What does the PAPI control unit do?
It switches the wing bar, sets its brightness and reports the health of each unit — but it never touches the colour change the pilot reads.
A PAPI is more than four lights. Something has to switch them on, set how bright they are and tell the airport when a unit has failed. That is the job of the control unit. It sits between the supply and control signal and the wing bar, and it never touches the colour change the pilot reads.
1It switches and dims the wing bar
The control unit turns the PAPI on with the runway and approach direction in use and sets the intensity step for daylight, dusk and night. On a series-circuit PAPI the brightness follows the CCR step; on a type fed from a low-voltage supply the control unit sets the lamp or LED drive itself. Depending on the type, it may be a cabinet in the equipment room, a box at the wing bar, or built into the units.
2It watches and reports
Most control units check the lamp or LED state and the supply, and some also check that a unit has not moved out of aim. A fault is shown with the identity of the unit, on a local indication and, where linked, to the ALCMS. A reading is a warning, not proof: an engineer confirms it at the unit before acting.
3Check the commands, the status and the failure state
Check that the unit follows every command from the local panel and the ALCMS, that each status matches the real state of its unit, and that on loss of control or supply the PAPI goes to the state the design requires. A control fault can leave the wing bar dark, stuck at the wrong brightness or showing a false healthy status. Treat the circuit as live until it has been isolated, locked out and proved dead.
| Element | Role | What to check |
|---|---|---|
| Command input | On/off and intensity step from the local panel or ALCMS | Follows every command; local and remote selection work |
| Brightness output | CCR step on a series circuit; unit drive on a low-voltage type | Each step changes the brightness, without flicker |
| Monitoring | Lamp or LED state, supply, and aim on some types | Each status matches the real unit |
| Fault report | Local indication and ALCMS alarm, with unit identity | Alarm is raised and clears correctly |
| Failure state | Behaviour on loss of control or supply | Goes to the state the design requires |
Frequently Asked Questions
Q1What does the PAPI control unit do?
It switches the PAPI on and off with the runway and approach direction in use, sets the intensity step for daylight, dusk and night, and watches the wing bar, reporting each fault with the unit's identity. It sits between the supply and control signal and the four units, and does not touch the colour change the pilot reads.
Q2Does the control unit set the glide-path angle?
No. The setting angle and aim of each unit are set mechanically on site and checked at inspection and maintenance. The control unit only switches, dims and monitors, so a control fault can darken or misreport the wing bar but cannot move the glide path.
Q3What should be checked on a PAPI control unit?
Check that it follows every on/off and intensity command from the local panel and the ALCMS, that each status and fault report matches the real state of the unit, and that the PAPI goes to the state the design requires on loss of control or supply. Treat a series circuit as live until isolated, locked out and proved dead.
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