Why would a PAPI and ILS glide slope for the same runway not coincide? 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.