How do you troubleshoot a PAPI?
The symptom shows whether a unit, the circuit, the aim or the optics failed, and a short set of visual, electrical and inclinometer checks confirms it.
A PAPI works only if every unit shows the right colour at the right angle. Each unit is one step of a four-step signal, so a single unit that is dark or dim gives the pilot a false slope indication, and a fault on one unit is a defect on the whole approach aid. Each unit is fed through its own isolating transformer: a lamp or secondary fault normally stays with its own unit, but an open primary can open the series circuit and put every unit on that loop out.
A unit that has moved after a knock, settlement or work on the lens or housing changes that unit’s transition angle, so the indicated path no longer matches the design; it is checked with an inclinometer. A dirty or cracked lens, water in the housing or debris in front spoils the sharp white-to-red change, and water also causes an earth fault. The ALCMS reports circuit faults but does not aim the beams. Work needs ATC or operations clearance and the circuit isolated at the CCR. The final proof after any correction is a flight inspection, as the authority requires.
Frequently Asked Questions
Q1Why is one failed PAPI unit a serious fault?
Each unit is one step of a four-step signal. A single unit that is dark or dim gives the pilot a false slope indication at its angle, so a fault on one unit is treated as a defect on the whole approach aid and is corrected before the PAPI is relied on.
Q2What does it mean if all four units go out?
Usually the series circuit has opened or the CCR has stopped. Each unit is fed through its own isolating transformer, but an open primary, connector or cable in the loop can put every unit on that loop out. The break is found by testing the circuit in sections, with the circuit isolated first.
Q3What if only one unit is out and the CCR is normal?
The fault is usually on that unit's own side: the lamp or LED, the secondary lead, its connector, or the transformer secondary. A lamp or secondary fault normally stays with its own unit.
Q4What does a wrong glide-path indication point to?
A unit that has moved. A knock, a settled foundation or work on the lens or housing can shift a unit by a fraction of a degree, which changes that unit's transition angle so the indicated glide path no longer matches the design. Elevation is read with an inclinometer and re-set to that unit's angle, then proved by flight inspection.
Q5What causes a poor or unequal white-to-red change?
A dirty or cracked lens, water inside the housing, a dim lamp or LED, or grass, snow or debris in front of the unit. The four units should look equal in brightness and colour from the approach, with a sharp change from white to red.
Q6Does the ALCMS fix an aiming fault?
No. The ALCMS controls power and brightness through the PAPI CCR and reports circuit faults, but it does not aim the beams. The angles are set mechanically on site and checked at maintenance.
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