Mainframe
".. the whole point of the Final Approach Fix is to provide a final check prior to committing to descent below the MSA"
OK, but .....
MSA with 10 nm of LHR NDB is 3100'. The point at which you need to commence your descent to get the 3.49 degree descent path is 1.7 nm from LHRWF. If you are on the suggested descent profile at LHRWF, then you will already be at 2860'.
On the LHR Rwy 30 Appr you actually start the descent 4.2 nm from LHREF.
The Garmin 430 changes to Appr mode within 2 nm of the FAF so for the Rwy 12 appr it would probably be in Appr mode at the commencment of descent point, but for the Rwy 30 appr it would still be in Terminal mode when the descent is commenced.
I check that the GPS is in Appr mode at the FAF, but I would have already commenced the descent.
Dr