How is a PAPI powered?
Normally one constant-current loop from the CCR, with an isolating transformer at each unit. Some types use a low-voltage feed instead. Either way, the supply sets the brightness and never the colour.
The four units of a PAPI need a supply that is steady, safe and always there. The supply decides how bright the lamps are. It does not decide what colour the pilot sees, because that is set by the angle of each unit.
1Many PAPIs run on the series circuit
A constant-current regulator (CCR) drives one loop of series cable, so the same current flows through every unit. An isolating transformer at each unit feeds its lamps through a short secondary lead, and the brightness follows the CCR step. Because the units share one path, an open primary can put out every unit on that loop.
2Some types use a low-voltage feed
Other types take a low-voltage supply from the airport distribution through a PAPI power supply, which protects, regulates and, with the control unit, dims it. A low or uneven supply only dims the lamps, but a wing bar of unequal brightness is harder to read, so the voltage is checked at the unit.
3Plan for standby, and prove it
Where the design requires it, a standby supply takes over if the main one fails. Check the current or voltage at every intensity step, the insulation resistance and earthing, that each unit has its own transformer or output, and that the changeover works both ways. Treat the circuit as live until it has been isolated, locked out and proved dead.
| Element | Role | What to check |
|---|---|---|
| Source | CCR on a series circuit; LV feed on other types | Correct output at every intensity step |
| Path | Series cable loop, or a radial LV feed | Continuity and insulation resistance |
| At each unit | Isolating transformer, or the unit's own LV supply | Rated to the lamp load; one per unit |
| Earthing | Bonds the unit, mounting and base can | Bonded and measured as the design requires |
| Standby | Takes over if the main supply fails | Changes over and back as the design requires |
Frequently Asked Questions
Q1How is a PAPI powered?
Many PAPIs are fed from the airfield series lighting circuit: a constant-current regulator drives one loop of cable, and an isolating transformer at each unit feeds its lamps. Some types use a low-voltage feed from the airport distribution through a PAPI power supply. The site design and the manufacturer decide which.
Q2What happens if the PAPI supply is weak?
The lamps dim, but the colour change does not move, because it is set by the unit's angle and not by the supply. A weak or unequal supply can still make the wing bar hard to see or unequal in brightness, and a break in a series loop puts out every unit on that loop.
Q3What should be checked on a PAPI power supply?
Check the circuit current or supply voltage at every intensity step, insulation resistance and earthing, that each unit is fed from its own transformer or output, and, where a standby supply is fitted, that it changes over and back correctly. Treat the circuit as live until isolated, locked out and proved dead.
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