Manual on how VNAV computes descent

[easymxp]

Hi all,

Just for my curiosity I'm looking for a detailed explaination on how VNAV computes the descent profile, like a step-by-step, I tried to find on the net but nothing. Do you have any link to a similar document?

[FlyingStone]

Which aircraft? C172 or PA28?

[easymxp]

I was looking for the B737, but I'm trying to understand deeply how a computer plans a descent path and which/how considers variables, so any aircraft would be welcomed...

[Amadis of Gaul]

Which aircraft? C172 or PA28?


Curtiss Jenny.

[FlyingStone]

Bill Bulfer's FMC guide should give you as detailed description as you could find:

Boeing 737 FMC Users Guide (http://www.afeonline.com/shop/boeing-737-fmc-users-guide.html)

[aterpster]

PFM, I believe.

[jack11111]

I think it has something to do with triangles.

[mustafagander]

By whatever PFM it uses, it's not very good at it. You always need to know a few gates and see when it is off on its own not making sense. Three times table is your best friend!

[easymxp]

Thank you all!
Of course I know, 3x table is always there to backup :). I just wanted to understand in dept how it really works.

[Amadis of Gaul]

It's kind of like SPICA fuel injection on older Alfa Romeos. It works, but nobody is real clear on how.

[aterpster]

By whatever PFM it uses, it's not very good at it. You always need to know a few gates and see when it is off on its own not making sense. Three times table is your best friend!

That doesn't work on high-traffic STARS. Case in point: Freedom Three STAR into KDCA from the west:

[RAT 5]

One very important thing to remember; no 2 VITR.

VNAV does not update actual winds during descent. It has computed a descent profile with what you told it to expect, or from actual TOD wind reducing an unknown algo' to zero at ground. It needs monitoring and some help; at times.

2nd most important thing to remember is it needs monitoring at all times.
3rd most......same as 2nd!!

There in lies the most oft heard ????? from cadets. Sadly 3x anything, too often, requires a japanese brain to find an answer.

[aterpster]

One very important thing to remember; no 2 VITR.

VNAV does not update actual winds during descent.



VNAV is a ground-referenced procedure when used on STARS. That takes care of IAS vs. TAS.

With a state-of-the-art VNAV FMS, if the pilot or the procedures places unacceptable constraints on VNAV, the FMS should provide a warning, and perhaps even a disconnect from VNAV.