What is an APAPI configuration?
The abbreviated PAPI: one wing bar of two light units that shows the pilot three signals, high, on slope or low, by colour alone.
An APAPI is the abbreviated form of the PAPI. It works on the same principle, each unit showing white above its set angle and red below it, but it uses two units in a single wing bar instead of four.
1Two units, one wing bar
The two units sit side by side beside the runway, normally on the left side of the runway (as seen on approach) and at the spacing the standard gives, and are aimed along the approach. Each is set to its own elevation angle, so the two change colour at slightly different heights.
2Three signals instead of five
The pilot sees both white when above the slope, the nearer unit red and the farther unit white when on or close to it, and both red when below. A four-unit PAPI gives five signals, so it shows deviation from the slope in finer steps. An APAPI is used only where the standard allows the abbreviated form, typically where the runway serves smaller aircraft (lower code numbers).
3Check both units, not just one
With only two units there is no spare. Check that both are lit and equal in brightness, in the right position, aimed parallel to the runway centreline and set to their elevation angles with a sharp white-to-red change. A dark or misset unit gives a false slope indication.
| Element | APAPI | PAPI |
|---|---|---|
| Light units | 2 in one wing bar | 4 in one wing bar |
| Signals shown | 3: high, on slope, low | 5: from high to low in finer steps |
| On-slope signal | Nearer unit red, farther unit white | Two nearest red, two farthest white |
| Principle | White above set angle, red below | White above set angle, red below |
| Setting angles | From the standard and the manufacturer | |
Frequently Asked Questions
Q1What is an APAPI?
An APAPI, or abbreviated precision approach path indicator, is a PAPI-type slope indicator with two light units in one wing bar instead of four. Each unit shows white above its set angle and red below it, so the pilot reads the approach slope from the colour pattern.
Q2What signals does an APAPI show?
Two units give three signals: both white when the aircraft is above the slope, the nearer unit red and the farther unit white when it is on or close to the slope, and both red when it is below. A four-unit PAPI gives five signals, so it shows how far off the slope the aircraft is in finer steps.
Q3What should be checked on an APAPI configuration?
Check that both units are present and working, that they are on the correct side and at the correct spacing, that each is aimed parallel to the runway centreline and set to its elevation angle, and that the two show the same brightness and a sharp white-to-red change. Setting angles come from the standard and the manufacturer.
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