Above or Below OPTIMUM ?

[Jetstream96 20]

David have you been looking at LRC in PD to get OPT altitude?

Yes. In Performance Dispatch -> LRC Max. Operating Altitude.

 

For example when I load the -800 with a gross weight of 75 tons, FMC shows FL354 as OPT ALT, while in the manual it's FL330. Not sure whether I misunderstood something, but this seems a little weird to me.

[Driver170 42]

Hmm been reading david and i think LRC OPT and FMC OPT ALT are different because the ECON OPT altitude is based on minimum cost per NAM by the entered cost index. The OPT ALT will decrease with high CI entries because ECON is looking for higher ground speed (lower alt equals higher TAS). Fixed-Mach and LRC yield different OPT altitudes than ECON OPT altitude because they do not consider cost optimization calculations found in CI.

[Spin737 102]

I think the OPT differs only based on Mach. If you change the Mach, regardless of if it's due to a high CI or simply selecting a high cruise, it'll change. I don't think the OPT isn't looking at how you derive the speed.

[Jetstream96 20]

Hmm been reading david and i think LRC OPT and FMC OPT ALT are different because the ECON OPT altitude is based on minimum cost per NAM by the entered cost index. The OPT ALT will decrease with high CI entries because ECON is looking for higher ground speed (lower alt equals higher TAS). Fixed-Mach and LRC yield different OPT altitudes than ECON OPT altitude because they do not consider cost optimization calculations found in CI.

I agree with you but the result in my previous post was with LRC. Actually I mostly use a CI of 20 and that's fairly close to LRC anyway. (I tried CI 20 and the OPT ALT is exactly the same with LRC.)

[Driver170 42]

Matt i'm not sure thats what i have been reading. Maybe someone who is an FMC geek can explain?

 

I always thought FMC OPT ALT is based on weight and CI?

[Guest]

Sam great stuff there what would be your call here in deciding what FL to use?

Again it depends entirely on the prevailing conditions. If it's smooth you might want to climb to be slightly above your OPT altitude for a while but if it's turbulent you'll have less of a margin between max/min manoeuvring speed so you'd stay lower. Worth mentioning if you've got a pretty strong tailwind at a lower cruise level you may opt to fly there instead of the FMC OPT. The flight plans used today are incredibly accurate and it will work out the most fuel efficient level to fly for you.

 

Selecting LRC gives you your maximum range speed for a given weight & altitude in still air conditions. We not use it unless diverting as it does not take into account operating costs because it ignores the existing wind, it's likely to give you a higher fuel burn than ECON. Why LRC chooses a higher FL I'm not sure, I'll have to do some digging.

​

[fnav77 56]

Try to be as close as possible to the Optimum. Assuming you are going to fly on odd FL, if, during the climb, the optimum shows FL388, climb FL390 as the optimum will go up as you cruise along and get to 390 fairly quickly. However, if it shows FL381 for example, then FL370 is better. Unless winds are much better above the optimum, then flying above the optimum is not great for the fuel.

If you can request block levels, then fly at the optimum and climb along with the optimum.

 

As for changing FL's in a flight plan, it is just that.... a planning thing, sometimes to do with the direction of the airways.

[Driver170 42]

Thanks sam and fnav. I guess with more experience flying through different European countries and facing a mix of step climbs and des i will soon know how to handle them. Also like what fnav mentioned, its about good airmanship when you know when to fly above or below the FMC OPT depending on the circumstances.

[BlueStar 323]

Guys check this out. surely no airline will have you at FL380 and then des to FL370 and back up to FL380.

Vernon,

 

I see it from time to time where altitude for direction (east/west vs. north/south hemispheric rule) of flight is an issue. Flying at FL380, direction of flight changes and too heavy to make FL390, so descend to FL370. Use to be a real issue in Russia before they adopted RVSM. May still be an issue in some of the "stans."

[skelsey 765]

Vernon,

 

I see it from time to time where altitude for direction (east/west vs. north/south hemispheric rule) of flight is an issue. Flying at FL380, direction of flight changes and too heavy to make FL390, so descend to FL370. Use to be a real issue in Russia before they adopted RVSM. May still be an issue in some of the "stans."

 

Just China and Mongolia using metric RVSM now, as far as I know (usually requires a 100ft climb at the boundary). Russia still use metres QFE below transition though, so that's one to watch out for...

 

Also I seem to recall that not all of Africa is RVSM: not sure if it's changed now but there certainly used to be a lot of level changes going on south or northbound through Algeria (at about 30N, if I recall correctly) to transition from RVSM to 2000ft separation.