RAT 5, Re “A misalignment of a PAPI is more likely than a G/S error”.
Maybe, but great care is required with most assumptions.
With PAPI (sharp transition lights), depending on the ground installation/location of the PAPI-GS transmitter, there may come a point during the approach where the difference between the aircraft GS aerial and eye height will, for a zero GS error, result in an indication of a visual error.
Also, remember that at very low altitude the GS beam is parabolic – it may never actually touch the ground along the flight path over the runway (the reflected beam, off the ground surface in front of the transmitter, is at the side of the runway).
Furthermore depending on the aircraft’s autoflight system, the vertical flight path phases out the GS signal and may phase in rad alt / flight path / attitude until the flare; the aircraft may deviate from the GS.
Then there may be complexities with the relative beam widths of the GS and PAPI.
Often very little about standard systems are ‘standard’, but the compromise is good enough; in the case of PAPI it is a considerable improvement over VASI enabling it to be used at much lower altitudes consistent with low visibility operations.