PAPI versus APAPI: what is the difference?
Four units against two. The same white-above, red-below code, but the PAPI shows the error in finer steps.
The PAPI and the APAPI (abbreviated PAPI) work in the same way. Each unit shows white above its setting angle and red below it, with a sharp change between the two. The units are set at slightly different angles, so the pilot sees a different mix of white and red at each height.
The difference is the number of units. A PAPI has four, so it gives five steps from all white to all red, and the pilot is on slope with two white and two red. An APAPI has two, so it gives three steps, and the pilot is on slope with one white and one red. More units give finer feedback; fewer units cost less and need less equipment.
| Feature | PAPI | APAPI |
|---|---|---|
| Units in the wing bar | 4 | 2 |
| Indications | 5 steps, all white to all red | 3 steps, both white to both red |
| On slope | 2 white · 2 red | 1 white · 1 red |
| Feedback | Finer: error shows in more steps | Coarser: fewer steps |
| Equipment and cable | More units to supply and align | Fewer units, less equipment |
| Typical role | Wider range of runways and aircraft | Runways with smaller aircraft |
In both systems the wing bar is normally placed on the left side of the runway, as seen by the approaching pilot, and a second bar may be added on the other side where the standard permits or requires it. The setting angles, the unit spacing and the distance from the threshold come from the standard and the manufacturer’s data.
Frequently Asked Questions
Q1What is the difference between a PAPI and an APAPI?
A PAPI uses four light units in a wing bar. An APAPI, the abbreviated PAPI, uses two. Both use white above the slope and red below it, but the PAPI shows more steps between all white and all red, so it gives finer guidance.
Q2How many indications does each system give?
A PAPI gives five: all white, three white and one red, two white and two red, one white and three red, and all red. An APAPI gives three: both white, one white and one red, and both red.
Q3Do both systems use the same colour code?
Yes. White means the aircraft is above the slope, and red means it is below. On the slope the pilot sees a mix of white and red: two of each on a PAPI, and one of each on an APAPI.
Q4Is an APAPI less accurate than a PAPI?
It gives coarser feedback, because there are fewer steps between high and low. The lights are aimed to the same approach slope in both cases. Where each system is used is set by the standard, and it depends on the aircraft that use the runway.
Q5Can I choose between them from this page?
No. The choice depends on the runway, the aircraft that use it and the applicable standard. Use this page to see how the two systems differ, and take the design decision from ICAO Annex 14, the FAA advisory circulars and the manufacturer’s data.
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