Entries in 777 FMC when mixing in specified step points?
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Entries in 777 FMC when mixing in specified step points?
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!
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!
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.
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.
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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"
Not 777 I might add.
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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?
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?
Last edited by Tordan; 24th Sep 2013 at 19:11.
So two valid 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).
Last edited by wiggy; 25th Sep 2013 at 06:24. Reason: Defining "best results"
