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Guest lvedin

B747 Strange climb gradient #2

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I can not understand why the FMC count so different for something that seams perfect equal. Do I do something wrong.If picture direct-link not work, please copy/paste to your web-browser.Two fix are entered ESSA01 (= ESSA 010/11) and ESSA02 (= ESSA 010/20), those fix are used in departure in case 1, while not used in case 2.Case 1.Picture http://hem.passagen.se/lvedin/1120nm.bmpTake notice the fix ESSA01 is 11 nm after departure and speed/alt is 176/3700.Case 2.Picture http://hem.passagen.se/lvedin/sidalt.bmpTake notice the fix SA402 is 11.1 nm after departure and speed/alt is 259/8011.So when I enter fix manual into the LEGS page, the FMC v-path counting is dramatic different, but for as it looks very same condition.Anybody, get similar ? know why ?Initially I only enter some basic required data to FMCPOS:ESSAROUTE:ESSA NTL EFHKPERF:Payload 122Fuel: 163Grossweight 680ZFW 516 RESERV 24COST 80CRZ ALT FL310THRUST LIMIT: nonePAGE TAKEOFF REF:L1: FLAPS 10/1000FT (i.e. not changed altitud)DEPARTURE:runway 01LThen I enter SID and/or FIX as in the pictures.PAGE TAKEOFF REF:execute all speed reference buttons R1-4, each time before take pictures./ Lennart Vedin

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The pictures indicate you are still in preflight and you are comparing FMS forecasts... ah! I thought you were actually flying the airplane! Silly me.The forecasts are just forecasts, get her in the air and see how things work out. Don't accelerate below 1000 agl, don't constrain flaps-up speed to below the FMS-provided ECON climb speed with constraint.Forecasts have to be taken with a grain of salt, for example it will often forecast an unable next alt when there is a AT OR ABOVE constraint in the SID but once you are airborne the problem evaporates.

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Forecasts in vertical is one of the main purpose with a FMS. It is a little late to find out climb gradient and "unable alt" after accept clearance, and even very late when airborn. The typical useage to insert alternative fix in the departure may be due to required standard departures are not able in an other way for a hevy 747.If I should take your message "Forecasts have to be taken with a grain of salt" in this example, the grain of salt is a forcaste climb gradient of 3.16 or 6.77 degrees for about same condition (And then I have not even take the speed difference that increase the error in counting). As well the first case PMDG FMS speed of 176 kts is of cause fatal error. If this is the typical PMDG FMS precision, then a PMDG FMS counted T/D of 40nm could be 80nm or 20nm as well and the PMDG FMS vertical forecast should be useless. Then better I count T/D on my fingers then use the PMDG FMS. But this seams not to be the case. I did fly the two departures, and as excpected the second forecast when insert the SID seams accurate much well as I cross both SA402 and ESSA01 at approx 259/8000. In addition I have fly several flights and I do not find the PMDG FMS vertical typical precision bad at all.The PMDG 747 seams me as an accurate well simulate product. And therefore it seams in this example there is some logical malfunction that make the PMDG FMS forecasts counting total failing, rather than think it is a typical bad precision. It is a little pity, and I dont find a work around no matter in what order I enter data !

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SOLUTIONFinally I find the solution/work-around. Just enter fixes in a standard interval such as 1.2nm 5nm and then as desired. At least it works in this case.See picture http://hem.passagen.se/lvedin/step.bmp and after 11nm (ESSA03) the speed/alt is correct 259/7862.I will use this primary to determine available climb gradient up to a required altitud. And in addition when required, to alternate the SID to meet requirement, typical to avoid "unable alt".

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I know this is a old thread, but I'll try anyway.I was wondering this as well, seeing as I usually get UNABLE NEXT ALT even though I meet certain alt. constraints for the SID I'm flying (finding this out by ignoring UNABLE NEXT ALT and flying the SID as published anyway). So in order to "fix" this "issue (?)", all I have to do is put in FIXES at rwy HDG 1.2nm 5nm and desiered. This is to make the PMDG FMC correctly forcaste climb gradient. Then check alt constrains to see if I meet them... when I'm done I erase the FIXES. Is this correct?I will test this later and hopefully update.

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Yep, can confirm this now. VNAV forecast seem to be failing, because putting in fixes manually at the exact same spot gives you the true alt. you will be at that point. So sad that this hasn't been fixed for a high quality product that is PMDG 744... but at least now thanks to Vedin I have a way to check if UNABLE NEXT ALT message is correct or not. Happy flying!

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