Yeah the numbers are a bit out O8 but they serve to illustrate the point well enough. The fact is that a PAPI is set up so that the onslope indication is within a range of 2.95 - 3.05 degrees, for a normal threshold crossing height of between about 48 and 51 feet.
Once the onslope angle is worked out for the desired TCH, the other indications are calibrated. The first indication of 4 red lights should occur at an angle of around 1.9 - 2.0 degrees. The first indication of 4 white lights will occur at an angle of about 3.5 degrees.
There can be variations on this, to cater for airports that only cater to smaller aircraft types, but that's not especially relevant to the question. Suffice to say that its possible to have a TCH down to (I think) about 35 feet.
Last edited by OzExpat; 14th Aug 2002 at 10:03.