FPA is another method of manually selecting a descent in the aircraft. Instead of using V/S, FPA can be selected instead. Depending on approach reference speeds, 700 to 800 fpm on an approach equals out to roughly a 3 degree G/S. But, if it is easier to think in terms of a 3 degree slope...one can just select FPA..simple as that.
With regard to VNAV approaches, using the autopilot...Yes, it is done! and.....it works wonderfully. The only requirement is that the approach has to be properly 'coded' for use. We can determine this on the legs page and having a look at the approach. If the approach is 'coded', there will be an FMS notation depicting the G/S angle...i.e. 2.99 deg. Along with the GPA, there are 'hard' crossing altitudes for the autopilot to follow, or fix crossing altitudes.
If the approach is coded, then a VNAV approach can be used. So for instance on a localizer only approach, all that is required is to load the approach..say a normal ILS approach....verify that the approach is satisfactory for use, configure the aircraft properly for the approach, using LOC for lateral guidance and then selecting VNAV for vertical guidance.
When the aircraft FMA is annunciating VNAV Path, one only has to sit back and watch the airplane 'do-its-thing'. The FMC calculates the angle and descent point to begin down to MDA, the fix crossing restrictions are all taken into account, thus the airplane flies a continuous descent of MDA....if one is looking out the window at the PAPI or VASI, the aircraft is right on the slope all the way down.
The most disconcerting part of flying a VNAV approach this way is that at the point that we precalculate the descent point using our 'natural computer'..our brain....sometimes the airplane waits a bit longer to begin the descent and it is difficult to resist the temptation to select either FPA or VS to force the airplane to descend. If another method is selected and we do away with VNAV, then the whole exercise becomes an FPA or VS approach and manual altitude restrictions need to be adhered to with vigor.
The system works very very well....NDB, VOR, LOC are all being coded for VNAV. The accuracy of the approach is so good, it is expected that the VNAV approach MDAs to be down to 250 ft.
Hope that helps a bit.....Safe flying.