Entries in 777 FMC when mixing in specified step points?

[Tordan]

Hi, flight simmer and former UL pilot here.
I´m having a discussion on what entries one makes in the 777 FMC when flying for instance a trans-atlantic route which has legs where the FL is set, such as NATs legs. As I understand it from Bulfers Guide any specified step points entered into the LEGS page(s) overrides the FMC calculated ones. Say that the FMC indicates a step climb at point D, but your flight plan says to make that FL later, at point E. When you enter that FL370S the calculations will use that, not the previously calculated one at D?
Also, I assume you always enter something else than zero as STEP SIZE since you´ll still want the FMC to calculate for the parts of the flight that are not restricted vertically for various reasons? The argument could be that the flight plan from dispatch should be used completely and not make use of the FMCs calculations. I think that sounds wrong since the FMC mixes in measured data as opposed to the dispatch plan that only uses predicted data (wind/temp/weights etc). Bulfers guide also says somewhere that the STEP TO fields are blank when STEP SIZE is zero?

Cheers!

[Wizofoz]

Correct regarding inserting a step point- but there is a bulletin warning against entering steps down route at LOWER than the current cruising altitude.

Step size defaults to "RVSM", meaning it looks to the next appropriate RVSM level (It used to be "ICAO").

You change that to zero when you do not intend to make any more step climbs, and FMC predictions are then based on maintaining current level.

[Tordan]

Thanks! a lot so there are more than 10 chars. :)

[JammedStab]

there is a bulletin warning against entering steps down route at LOWER than the current cruising altitude.



Does anyone know if this applies to other Boeing aircraft type FMC's?

[wiggy]

I'm open to correction but I'm not sure that particular bulletin is still in effect.

[Broomstick Flier]

JammedStab,

Have not heard about mentioned bulletin for the 767, was not mentioned either by my TRIs during initial training.

[Wizofoz]

Wiggy- My bad, that has actually been removed.

[JammedStab]

Wiggy- My bad, that has actually been removed.

Thanks. Don't you set your step climb to 0 before departure or else the FMC in some cases may show higher altitudes after the last step climb and base it's fuel prediction on the climb that you won't actually do?

[JammedStab]

From the bulletin:

"When required to cruise at an altitude below optimum due to air traffic, atmospheric conditions, or non-normal procedures, and no further step climbs are planned, set the STEP (L4) size to zero on the ACT ECON CRZ page. This ensures best available FMC FUEL and ETA predictions.

The Boeing Company Seattle, Washington 98124-2207"

Correct....that is what we tend to do as well. As the company tends to want you to just follow the flight plan, the step size can be set to zero prior to takeoff.

Not 777 I might add.

[Tordan]

So two valid procedures.
1. Change step size to zero when no further step climbs will be done.
2. Set step size to zero prior to departure. Only enter specified step points.

Up to company SOP which is used? Or is it also dependent on type of aircraft?

[wiggy]

So two valid procedures.

There are no doubt more so I don't think you should get to hung up in thinking there are valid and invalid procedures..........

Our SOP is to fly the flight plan if we are close to the flight planned weights, so in that case we'd put the steps in to the FMC as per the plan. OTOH if we are adrift from the planned weights (thinking a heavy, perhaps more than 5 tonnes) you might consider following the FMC steps. If ATC force you to fly at a non-planned level you'll probably end up "playing" with the FMC steps to see what gives you the best results (in terms of ETA and Fuel).

[Tordan]

Gotcha. Use the tools available when and as they are needed.