) in the sense that, as the FMC Guide indicates, these speeds will take into account wind if a CRZ WIND is entered after PERF INIT has been executed, but, subsequently, the TGT SPD's will not adjust for wind until you either enter a new CRZ ALT or you reach the CRZ whereupon the CRZ TGT SPD speed is adjusted depending on the
actual wind.
In my case, MAX ALT changed early in the climb.
What might be the reason for that? Perhaps a VNAV performance iteration that resulted in a computed ECON TGT SPD that was a little higher, with the corresponding effect on MAX ALT? What other factors affect the MAX ALT calculation?
I'm not sure whether the above statement is gobbledigook?
I've not actually noticed whether the ECON CRZ TGT SPD changes during the climb phase. I hazard a guess, not, for the reason already suggested.
However, it may be that the relatively simple problem of determining MAX ALT is done
real time via the FMC using a look-up table that depends on
actual instantaneous OAT as contributors suggest. Can anyone confirm this?
My next question concerns the statement on Pg 11.8 of the above FMC Guide which alleges that the use of OPT ALT when flying in ECON is mis-guided.
Why is this?
Sure enough today, at M=0.741 in ECON, OPT ALT = 354. Entering a fixed Mach cruise of M=0.741 gave OPT ALT = 361.
Whilst I understand that ECON flying seeks to minimize
cost per nautical mile and LRC/fixed Mach cruise seeks to maxmimize
nautical miles per pound of fuel, I'm not sure I understand the above discrepancy.
Cheers.
