Hello!
Airbus A310, Non-Precision-Approach, selected vertical Guidance (e.g. RNAV-Approach with LNAV-Minimum only):
There is a „FPA“-Button and when pressed a Window next to it where you can enter the desired Flight-Path-Angle (e.g. 3 Degrees) on the Glareshield.
Question: how can i command the Autopilot / Flightdirector to follow the selected FPA?!
Of course i can select V/S as vertical mode, but why than this FPA-Selector?!
Thank you for any advice! 
See this thread:
The A310 has no such Button, like the A320 has one. Maybe you mixed that up?!
My apologies, I should have checked before linking to that post.
The poster must have been thinking A320.
Although I fly jetliners (a lot) I never use FPA, mostly visual, or in ucky weather ILS/ RNav.
I figured that I should find a copy of an A310 manual and find this out for sure, and a google search revealed this:
The introduction is not in English, but the manual itself however, is English.
I couldn’t find any mention of FPA in the manual, so I it seems it may not be a function the A310 has.
There is a lot of good info on the A310 in this manual.
There is a sort of FPA mode. The FPA button below DH will allow you to enter an FPA, which you can see on the PFD by flicking the FD/FPV switch up to toggle on FPV. This is for visual reference ONLY, the autopilot will NOT fly according to the Bird, unlike the newer Airbus aircraft
I have also been pointed to this site which has an excellent manual.
https://www.smartcockpit.com/plane/AIRBUS/A300-A310.html