Implementing VNAV support in FlightGear
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Problem
Vertical modes (VNAV) also tend to depend more on fuel/load and engine performance data from the FMS, which is an area that's been lacking in FlightGear, so hard to model the climb and descent profiles meaningfully.
It is the single most important reason why this isn't yet supported by FlightGear, which boils down to a lack of support by the FDMs (JSBsim/YaSim) actually. So far, the general consensus has been that it simply isn't yet possible in FlightGear to properly implement VNAV.
I've seen the use of simple simulation within a simulation, where performance data is being calculated. It was not within JSBSim, however. This is an interesting prospect. Nobody has ever done this, up to now. My initial reaction was pessimistic. However, the flight control components within JSBSim are fairly capable, and you are permitted to define arbitrary functions. So, my guess at this time is, yes, you should be able to set up JSBSim to calculate *some* performance data. It may be a lot of work, but it should be possible.
Without being very familiar with the FG source code, the usual way to do this is to call the FDM. Isn't that sort of thing exactly what JSBSim is for? The distinction between a flight planner and a flight simulator is the bounds on the integrations. A simulator will integrate over a frame; a flight planner much longer.
It is one of those "holy grails" in FlightGear, I am not aware of any aircraft that currently support a proper FMC/CDU "progress" page - creating the page is trivial, but making it functional requires some more work - look for threads with the keywords "performance database", and you'll see that this has been talked about for over half a decade meanwhile. (computing remaining fuel vs. fuel consumption per hour is fairly trivial, but once you want to compute range, you need to be fully aware of the flight plan (speeds, altitudes) and weather/configuration (and aircraft/engine performance in particular) to make a reasonable guess at remaining range).
So, we currently do not have any aircraft with a properly working FMS and performance database support.
Overall, airliners like the 777 in FlightGear remain among the most developed aircraft/airliners in FG, especially in comparison to hundreds of other aircraft. But compared to airliners supported by FSX or X-Plane, we are still lacking in that department and do not provide a complete solution (yet).
And it is also not foreseeable when that will change, because it will require changes in the FDMs we're using or use of some 3rd party/proprietary performance database. Otherwise, it is not currently feasible to come up with VNAV/LNAV modes that properly work for different aircraft. The 3rd party approach is being used for TPPs data, but in the case of FDMs, we would need 100% accurate FDMs, too - so it would be better to directly use the FDM for computing a performance DB. Supporting VNAV/LNAV still is tricky for other reasons, and currently not on anybody's agenda - it's not just something that can/should be tackled by a single aircraft developer, it requires lower-level changes in FlightGear's architecture and the way FDMs are used.