Some pilots are reporting the glide slope on our PAPI is not coinciding with the glide slope on our ILS even though FAA did a flight check of our PAPI's. Are there any reasons why a PAPI and ILS would indicate different approach glide slopes?
From the FAA AIM (section 2-1-2(b) Visual Glideslope Indicators): PAPI angles are based on obstruction clearance +/- 10 degrees from the runway centerline, up to 4 miles from the threshold. The ILS provides for obstruction clearance in a 3-6 degree angle on a 3 degree glide path. Therefore, if there are obstructions in the PAPI clearance area (e.g. a tower off to the side of the rwy) that don't exist in the narrower ILS clearance area, the PAPI will have to be a steeper angle.
As a simple word: Height of eyes vs height of ILS antenna.
The extract from Transport Canada is as follow,
However, this is not possible to achieve completely because the pilot eye is above the aircraft antenna which senses the ILS signal and the angle "B" of the lower edge of the PAPI approach corridor is less than the angle of the ILS signal.
At various airports e.g Paris De Gaulle (Lfpg), the PAPI's are set for cat D I.e B747, so the medium to light A/c class, will on a Cat 1 approach, appear to below PAPI 3% glide angle.