GS vs PAPI/NPA vs PAPI

[Capn Bloggs]

Assymetricdrift,

Going into somewhere like Gatwick, if you follow the papi indication on the way down, you will find that your glide slope indication goes far too high. If you follow the ILS indication below minima you'll see that at 150 feet or so, you hit 3 red lights.

What is the TCH for the ILS and the MEHT for the PAPI? This will probably explain what you see.

Now, my question would be if you were doing a Cat II/Cat III landing, and with the autopilot disconnected at MUH, what the indication on the PAPIs would be? We did a practice Cat II into an airport recently, and when we came out of the simulated approach at 100 feet RH, the PAPIs were showing 3 red and 1 white.

From a previous thread on this issue, the Canadians switch off the PAPI during low vis ops so this phenomenon doesn't occur. If you're on-slope on the ILS at 100ft, doing 10ft/sec, you must be pointed at the correct aim point and unless you do something crazy with the flight path you're going to "hit" the runway at the right point. What the PAPI shows is irrelevant.

[planeboy_777]

I think this will sort out my doubt

I'm flying an A320 whose eye-to-wheel height is 24 feet.
the Airports TCH is 50Feet


Situation A:
if i'm approaching with the help of PAPI as my refrence
what will my Radio altimeter read?

Situation B:
if i'm approaching with the help of ILS GS indication as my refrence
what will my Radio altimeter read?



Please help

[Centaurus]

I am going back many years here but in another time I was working with the Australian Dept of Civil Aviation Navigation Aid Calibration Unit. This involved flight testing of ILS and also T-VASIS. Although the ILS was flown down to 50 feet to look for aberrations below the nominal Cat 1 of 200 feet, it was not intended that aircraft would follow the ILS glide slope below 200 feet since measurements below 200 ft were not always reliable.

I am sure measurements are more reliable now with the advent of lower DH. On the other hand the T-VASIS was only flight calibrated to 200 feet and it was never intended to be used below 200 feet because of lateral siting.

VASIS and PAPI are visual aids to give an accurate visual slope from 4 miles and again it was never intended to fly an ILS in IMC to 200 ft and then use the PAPI/VASIS below that. In any case if the aircraft is stabilised on the ILS at 200 feet there should be no need to further "fine tune" below 200 ft as the aircraft is within seconds of crossing the fence and heads should be up unless Cat 2 or 3 ILS.

Attempting to switch to a PAPI glide path indication from an ILS at 200 feet is asking for trouble as the siting of the PAPI may not be the same position as the ILS glide slope transmitter. In other words don't make things unnecessarily complicated when there is simply no need.

[compressor stall]

By my calculations, for every foot your eye height (published in your AFM) is less than that of the papi (in some countries published on the Jepp charts) you will land 7 meters long.