First of all, which height can we rely on glideslope indication?
From what I know, we can trust and use glideslope to Decision Height, not even below Decision Height.
If we use glideslope even passing Decision height, Glideslope signal can become unreliable and it can impose some problem related to signal interference.
I can't find any reference on it. It's just my inference.
Second, PAPI is for a visual glide path. What I mean is, PAPI provides a descent angle when we see at our eye height at the cockpit.
However, Glideslope provides a descent angle that the airplane's glideslope antenna passes sited angle.
But, according to ICAO Annex 14 vol 1, aerodrome design, it says that PAPI collocated with ILS should be installed that it coincidences with the glideslope indication.
So, I think below decision altitude, relying on PAPI indication will make the airplane descent even as same as glideslope descent angle.
I know that every airplane's eye height is different. And, in some airplane that has a lower cockpit than the standard operating airplane at the airport, if the pilot uses papi after descent altitude, the airplane will need to climb for papi glidepath which should be higher than glideslope.
However, is using papi after decision altitude not okay even for standard operating airplanes?
What I want to say, in every case, only following glideslope's descent angle even after the pilot can see PAPI is allowed or not.