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northridge

FMC "Opt" FL and real world climbing

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LSZH-SBGL in a payware MD11: TOGW is 615.7, 218 of which is fuel: I have filed FL350.My FMC tells me that my Max FL will be 325 but I know that that will rise as I lose fuel. However, approaching the bottom end of Spain my "Opt" is still only 315 and Max 335. Surely I should be higher by now? In the World Air Routes brilliant MD11 Swiss video on the same route they climb to their filed FL as soon as possoble. So, is this a carrier thing - burn expensive fuel early climbing heavily laden to the filed FL in order to save expensive fuel later by being high? ThanksNicholas

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I have found that most RW carriers stay on their optimum altitudes as often as possible. If you are flying all the way down to Rio, I would want to stay on the optimum altitude to be the most fuel efficient.


Joe Sherrill

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LSZH-SBGL in a payware MD11: TOGW is 615.7, 218 of which is fuel: I have filed FL350.My FMC tells me that my Max FL will be 325 but I know that that will rise as I lose fuel. However, approaching the bottom end of Spain my "Opt" is still only 315 and Max 335. Surely I should be higher by now? In the World Air Routes brilliant MD11 Swiss video on the same route they climb to their filed FL as soon as possoble. So, is this a carrier thing - burn expensive fuel early climbing heavily laden to the filed FL in order to save expensive fuel later by being high? ThanksNicholas
I'm not a specialist as far as MD-11 is concerned, however in 737-800 the opt and max FL depends (of course besides gross take-off weight) greatly on the "Cost Index" that you fill in the performance page of the FMC. Maybe you enter some irrational value and the FMC wrongly calculates the opt FL :DRemember also that the opt and max altitudes do not change 'quickly'. E.g. when I flew the route EPWA-GCLP, the opt FL increased by just 1000 feet only when I was over about the middle of Spain :)

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Thanks for the replies.I posted over at the PMDG forum too, wondering if there was some aircraft specific thing going on here, and got the same answers, but the CI point is interesting - mine was 80 so I dont think it was too far off the mark.I eventually got to my filed FL350 shortly after reaching Brasil landfall, and by the time I was about to land it was FL375. It was interesting to watch the occasional "Insufficient Fuel" messgaes appear, especially as I climbed to a new Opt, which then disappeared as I got into the cleaner air. I landed with with a fraction over 22,000 fuel remaining. Is that enough or more than enough for this length of flight?ThanksNicholas

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Guest raikky

im not sure if this relates but i fly the pmdg 747. if your heavy with fuel, your inital FL will be much lower than if you were light with fuel. your FL will change as you burn more fuel off and by step climbing to new Flight levels. e.g.lets say you are heavy with pax and heavy with fuel, the FMC will indicate a FL of just 31000 for example. AS your burn off more fuel, the aircraft will be lighter, and new optimum flight levels will be suggested. The next FL will already be shown in the FMC progress tab. for example, step to FL33 in 1024 miles. sometimes my last step climb will be just 100 miles before rthe TOD by which time i will be at FL370.'Step-climbs are done to optimize fuel consumption. 2,000' increments are done to adhere to RVSM (Reduced Vertical Separation Minimums) requirements. To describe what RVSM is, I have to describe what cruising altitudes you need to be at to operate under IFR.If you look at CFR 14 part 91.179, this part describes IFR cruising altitudes. It notes that on a magnetic heading of 0*-179* and under FL 290 your altitude needs to be a odd number (IE 1,000, 3,000, 5,000, FL190, FL210, etc).On a magnetic heading of 180*-359* and under FL 290 your cruising altitude needs to be a even number (IE 2,000, 4,000, 6,000, FL180, FL200, etc).RVSM airspace will be the same. RVSM airspace is listed as airspace between FL290 and FL410 where 1,000 FT separation minimums between aircraft traveling in opposite directions can be applied due to the pressure altitude reporting equipment installed into the aircraft and ground equipment.Under RVSM airspace with a magnetic heading of 0*-179* your cruising altitude needs to be at odd 2,000 foot intervals beginning at FL290 (IE FL290, FL310, FL330, FL350, FL370, FL390 and FL410).Under RVSM airspace with a magnetic heading of 180*-359* your cruising altitude needs to be at even 2,000 foot intervals beginning at FL300 (IE FL300, FL320, FL340, FL360, FL380 and FL400).Non RVSM airspace obviously doesn't adhere to 1,000 foot separation minimums and therefore the separation minimums are increased to 2,000 feet.In non RVSM airspace and a magnetic heading of 0*-179* your cruising altitude needs to be any FL at 4,000 foot intervals beginning at FL290 (IE FL290, FL330, FL370, FL410)In non RVSM airspace and a magnetic heading of 180*-359* your cruising altitude any FL at 4,000 foot intervals beginning at FL310 (IE FL310, FL350, FL390).RVSM step altitudes are already programmed into the FMC by leaving the step climb profile to ICAO. The ICAO step climb profile will give you steps in 4,000' intervals above FL290, and 2,000' intervals below FL290. You may need to change this value if you're flying in non RVSM airspace as the FMC may suggest a step altitude that contradicts part 91.179.Long story short, your cruising altitude is regulated more by ICAO than it will be by company dispatch. wink.gifIt's also worth a note that RVSM altitudes and standard vertical separation minimums doesn't adhere to NAT's. This is because NAT's are one-way roads (hypothetically speaking). Cruising altitudes and the current NAT can be reviewed by looking at the route NOTAM's.'i took this from another forum written by THBatman8

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Guest raikky
LSZH-SBGL in a payware MD11: TOGW is 615.7, 218 of which is fuel: I have filed FL350.My FMC tells me that my Max FL will be 325 but I know that that will rise as I lose fuel. However, approaching the bottom end of Spain my "Opt" is still only 315 and Max 335. Surely I should be higher by now? In the World Air Routes brilliant MD11 Swiss video on the same route they climb to their filed FL as soon as possoble. So, is this a carrier thing - burn expensive fuel early climbing heavily laden to the filed FL in order to save expensive fuel later by being high? ThanksNicholas
also i havnt flown the MD11 but with that starting fuel and pax seems your FL should be starting around FL 31 and not FL350.Part of the idea of step climbing is effiency. as your plane burns more fuel, the plane should climb to 'new' optimum FLs and take advantage of higher altitues.

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