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Optimum FL

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Hi Captains,A question about the optimum FL :When in cruise i have in my VNav page "ACT ECON CRZ 2/3" a stepclimb announcement : ex : Step to FL310 at 0806z/15nmAnd in the lower richt corner of the same page you have the OPT and MAX FL => FL301 MAX FL362Now here it comes : when my FMC tell me to go to the nex FL : "Step to FL310 NOW", my optimum FL is still only "FL301" !So am i alowed to go to the next FL as soon as the FMC tells me "NOW" or do i have to wait for the OPT FL to be at FL310 ?I know there can be a difference, but can it be that much ?ps : the numbers i gave are examples !


Regards,

William Vrielynck

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When you are at FL290 and your optimum is at FL301 you will get "goto 310" because the optimum is nearer to 310 than to 290. The msg is perfectly ok.If that's alright with ATC get to 310 as soon as the FMC tells you.


Cheers - Stefan Maus

 

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Also take into consideration that, by the time you reach your new altitude, you will have burnt off more fuel, the aircraft gross weight will be lower and your recalculated optimum altitude will be even closer to new altitude (if not a perfect match).Cheers.Q>

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Hi Q!May I add that I find that an en-route climb within about 500nm of destination airport is not an efficient use of fuel.Regards, Richard McDonald Woods


Cheers, Richard

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"May I add that I find that an en-route climb within about 500nm of destination airport is not an efficient use of fuel."In real life, Richard... or in MSFS?The FMC allows for a 1 minute cruise time. I wonder what the significance of this is?I don't think you'd lose too much fuel climbing... Don't forget that you spend extra time at idle during the descent (further away from your destination). I guess it depends on how much extra thrust is required for the climb and how long it takes to climb.Cheers.Q>

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The 500nm closed window for step climb is an interesting hypothesis. Assume that this is approximately 1 hr out, and the descent will involve about 20 min of that. Then the issue becomes is it worth the climb for 40 min? Comparing the fuel flows for the aircraft in the two different scenarios will give you the answer, which I expect to be no... the small increase in fuel required to climb will not be offset by the even smaller decrease in fuel used for the remaining cruise (40min less the time to climb).The plus of getting to throttle-back for a descent sooner because your're higher is probably nil since we are only talking 2,000 ft -- or maybe a minute.Just my guess.


Dan Downs KCRP

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Hi Dan,Well reasoned argument (what else should I expect?) and well beyond my original rule-of-thumb argument.Many thanks,Richard


Cheers, Richard

Intel Core i7-7700K @ 4.2 GHz, 16 GB memory, 1 TB SSD, GTX 1080 Ti, 28" 4K display

Win10-64, P3Dv4.5, PMDG 748 & 777, ASP4 + ASCA, vPilot, Navigraph, PFPX, ChasePlane, Orbx 

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A real life story on an Emirates flight B777 from HAM to JFK in FEB I was very astonished that about 30 mins prior TOD ( in reality it was 28 mins ) we stepped up one floor from 360 to 380 ! Could have been ATC or so . . .?Karl-Heinz - EDDI/THF " Tempelhof "" The mother of all airports " - Sir Norman Foster


Karl-Heinz Wichmann - EDDI/THF "Tempelhof"

"The mother of all airports" - Sir Norman Foster

Now CLOSED - its a shame !

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