What is the PAPI unit arrangement?
How the four units of a wing bar stand beside the runway: one straight line, equally spaced, in a fixed order, so the pilot reads them as a single bar of lights.
A PAPI wing bar is a row of four light units. They stand in one straight line at right angles to the runway centreline, equally spaced, beside the runway and a set distance beyond the threshold, normally on the left side as the pilot sees it on approach. The nearest unit is offset from the runway edge by a fixed distance.
The layout matters because the pilot does not read one light but the four together. Each unit is set only about 20 arc-minutes from its neighbour, so the four must be seen from the same direction and at the same moment. A unit that sits out of line, or at the wrong spacing, is seen from a slightly different angle and changes colour too early or too late.
The order matters too. Each unit has its own elevation angle, with the highest setting at the unit nearest the runway and the lowest at the farthest, so two units that are swapped give the wrong colour pattern. A second wing bar may be added on the right side, as a mirror image, which gives eight units in total. The shorter APAPI uses two units per bar.
Frequently Asked Questions
Q1What is the PAPI unit arrangement?
It is how the four units of a PAPI wing bar are laid out on the ground: in one straight line, at right angles to the runway centreline, equally spaced, beside the runway and a set distance beyond the threshold. The pilot reads the four lights together as a single bar.
Q2Why must the four units be in a straight line?
Because the pilot compares the four colours side by side. Each unit is set only about 20 arc-minutes from its neighbour, so a unit that sticks out, or sits further along the runway, is seen from a slightly different angle and shows its colour change at the wrong moment.
Q3Which side of the runway does the wing bar go on?
Normally the left side as the pilot sees it on approach. A second wing bar may be added on the right side, which gives eight units, and the two bars must then be laid out as mirror images and set to the same angles. Take the rules for your runway from the standard and the design.
Q4Does the order of the units matter?
Yes. Each unit has its own elevation angle, with the highest setting at the unit nearest the runway and the lowest at the farthest. If two units are swapped, the colour pattern the pilot sees no longer steps correctly as the aircraft moves above or below the slope.
Q5What sets the spacing and the distance from the runway?
The standard and the design for the runway. The spacing between units, the offset of the nearest unit from the runway edge and the distance beyond the threshold depend on the aircraft category and the approach slope, so they are taken from ICAO Annex 14 or FAA AC 150/5340-30 and the manufacturer's installation data.
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