if you are on a Cat 2/3 approved rwy for autolands the a/c will follow the ILS G/S to touchdown; so why not do the same on a visual day and manual landing if there is a difference between it and PAPI.
The GS will only be working correctly when the GS sensitive area is clear of aircraft and vehicles. If you have a CAT3 system and want to use the GS below 200ft then you have to ensure that the protections are in place on the ground. Can't put my finger on the piece of paper but we hab a remider about practice autolands some time back that cautioned about doing them without the normal protections in place on the ground.
In the old days of VASIS 2 & 3 bar we flew 2 red 1 white on 3 bar systems in a medium a/c. I always wondered why not the same on PAPI; 3 red 1 white.
PAPI system is angular and radiates from a point - just like your ILS. If you fly a constant 3 white and 1 red you are flying a steeper approach angle.
VASI was a multiple plane system and each unit was set to say 3 degrees. What you did with VASI was to put yourself above a 3 degree slope towards the near unit (white) and below a 3 degree slope to the far unit (red) thus you made an approach between the two 3 degree sloped planes indicated by the units.
When flying a large aircraft, you placed your eyes between the 3 degree planes of the two furthest units (ignored the nearest one) and thereby increased your threshold crossing height but also moved your visual aiming point further down the runway.
This touchdown point is not published, but by nature if it is safe for a BIG a/c it is safe for a smaller one.
The touchdown point is certainly published. It is very obvious and is very well marked on the runway. The PAPI are required to take you towards the touchdown point.
The ILS GS and PAPI should always tell you the same (when flying above the certified minima for the ILS GS) ICAO Annex 14 makes provision for widening the PAPI on-slope indications to allow for differences in PAPI and ILS GS set-up.
The problem with using the GS below 200ft in a CAT 1 system is that the GS may not always be at 3 degrees and there can be reflections etc etc
However, the PAPI guarantees you obstacle protection until you cross the threshold.
Simce EBCI was used as an example earlier, have a look at the AIP charts. You can see that the touchdown markings are more than 300m beyond the threshold and this will give a high threshold crossing height for small aircraft. You can also see from the chart where the PAPI and GS antenna are located.
Regards,
DFC