Originally Posted by
Capn Bloggs
The Threshold Crossing Height (TCH) for the ILS and the Minimum Eye Height (MEHt) for the PAPI have to match for the PAPI to line up when you are on the GS.
Wish this was true, but it isn't. FAA does not recognize (good!!) the ICAO standard for 2400+ runways and puts the aiming markers on the second position (300 mtrs) - then it works as you describe.
The ICAO standard is 150+150+100 = 400 mtrs where the PAPI becomes too deep. Some airports even have 150+150+150 = 450 and everything is all over the place.
N.b. the PAPI (and markers) displacement does not affect the GP antenna or GS calibration of 50' over TRH.
Yes, the GP antenna is not located at the theoretical 3° intersect point due to asymptomatic GP bending close to the ground (below 10 ft or so, on a modern-day installation which is CATII capable it is darn precise to very very low) but slightly farther out. This is not relevant to the discussion, the GP signal itself is OK.
When the PAPI are not at the 300 meters mark, as suggested by the OP (300*66/50 = 400 m), flying below the PAPI 3° signal is a geometrical necessity as GS and PAPI don't align. From below 100 this will become visually apparent (3 reds) and due to the aperture / parallax / sideways distortion 4 reds are a possibility.