747-8: RCMD ALT calculation in FMC

[berts 97]

2 hours ago, downscc said:

Any perceived differences are due to a different wing, weights and possibly winds given in the cited examples.

I think you have hit the nail on the head here, Dan, because the 744 and the -8 are not the same beast even though they may look alike.  It is therefore unsound to assume these two aircraft will be more or less identical in terms of their performance.  Like you, I am sure most of these differences can be explained by their different wing design, engine fit, zero fuel weights etc.etc.

Simon's notes presumably relate only to the -8 because they do differ to those of the original B744 Honeywell FMS fit.   For example, the 744 FMC's Cruise page has an OPT/MAX line but there is no RCMD value; perhaps because the pilots were treated as intelligent in those days and expected to work it out for themselves?! 😁  Anyway, the OPT ALT in the 744 was useful because it would display the altitude which minimizes the trip fuel when LRC or SEL speed is displayed, or the altitude which minimizes the trip cost when ECON speed is displayed.  Another difference seems to be the MAX Altitude line, which on the 744 takes into account the number of engines operating, current gross weight and the SPEED line value, but disregards any altitude or speed constraints. .

3 hours ago, downscc said:

Does the RECMD algorithm include the cost of the step?

Common sense would say it does although not within 500nms of TOD, but I am just guessing.  

5 hours ago, skelsey said:

This is where a degree of pilot interpretation is required -- i.e. "over what distance am I going to get better winds at lower levels and if I do descend will other traffic block me from climbing again later" -- using the forecast wind data on the paper OFP may be useful here.

This has a lot to do with pilot experience and airmanship over and above what the FMC is telling you.   When flying longhaul across the Middle East from say India to Europe it is sometimes possible to save a considerable amount of time and fuel by descending to a relatively low FL in the mid 200's because of strong headwinds at the normal cruising altitudes (i.e. in the high 300's) for that aircraft's weight. 

[skelsey 765]

4 hours ago, downscc said:

Does the RECMD algorithm include the cost of the step?  I was under the impression that the STEP TO did but not RECMD.

A good question and one which is not directly addressed in any of the manuals -- however, like Bertie, I would assume so. From the FCOM:

Quote

RECMD - displays the most economical altitude to fly for the next 500 nm based on gross weight, selected speed, pressure altitude, and entered forecast winds and temperature at cruise altitudes. The FMC evaluates altitudes up to 9,000 feet below the current CRZ ALT and up to less than MAX altitude. Recommended altitudes are consistent with the specified step size and step climb schedule. If the step size is zero, the recommended cruise level is calculated assuming a 2,000 feet step size. The recommended altitude is set to the CRZ ALT when within 500 nm of the T/D.

 

43 minutes ago, berts said:

the OPT ALT in the 744 was useful because it would display the altitude which minimizes the trip fuel when LRC or SEL speed is displayed, or the altitude which minimizes the trip cost when ECON speed is displayed. 

OPT - as far as I know - is just a function of weight and winds are not taken in to account for that calculation.

STEP TO (AT), however, functions in the way you describe -- calculating the step point to minimise fuel or cost depending on the speed selection.

43 minutes ago, berts said:

Simon's notes presumably relate only to the -8 because they do differ to those of the original B744 Honeywell FMS fit.   For example, the 744 FMC's Cruise page has an OPT/MAX line but there is no RCMD value; perhaps because the pilots were treated as intelligent in those days and expected to work it out for themselves?!

😂!

The notes in question relate to the 'NG FMC' which is fitted to the -8 as standard, but can be (and has been in many cases) retrofitted to the -400. Perhaps one day PMDG will give us the option to retrofit our -400s in the virtual world too!

[Woozie 732]

Excellent information, it all makes more sense now. Thank you gentlemen!

[emvaos 13]

Simon correct. OPT ALT is basically a function of weight. It will not take into account forecast wind and it will also do not take into account the fact that for a given cost index ECON SPD will vary between current CRZ ALT and STEP TO ALT.

 

RECMD ALT on the other hand is quite a bit more technical. The algorithm performs simulated runs (ie solves the equations of flight) from 9000' below to MAX ALT above as if the airplane was cruising at those altitudes for the next 500nm of the active flight plan (that is for about the next hour or so of flight). RECMD ALT takes into account foreact wind (and temperature) and the variation of ECON SPD changes. The fuel consumed at the end of all those "simulated" runs is recorded and compared and the most economical run (even by 0.1 kilo) is the answer (for the next hour I repeat). RECMD ALT will NOT take into account the extra fuel burn cost of the step.

Now what might happen mathematically under some circumastances is this : for certain outside air pressure distributions and ISA deviations at very high altitudes and cost indices, near MAX ALT, the VNAV operational envelope might be enforced ie. mach number might be restricted resulting thus in an equivalent lower cost index number. That is while ECON mach number at MAX ALT might be say .855 the environment might result in a 0.3g-buffet limit of .845. As a result the RECMD ALT solver will calculate a run for this "protected" lower Mach number (at least for a part of the 500NM) and save a couple of kilos. So care should be applied.

NOTE : The cost of extra fuel burn to climb is accounted for when you create a MOD/ACT with a new CRZ ALT. The fuel remaining figure (as all other variables such as time) does take into account the CRZ CLB part.

 

Best,

Vangelis

Edited October 21, 2018 by emvaos