FMC Prediction Question

[Will]

So let's say that I enter FL310 for my initial cruise altitude because ATC will want that, and am expecting to stay there for a good long while. I enter 2000 into the FMC for step climb in PERF INIT. In the LEGS pages, I see entries on the right side as looking like 0.830/FL350 for several waypoints, small font, and then it changes to 0.853/FL370 for several waypoints, and then 0.858/FL390, all in small font.

Correct me if I'm wrong, but I'm thinking the FMC is showing the optimum altitudes in the right side of the legs pages, even if those altitudes are higher than the FL310 that I put into the FMC for my cruise altitude.

Question 1: Are the FMC fuel predictions based on these small font optimal altitudes, and not on the FMC cruise altitude?

Question 2: Let's say you're going to stay at FL310 for a long time for whatever reason, long after the optimum altitude is higher. Would you go into the LEGS pages and manually enter "FL310," overriding the calculated optimum altitudes?

Thanks.

[skelsey]

Short answers:

1. Yes. The FMC predictions are based on the optimum profile it has calculated.

2. No. Change STEP SIZE to zero, or if you know the point at which you will be climbing you can enter /FL330S to force the step climb at that point (the FMC will continue to compute an optimum profile beyond that).

[Will]

Thanks, skelsey.

[Jeroen Hoppenbrouwers]

Sometimes your filed flight plan will indicate the dispatcher-requested step points to ATC ahead of time. A typical step point is when leaving the Atlantic tracks. Inside the tracks the chances are slim (but not zero) to get a step; when the whole bunch fans out over the continent and needs to be inserted into the national system(s), it usually gets easier.

A good sport is to haggle for flight level just before getting into the tracks tunnel. If you are close to the step point to the next optimal level, but not there yet (or even two hours from getting there yet), you still may benefit from requesting one step up, burn more fuel for a while, but  then get the prize for many more hours (and screw the competitor below you).


Hoppie