Without wishing to plough through all of the very learned data on this thread, PAPIs are only as good as their last flight check, and that is a very variable standard! As pointed out previously, they are unreliable, at best, below CAT 1 minima, and also depend upon which Aircraft Cat they are calibrated. For instance, NCE 04L is set for Cat D, presumably to prevent them digging their tail into the Med. On my Cat C aircraft, the GS equates to 3 red/1 white.