How is a PAPI selected?
Four decisions, taken in order, that narrow the choice to the right precision approach path indicator for a runway approach.
A PAPI marks the approach path with light, so a wrong choice gives the pilot a wrong slope without any sign of it. The form sets the number of units and signals, the approach slope sets the setting angles, the aircraft that use the runway decide how the wing bar is sited, the supply decides how each unit is fed, and the site decides the mounting and foundation.
The flowchart only narrows the choice. The final PAPI is confirmed against the design for unit count, setting range and angles, transformer and lamp or LED load and frangible mounting, and against the standard it must meet.
Frequently Asked Questions
Q1How is a PAPI selected?
By working through a few decisions in order: whether the full four-unit PAPI or the two-unit APAPI is required, what approach slope sets the angles and which aircraft the runway serves, how the units are powered, and what site the wing bar must suit. The result is then checked against the data sheet and standard.
Q2What is the difference between a PAPI and an APAPI?
Both work on the same principle, white above a set angle and red below it. A PAPI has four units in one wing bar and shows five signals, with two white and two red on slope. An APAPI has two units and shows three signals, with one white and one red on slope. An APAPI is used only where the standard allows the abbreviated form.
Q3What sets the PAPI setting angles?
The approach slope, normally about 3 degrees, within the rules of the standard. Each unit is tilted to its own elevation angle around that slope (about 2°30′ to 3°30′ across the four units for a 3° slope), and a small error moves the whole indicated path, so the setting is verified at commissioning and after maintenance. The aircraft that use the runway decide how the wing bar is sited.
Q4How is a PAPI usually powered?
Commonly from the airfield series circuit, with an isolating transformer for each unit, its primary in the series loop and a short secondary lead to the lamps. The transformer is rated to the unit’s lamp load. Where another supply is used, the unit must be rated for it and the design states which.
Q5What should be checked before the choice is final?
That the number of units is permitted for the runway, that the unit and its setting range suit the approach slope and the aircraft that use the runway, that the supply and transformer match the circuit, that the mounting is frangible and the foundation firm and level, and that the equipment meets the applicable standard. The manufacturer’s data sheet and the standard decide.
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