Today, during a technical conference it was mentioned by a highly reliable source that the B737NG FMS has two programming options for VNAV, one based on descent performance and the other based on geometric path.  It appears that the performance option is simulated in the NGX.  It was said that most FMS installations use the  performance option, but at least one prominent B738 operator in the US uses the geometric path option.

 

Just curious if PMDG included this option in the NGX and I just haven't found it yet, or if they considered it and decided not to include it.

 

Thanks,

 

Rich Boll

Wichita, KS

Rich,

 

We did model the geometric path option actually, not the performance/idle path one. It follows an idle path from T/D until the first restriction and then goes to geometric for the rest of the path (which is how the real one also works). You can see this happen when the A/T goes from HOLD to SPD after the first restriction.

 

The performance/idle path option works quite differently - it constructs level flight segments followed by additional T/Ds beyond the original one. (they show up as things like "T/D-9000" on the ND) Essentially it considers every restriction to be a new cruise altitude with an idle path descent from a new T/D down to the next restriction.

 

The geometric path option is the more advanced and complicated one programming wise and that's what we chose to model. (it's also closer to what the 777 does, which is a hybrid mode) It's possible we may do the idle path one for SP2, but I doubt it as it's a whole bunch of extra coding and testing for little tangible benefit to anyone.

Indeed it has been mentioned that PMDG chose to model the geometric one, only to find out later that it is very, very rare in real world and only a handful of operators use it as compared to the other one...

Tabs & Fabio,

 

Thank you very much for the quick response and the information on the NGX programming. It was interesting to hear about the options for the VNAV descent, and more interesting to hear that the NGX uses the Geometric Path, which if I understood correctly from yesterday's had some advantages when flying Optimized Profile Descents.

 

Once again, thanks for the reply. I enjoy the NGX and eagerly await the B777.

 

Thanks!

 

Rich Boll

Indeed it has been mentioned that PMDG chose to model the geometric one, only to find out later that it is very, very rare in real world and only a handful of operators use it as compared to the other one...

 

I don't know about "very very rare" - more have switched to it since the advent of all the RNAV arrivals and RNP approaches.

IIRC you have told so at the time (or was it Robert?). It could have definitely changed since.