Hi DFC. You'll find that I said "The PAPIs may be disregarded below 200 ft AGL provided your aiming point is safe." Please don't selectively quote out of context.
A person complying with your statement could make a very shallow approach (4 reds on the PAPI) and think that their aiming point was safe because the can see it and see no obstacles between their eye and that point. Unfortunately, their eye is not at the wheels which could be dragging through the trees.
At airfields where the MEHT is less than 45 feet, then I have to fly 3 whites 1 red (or whatever it takes) to ensure my aiming point is safe.
You really need to look again at how PAPI work.
With a 45ft MEHT, flying with 1 white and 3 red gives you an extra 5.3ft.
Because unlike VASI based systems, all lights have a common origin.
So if the MEHT is 40ft, flying 1 white and 3 red only gives an extra 4.7ft. So you will not get your required MEHT by flying 3 white and 1 red.
For PAPIs, the method of increasing MEHT is to move the lights deeper into the runway.
These issues are not something that can be done on the fly. They require someone to sit down and wprk out the issues. Remember that a lower MEHT at the threshold can be acceptable in the case of a displaced threshold.
So if you are flying a B737, the ILS at LGW will put you in the 3 reds 1 white PAPI (which is set for B747) when you are below 200ft.
A totally separate issue. The PAPI are set for a B747 (A380?) but so too is the ILS. If that was not the case then how would the B747's wheels clear the threshold during a CAT 3 autoland?
I think here you are confusing eye to wheel height with eye to GS antenna height. Not only do these vary from type to type but they can also vary quite a bit in the same type.
Even forgetting PAPI and ILS for a minute, dont forget that the distance from the threshold to the aiming point varies with runway length. -
150m for less than 800m
800 to 1199 is 250m
1200 to 2399 is 300m
2400m plus is 400m
So even with no PAPI and no ILS if you fly a normal 3 degree approach to a 1100m runway you will cross the threshold lower than when you fly into a 3000m runway if you use the standard aiming point.