How do PAPI's and VASI's work?

[shon7]

How do PAPI's and VASI's work? One response I got for (VASI) is that they are divided and thus from a higher altitude you can only see white and lower you see red? But how would this work in the case of a PAPI with 4 separate lights?

Thanks for your answers. Sorry if the question is somewhat ambigious.

[Oktas8]

This is what I've been told for PAPI's:

Each light box projects through a lens. The lens is designed so light passing through the lower half of the lens is red, and the upper half, white. There is a sharp dividing line between the two - no blurring!

The four lights in an array are all calibrated to point up at different angles. For example, (in degrees above horizontal) the four lights might be set for 2°, 2.5°, 3.5° & 4° respectively. Anyone on a glideslope of 2.5° to 3.5° sees two red, two white. A pilot descending lower will see three reds, or four reds if below 2° glideslope.

Similarly, a high glideslope will give three or four white lights, with one or nil red lights.

Note that my calibration numbers are fictitious - perhaps someone else can provide "real" angles?

cheers, O8

[OzExpat]

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.