PAPI - Precision Approach Path Indicator

Today i seem to have some spare time so i decided to actually put something intellectual up for a change. Over the last 2 weeks I managed to do some night flying, just lucky to get a couple of hours. Since its night time and we do not really have any visual with the ground, the instructor was telling me to maintain 2 white and 2 red on the PAPI lights on finals (approaching the runway from around 4 nautical miles out). So what is PAPI you may ask? Well I've got a little write up on it below. Incase you dont want to read it, its basially 4 lights (either white or red) that tells you that you are high or low on the approach.

Not interesting? Nothing special you might say.. Thats true actually. Its based on a simple concept of shinning 2 lights, upper white, lower red. If you are high you will see more white, low you will see more red. The ideal approach would be 2 white and 2 red.

Now its time for the slightly more 'interesting' write up..Enjoy!

Development of PAPI

The Precision Approach Path Indicator, commonly known as PAPI, was devised by Tony Smith and David Johnson at the Royal Aircraft Establishment Bedford in 1974, and developed by them over the ensuing two years. Research Engineers (RE) were also pioneers in the development of PAPI, having produced and supplied PAPI units for the first trials conducted. The same design is still in use today.

Function of PAPI

PAPI is a visual aid that assists pilots landing aircraft. PAPI enables a pilot making an approach to a runway to acquire and maintain the correct glide path from the time that the PAPI system becomes visible until the aircraft has crossed the runway threshold. PAPI systems utilize a set of two colour high intensity light projectors. The upper half of the light beam from each projector is white and the lower half red. The transition between the two colours occurs over a very small angle. This sharp transition is an integral feature of the PAPI system.
PAPI projectors are inclined at the nominal glide path angle. Typically, the elevation setting angles of each PAPI unit vary by 20 minutes of arc, the nominal 3 degree glideslope being midway between the centre pair. The approaching pilot sees either a red or white light signal from each PAPI light projector.

Advantages of PAPI

PAPI is a digital system and has the well proven advantage that it can be interpreted as soon as the white signal becomes visible. It therefore has a greater usable range than the older VASI system which relied on the red signal being seen.