approach indicators  (VASIs & PAPIs)

A pilot should always be able to control his/her descent down onto the runway in good visibility. However, many airfields operate approach indicators. These are a range of lights alongside the runway placed just after the threshold. T

Visual Approach Slope Indicators (VASI) are normally used both during the day and night. They provide the pilot with an “onslope” glideslope angle of approximately 3 degrees, depending on the local authority rules, which normally are based on the most common class of aircraft that the airfield is used for. If the airfield is for military fighter aircraft the slope is normally high, 3 degrees or just a fraction above. If it is more commonly used for say Jumbo Jets the glidepath is more likely to be in the region of 2.75 degrees.

The 2-bar VASIS has 2 ranks of lights. Each rank may consist of one light or two lights side by side. The pilot is “on glide slope” as shown in the centre diagram (red over white). If he is too high both ranks will show white, as on the right in the diagram. If too low then both ranks will indicate red as shown on the left.

The 3 bar VASIS has 3 ranks of lights, The two centre show “on glide path” indications. The leftmost is a low path, and the rightmost is a high path. The all red is too low. The all white indication is too high.

Precision Approach Path Indicator (PAPI)

Another approach slope indicator is the Precision Approach Path Indicator. The system provides a more precise glideslope indication than does VASIS when all 4 lights are white, you are too high. When all are red, you are too low. When 2 are red and 2 are white you are on the correct glideslope. Three white on the left indicate that you are slight too high while three red indicate that you are slightly too low. Again the system is set for the average aircraft using the particular airfield.

The Tri-Colour system is a single light that projects 3 colours. The above glide path indication is amber. On glide slope is Green. Below glide slope is red. When the aircraft descends from green to red, the pilot may see amber during the transition.

There is a similar system called the Pulsating Visual Approach Slope Indicator. It is somewhat similar to the Tri-colour except a solid white indicates on glide path; steady red on a slightly low path. Pulsating white indicates too high. Pulsating red means too low.