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Guest AJ

Entering Cruise Alt on Perf page

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Guest AJ

Quick question. I'm planning a long flight with a heavy fuel load and in turn will be using a step climb. When entering a cruise alt on the perf page, should this be my initial cruise alt or the final cruise alt? Thanks,Andrew

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Guest D17S

Initial. Check Vnav's cruise page for the OPT and MAX altitudes. OPT is the one you want. OPT-imum means you will get the best performance here. Try MAX just to check it out. You

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Guest dt1951

If you are pretty heavy at takeoff, then I would highly recommend that you not go the MAX altitude as shown on the VNAV page. It's doubtful that you would ever get there.

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Guest AJ

I just wanted to bump this back up, because I would like mroe feedback. Is it for certain that the cruise alt entered here is the initial one, and not the final alt after all step climbs are complete? I would think that in order for the FMC to make accurate fuel projectiosn, that it would need to know what your final crusise alt would be. I realise that maybe I am missing something here, but I am just trying to get my head around how the FMC makes its calcs. Most of my fllying with the 744 in the past has been on relatively short trips that did not require step climbs.Thanks again,Andrew

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Guest D17S

Whatever you enter as your cruise altitude, the FMC will obey . . . at least try to. There is no automated step climb procedures programmed into the FMC. All the FMC can do is just tell you what it has calculated to be the OPT-imum and MAX altitudes.The FMC's Vnav cruise page will update step climb points based on ongoing calculations, but it will not take you there. In any case, for better or worse, for why or why not, the OPT-imum altitude is where the airplane really wants to be. I know this might seem a bit unsatisfying. They used charts for years to get these altitudes, but the charts were just as mysteriously constructed as the FMC's digital presentation on a Vnav cruise page. We can draw lines on a chart, but do we know what equation THAT line represents and what it means . . . and why that Fx result needs to be used as an X or Y coordinate for this other equation represented by this other intersecting line (i.e, "where the lines cross"). I get that (NOT!). Vanglis, help. I'm getting dizzy. These are serious computations. Faith is always an absolute last resort, but this may be just such a case that needs a little faith . . .and as always, it is wholy unsatisfying. The procedural drill is to watch Vnav's cruise page for the "climb now" annunciation. When you get it, dial up the MCP and double tap the altitude knob. That will reset the FMC's Vnav cruise altitude and up you'll go in a VNAV SPD/THR REF climb. A kinder gentler climb might be facilitated with FLCH/THR. Dial up the MCP then press FLCH. When you capture in ALT, double tap the MCP altitude knob, press Vnav and you will get back to VNAV PATH/SPD cruise.You want to stay at opt-imum if at all possible. Max is just something available for emergencies like thunderstorms and the like. Even so, I feel we need more conversation on this so chime in guys. The actual arithmetic that is happening in the FMC would be interesting to dissect. Also, is there an indication on the ND too, that alerts to a step climb point. I forget.

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Guest mortimer

Sam, I just want to thank you. The couple of paragraphs you just wrote have clarified things immensely for me. Thank you sir!!Wayne

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Guest AJ

Ditto. Hopefully, Robert's type-course will get into this in greater detail.Andrew

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Guest bruchpilot

Hello,thanks for pointing that out. However sometimes even the OPT altidute cannot be reached. I always thought it could be the ZFW value that was wrong but I didn't change anything since my last flight. What else could be the cause?GreetingsEdgar

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Guest D17S

I've found that sometimes the airplane will refuel itself. Check the GW too.

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A possible cause is tailwind/insufficient engine power.If you leave thrust limit at 'crz' when level or at too-derated climb and encounter strong tailwinds the aircraft will not climb and may even stall at current altitude - change thrust limit to 'con' - idea being to make more power available to compensate for loss of (indicated) airspeed. Learned that one the hard way over the weekend :7 Love this plane - forces you to pay attention and obey the rules! :) regards,MarkXPHomeSP2/FS9.1/3.2HT/1024mb/X700pro256


Regards,

Mark

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