I remember that the two-bar-VASI was the first “visual aid” then came the three-bar-VASI with the wide body transports and at last the PAPI which contains the
word precision in its name …
However the systems should not be used below 200ft GND.
However the systems should not be used below 200ft GND.
Huh? And why is that? I thought the main reason of existence of PAPIs was that you could smoothly transition from vertical guidance using the glideslope to vertical guidance using the PAPI at the Cat I DA.
Excerpt from an Airline Operations Manual:
Use of VASIS, T-VASIS or PAPI
On runways equipped with VASIS, T-VASIS, their abbreviated versions (AVASIS, AT-VASIS) or
PAPI the visual glide slope must be adhered to as closely as possible. They must not be used
below 200 ft above threshold elevation. For details refer to OM-C.
The use of non-standard visual glide slope indicator systems (e.g.: VGPI) is prohibited.
The use of 2-bar VASIS by B 747, B767, B757, MD11, A 340, A 306 and A 310 is prohibited.
Note: In conditions of ground fog, mist or snow T-VASIS may provide erroneous indications due to light refraction.
Another (printed) older version says:
Due to glide slope tolerances, however, VASIS, T-VASIS or PAPI indications shall not be used below 200 feet above aerodrome elevation.
Reaching the CAT 1 minimum you basically keep attitude, speed and descent rate aiming at the touchdown point which is about 1000-1500ft inbound from the threshold.
You do not follow the VASI indication. Often the glide slope ( if it is not a CAT II/III quality beam) is also not reliable beyond the CAT 1 minimum (heavily bent sometimes).
Why would they be invalid below 200ft?
I know that you will get all reds if you are trying to land on the start of a runway. The two reds plus two greens is what you get for the touchdown zone which is about 1/3 down the runway, but in GA one rarely goes for the TDZ.
Ah, those wonderful Operations Manuals… 
Frankly, I think the main advantage of these devices comes during approaches which don’t have any glideslope at all. Especially at night.
Once at the CAT I ILS minimums, one really has essentially made it. As said, don’t change much, just continue and that will get you to the touchdown zone.
Excerpt from an Airline Operations Manual:
I’m not an airline, and I don’t have and don’t have to abide by an OMA. (And my grandmothers have died, too).
Granted, the exact impact point depends on the “height of your seat”, so it’s certainly different for an A380 compared to a C152, but I don’t see where the problem lies for small GA.
I think what this restriction of 200ft refers to is a night time VFR approach. No ILS glide slope … The change from correct red/ white indication (= on glide) to all red is so gradual that the moment you realize you are too low the threshold might just cut away your undercarriage – whether you are an airliner or a Swiss fighter pilot 
