Where a PAPI is installed on a runway served by an ILS, ICAO requires the PAPI to be "harmonised" to the ILS GP. That way, everyone should, in theory, see the same two whites and two reds.
Not quite! As ´FEH´ says, a 747 following NORMAL vasis will be below the calibrated approach due to the fact that where the VASIS are ´seen´(ie the cockpit) is significantly higher that where they are ´seen´ 'n a ´normal´ a/c. Hence the so-called ´long-body´VASIS?
Perhaps Jumbo drivers could help by telling us what they do if landing on ´short-body´ VASIS?
I THINK the VASI/PAPI approach path ´harmonistaion´ is based on wheel height? Anyone know?