Different PAPI systems
Four-unit, two-bar, two-unit and VASI: four ways to show a pilot the approach slope by colour, differing in units, signals and layout.
A visual glide-slope indicator tells the pilot, by colour alone, whether the aircraft is above, on or below the approach slope. The PAPI does it with a wing bar of four units, each showing white above its set angle and red below it. The APAPI is the abbreviated form, with two units. The two-bar PAPI adds a second bar on the other side of the runway. The older VASI reaches the same end with light bars instead of split beams.
More units give finer steps. A four-unit PAPI shows five signals, and an APAPI shows three. Because the signal depends only on the elevation angle of the aircraft, each unit’s setting matters: a unit that is mis-set, dark or dirty gives a false slope indication that the pilot cannot see is wrong. That is why setting angles are verified at commissioning and after any work that could disturb a unit.
PAPI, two-bar PAPI and APAPI use units whose beam is white above and red below. The number of units sets how finely the slope is shown.
The VASI shows the slope through the colour of near and far bars. It is an older design that PAPI has largely replaced.
| Aspect | Four-unit PAPI | Two-bar PAPI | Two-unit APAPI | VASI |
|---|---|---|---|---|
| Units | Four units in one wing bar | Two wing bars of four units, one on each side of the runway | Two units in one wing bar | Light bars, in near and far positions, rather than a single wing bar |
| Principle | Each unit shows white above its set angle and red below, with a sharp change | The same as the PAPI, with a second bar repeating the signal | The same as the PAPI, with two units | Colour of the bars seen by the pilot: red over white, white over white or red over red |
| Signals | Five, from four white (too high) to four red (too low) | Five, repeated from both sides | Three: high, on slope and low | Three: high, on slope and low |
| On slope | Two white, two red (the two nearest the runway are red) | Two white, two red on each bar | One white, one red | Red over white |
| Placement | One bar beside the runway, about 300 m beyond the threshold, as designed | A bar on each side of the runway, as the design requires | One bar beside the runway, on the side the standard gives | As the design requires; the arrangement is different from a PAPI |
| Where used | The usual choice on runways that need a visual slope | Where the design or the standard calls for a second bar | Only where the standard allows the abbreviated form | Older installations; PAPI is the usual choice for new work |
| Watch points | Each unit set to its own angle, a sharp colour change, all four equal and lit | Both bars set and matched, the same on each side | Both units lit, equal and set; there is no spare unit | Bars aimed and lit as designed; follow the maker’s data |
What to check on PAPI systems
Setting angles
Check each unit is set to its own elevation angle and aimed parallel to the runway centreline, as the design gives.
Sharp colour change
Check each unit changes from white to red sharply. A soft change, a dirty lens or a blocked beam moves the indicated slope.
All units lit and equal
Check every unit is lit and equal in brightness. With an APAPI there is no spare, and a dark unit gives a false slope.
Line and position
Check the units stand in one line, at the right spacing and offset from the runway, on a firm and level foundation.
Flight inspection
Treat the flight inspection as the final proof of the indicated slope, as the authority requires.
Isolate before work
Isolate and lock out the circuit before working on any unit, its transformer or its connectors.
Frequently Asked Questions
Q1What are the main visual glide-slope systems?
Four to know are the four-unit PAPI, the two-bar PAPI, the two-unit APAPI and the older VASI. All show the pilot whether the aircraft is above, on or below the approach slope using colour alone, but they differ in how many units they use, what signals they show and how they are laid out.
Q2How does a PAPI show the slope?
Each unit shows white above its set angle and red below it, with a sharp change between the two. Four units are set at slightly different angles, so as the aircraft descends the units turn red one at a time. Two red and two white means on the slope, with the two red units nearest the runway; more white means high, more red means low.
Q3What is the difference between a PAPI and an APAPI?
An APAPI is the abbreviated PAPI. It uses two units in one wing bar instead of four, and shows three signals (high, on slope, low) instead of five. It is used only where the standard allows the abbreviated form.
Q4Why would a runway have two PAPI bars?
A second wing bar on the other side of the runway gives the pilot the same signal from both sides, which can make it easier to see and read. Where it is used, and how the two bars are arranged, comes from the standard and the airport's design.
Q5What limits and requirements apply?
Unit numbers, setting angles, spacing, tolerances and positions come from the standard, such as ICAO Annex 14 or the FAA specifications, and from the airport's design. Use the requirements for your site, and isolate and lock out the circuit before working on any unit.
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