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frank99

VNAV PATH expected and calculated

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I have a question about 737 and VNAV PATH.I've been reading through dozens of VNAV posts, but couldn't find anything (yet). In the picture below I have tried to draw a VNAV PATH with multiple altitude constraints (not to scale).I was surprised to see that the FMC calculates the VNAV PATH as in the red dotted line. This means that, when flying, the initial descent is at (app.) VS -2400, whereas the following descents are at (app.) VS -800. It seems that the FMC calculates the VNAV PATH from one constraint to the next one.http://forums.avsim.net/user_files/187236.jpgI was expecting the VNAV PATH to be as the blue line, where the FMC is calculating a TOD for each segement between constraints.BTW This is including getting rid of the magenta vnav bug after each constraint. Also, I'm not a RW pilot and I have no knowledge of 737's whatsoever, except from what I learn in this forum.Am I doing anything wrong or is this normal (PMDG) FMC behaviour?

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Frank,If I understand what you are talking about, this is normal behaviour. Some early versions of FMC software did calculate the descent with constraints like your blue line. It would descent at best rate down to the next constraint and level off until approved to descent further. On the newer software versions the path is calculated direct from one constraint to the next (within limits of course). It makes for a smoother descent.A popular 737 Classic sim is performing the older type of VNAV descends.Hope it helps,

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Mats,it is exactly what I was talking about, thanks for the answer.

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In your example, the FMC provides a "better" profile. But there are many cases with multiple constraints where the "at or above" altitude is acutally lower than the desired crossing altitude. Ideally, the FMC wouldn't descend all the way down to an "at or above" but down only so far as required to maintain the best profile considering all of the multiple constraints. However, the FMC seems to calculate a profile for each segment rather than a path to the lowest constraint.I've always wondered if the PMDG FMC is representative of real world descent profiles, or a simplification of a more complex algorithm?

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The 744's VNAV is much closer to what the real life thing does - there's admittedly some shortcomings in the original NGs. The NG 2.0's should be the best yet though. PROF mode on the MD-11 is being done very realistically too.

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I'm curious now: what's the real life 737 FMC VNAV doing?And: get that NG2.0 out :-lol

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Frank,I think Ryan is talking about the shortcomings most are aware of. The VNAV speed bug bug, path calculations over altitude constraint points to name the ones I can think of off the top of my head.And I second your second statement!Cheers,

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