Another scenario is an instrument failure. This happened to me once when I was teaching IFR
The scenario:
ILS no DME with a NDB at the FAF. DH 400 feet, FAF ctossing alt ( gp check)
1400. We were on vectors at 2500 feet and told cleared for the approach, not below 2000 until loc intercept and to expect to join at or slightly above the glide slope.
I knew we were in close so I told the student he had better expedite to 2000, but he was slow to react and we were at around 2200 when we intercepted the localizer with the glide slope almost full down. He then started an aggressive descent. Very quickly we were approaching 1400 with no sign of beacon passage. I then took control and levelled at 1400. The glide slope needle remained full down even when we passed the FAF so I imediately started climbing and followed the missed approach procedure.
On return to our base the system was tested and an internal failure of the instrument was confirmed. At no time did the instrument display a flag.
This scenario developed very rapidly and the student realized when I took over we were maybe one minute from flying into the ground due to the high descent rate he had established. He said all he was thinking of was chasing the gs needle. The glide path check point can literally be your last chance to save your life.
The final lesson was if you experience an abnormality during the approach, do not try to troubleshoot it, go around.
Ignore the bollocks taught in flight schools about switching to a non precision approach if the gs fails
