Acegreaser,
Flying a 'one dot low' approach on a PAPI will bring you in on a slightly lower/flatter approach to the same point on the runway because all the lights are in the same plane. With the undercarriage behind and below they will touch down closer to the threshold than on a normal 2 red/2white approach,presuming no 'flare'.
With a T-VASI flying one dot low(upright 'T') you'll be coming down a similar approach angle to a point closer to the threshold so will touchdown 'short'.
The point of flying a one dot low approach onto wet/short runways is to ensure touch down at the correct place AFTER you flare,IMHO. In other words the PAPI/VASIS approach plane intersects the flightstrip at a specific point(the 1000' markers)if you're on slope, having rounded out and flared the aircraft to the correct attitude for touch down you'll touchdown beyond that point(say 1200'-1400' in). Flying a 'low' approach simply brings the touchdown point back towards the 1000' markers, if done correctly.
Can anybody comfirm/deny something I vaguely remember hearing/reading many moons ago that visual approach aids are 'calibrated' to give a specific threshold crossing height for a specific size of aircraft? Say a 737 or 727 ? If not you would think that the same height of eye on a 747(assuming on slope) would bring the dunlops perilously close the threshold as opposed to the touchdown zone. I vaguely remember hearing that 747s make a 'one dot high' approach for this reason.
Chuck.