What is the difference between VASI lights and PAPI Lights?

Question

VASI - Visual Approach Slope Indicator

PAPI - Precision Approach Path Indicator

Answer 1

Well, first I'll tell you what's the same. They're both lighting systems that provide vertical guidance for aircraft on final approach. They both provide a unique visual indication of 'too high', 'too low', or 'on path'. The difference between a VASI and a PAPI is that a PAPI also has a 'slightly high' and a 'slightly low' indication.

In a typical VASI installation, you will see both lights white if you are too high, both lights red if you are too low, and one red and one white if you are on the glide path. There may be additional lights to provide an upper glideslope (for larger planes) and a lower glideslope. There is no way to tell how low or high you are.

In a typical PAPI installation, you will see four white lights if you are very high (.5 degree or more) and four red lights if you are very low (.5 degree or more). If you are within .2 degrees of the glideslope, you will see two red lights and two white lights. Slightly low will be three red, one white. Slightly high will be three white, one red.

Both a typical VASI and a typical PAPI simply consist of light boxes with two lights in each box and a metal divider such that which light you can see depends upon whether your sight line is above or below the divider. So they are very simple mechanical device, but they do have to be installed with great precision by people who know exactly what they're doing.

Answer 2

Great first answer, I won't repeat anything. If you want to tell the difference just by looking, VASI is usually stacked, 2 on top of 2 (more for larger aircraft) and PAPI are lined up in a row, usually 4.