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.
Pilot complaints about mismatch between ILS and PAPI information are very frequent, and not necessarily a sign of malfunction.
There can be many factors at play, but assuming good design and good operating condition of both systems, this mismatch is due to the differences in aircraft geometry amongst aircraft types, and more specifically due to where the Glide Path (GP) aerial antenna is located with respect to the cockpit for each type of aircraft.
The Glide Path signal is perceived by the antenna, which is usually on the nose of the aircraft, whereas the PAPI, being a visual aid, is perceived by the pilot's eyes. Each aircraft type will have a different location of this antenna with respect to the cockpit and the eyes of the pilot, but they are never at the same height. This causes misharmonization of the ILS and the PAPI.
PAPI siting, for an ILS runway, is done such that the PAPI provides as close signal as possible to the GP signal for the majority of aircraft types that use that particular runway.This process is called harmonization, and by definition the degree of harmonization will differ for each aircraft type.
Pilots will therefore perceive that the information visually acquired from the PAPIs will differ from the on-path information obtained from their on-board instruments. This is normal and to be expected at any airport where PAPIs are used in conjunction with ILS, and it is more accentuated at airports where there is a wide range of aircraft commonly using the runway.
That said, a good harmonization will aim to err on the side of safety, meaning it should never give a fly-down (three whites and one red) due to lack of harmonization for a given aircraft type, but a fly-up signal.