How is a PAPI re-aligned?
Making sure each of the four units still shows its white-to-red change at the right angle and points parallel to the runway, and re-setting it when it has moved.
A PAPI gives the pilot a slope only if each unit changes from white to red at its own set angle. A knock, a settled foundation or a lens change can shift a unit by a fraction of a degree, and that moves the whole indicated path. PAPI alignment is the maintenance routine that checks and, where needed, re-sets each unit.
1Re-check after anything that can move a unit
Re-align after work on the lens, lamp or LED, or housing, after an impact or settlement, and whenever an inspection or test shows a reading outside tolerance. Routine checks follow the airport’s maintenance programme.
2Set elevation and azimuth
Elevation is read with an inclinometer on the unit’s reference surface and set to that unit’s setting angle, which is the nominal slope plus or minus a fixed offset for each unit. Azimuth is set so the beam points parallel to the runway centreline. Tighten the fasteners, then read both again, because tightening can move the unit.
3Confirm and record
Check that the white-to-red change is sharp, compare the four units, record the readings and treat any unit left out of tolerance as a defect on the whole approach aid. The final proof is a flight inspection, as the authority requires.
| Check | How | What it protects |
|---|---|---|
| Elevation angle | Inclinometer on reference surface | Correct slope for each unit |
| Azimuth | Beam parallel to centreline | Signal seen from the approach path |
| Level and mounting | Mount, frangible parts, fastener torque | Setting that stays put |
| Colour change | Sharp white-to-red at the setting | Clear slope indication |
| Record | Readings and adjustments logged | Traceable maintenance history |
Frequently Asked Questions
Q1What is PAPI alignment in maintenance?
It is the check, and where needed the re-setting, of each PAPI unit's elevation angle and azimuth so that the white-to-red change occurs at the correct angle and the beams point parallel to the runway centreline. It is done after work that could disturb a unit and when inspection shows a unit has moved.
Q2When does a PAPI need to be re-aligned?
After a lens, lamp or LED, or housing has been worked on, after the unit has been struck or has settled or shifted on its foundation, and whenever an inspection or test shows an elevation or azimuth reading outside the allowed tolerance. The airport's maintenance programme sets the routine checks.
Q3How is the alignment confirmed after adjustment?
The elevation angle is read again with an inclinometer and the azimuth is checked, the fasteners are checked at their specified torque, and the result is recorded. A flight inspection is the final proof of what the pilot sees, as the authority requires.
Q4Why is each PAPI unit set at a different angle?
The four units are set at stepped angles either side of the nominal approach slope, so that each changes from white to red at a slightly different height. This is what lets the pilot read the slope from the number of red and white lights. If one unit is out of tolerance, the indication is wrong for every pilot using that slope, so it is treated as a defect on the whole approach aid.
Q5What does the pilot see on and off the slope?
On the approach slope the pilot sees two white and two red lights. Slightly high shows three white and one red, and well above the slope all four are white. Slightly low shows three red and one white, and well below the slope all four are red. Re-alignment keeps each change-over at its set angle so these indications stay correct.
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