How do the APAPI beams work together?
Each of the two units splits its beam into white above and red below. Set at two different angles, they give the pilot a high, on-slope or low signal.
The two units of an APAPI are identical, yet together they show three different signals. The reason is the beam arrangement: each unit splits its beam, and the two are set at different angles.
1Each unit splits its beam
A unit projects one beam that is white above its setting angle and red below it, with a very sharp change between the two. The pilot reads only the colour, not the brightness or the distance.
2Two settings make the on-slope band
The unit nearer the runway edge has the higher setting and the farther unit the lower, typically about 15′ above and 15′ below the nominal approach angle. Descending, the pilot first sees both white, then the nearer unit turns red to give nearer unit red, farther unit white, and finally both red. The narrow band between the two settings is the on-slope signal. The two units sit side by side across the runway, so both are the same distance from the threshold.
3Check the change, the angles and the aim
Check that both units show a sharp white-to-red change, that each is set to its own elevation angle and aimed parallel to the runway centreline, and that the two are equal in brightness. A dirty lens, a blocked beam or a shifted unit moves the band and gives a false slope indication.
| Element | Setting or signal | Effect on the pilot's view |
|---|---|---|
| Unit 1 (nearer the runway edge) | Higher setting angle (about θ + 15′) | Turns red first on descent |
| Unit 2 (farther from the runway edge) | Lower setting angle (about θ − 15′) | Turns red second on descent |
| Above both settings | Both units white | Above the slope |
| Between the settings | Nearer unit red, farther unit white | On or close to the slope |
| Below both settings | Both units red | Below the slope |
Frequently Asked Questions
Q1How is the beam of an APAPI unit arranged?
Each unit projects a single beam split vertically, white above its setting angle and red below it, with a sharp transition between the two. The pilot reads the colour of each unit, not its brightness or distance.
Q2Why are the two APAPI units set at different angles?
The unit nearer the runway edge has the higher setting and the farther unit the lower one, around the nominal approach slope. As the aircraft descends the nearer unit turns red first and the farther unit second, so the band between the two settings gives the on-slope signal: nearer unit red, farther unit white.
Q3What should be checked on the APAPI beams?
Check that both units show a sharp white-to-red change, that each is set to its own elevation angle and aimed parallel to the runway centreline, that the two are equal in brightness and that nothing blocks or dirties the beam. Setting angles come from the standard and the manufacturer.
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