What is a PAPI?
The Precision Approach Path Indicator: four light units beside the runway that show the pilot, by colour alone, whether the aircraft is on, above or below the approach slope.
A PAPI is a visual approach aid. It does not guide the aircraft by radio. A wing bar of four light units stands beside the runway, and each unit shows white above its own setting angle and red below it. The pilot reads the four lights together: two red and two white is on the slope, more white is high, and more red is low.
The system is more than the four lights. Power comes from a constant-current regulator through the series circuit, with an isolating transformer feeding each unit. Each unit holds a light source, a lens and a colour filter, stands on a frangible mounting, and is aimed in elevation and azimuth. The colours depend on the angle at which the aircraft sees each unit, not on its distance, so the same indication holds along the approach within the coverage of the beams.
Because the pilot trusts the colours to judge the slope, every part must be right: the layout, the setting angles, the aiming, and the supply. The entries that follow take them one at a time, from the unit arrangement and the optical principle to installation, inspection and testing.
Frequently Asked Questions
Q1What is a PAPI?
A Precision Approach Path Indicator is a row of four light units beside the runway that shows the pilot, by the number of red and white lights, whether the aircraft is on, above or below the approach slope. It is a visual aid read by eye, and it is separate from the radio signal of the ILS.
Q2What does the pilot see on the approach?
Two red and two white lights means on the slope. More white lights means the aircraft is high, and more red lights means it is low, with all white too high and all red too low. Because each unit changes colour at its own angle, the pattern steps from one unit to the next as the aircraft moves off the slope.
Q3What is a PAPI made of?
A wing bar of four units, each with its own light source, a lens and a colour filter that split the beam into white above and red below. Each unit stands on a frangible mounting and is aimed in elevation and azimuth. Behind them are the cable, the isolating transformers, the earth and the power supply.
Q4How is a PAPI powered?
On a series-circuit airfield, from a constant-current regulator through the series circuit, with an isolating transformer feeding each unit. The exact arrangement follows the design for the site, and the PAPI is normally controlled and monitored with the rest of the airfield lighting.
Q5What is the difference between PAPI and APAPI?
A PAPI has four units in a wing bar. The APAPI is the shorter version with two units per bar, used where the standard allows a simpler indication. The principle is the same, and the PAPI Versus APAPI entry compares them.
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