PAPI angle setting calculator

The four unit elevations, the minimum eye height over threshold, and the nominal siting distance. The unit nearest the runway carries the highest setting — the opposite of what most secondary sources say.

Why the near unit is set highest

The four units sit two arcminutes apart in a symmetrical pattern about the nominal slope, and the unit nearest the runway carries the highest angle. That is the opposite of what most secondary sources say, and a plain web search will confidently tell you the wrong thing.

It matters because it determines which way the reds spread. As an aeroplane goes low the reds appear from the runway outward. On slope, the two units nearest the runway show red and the two farthest show white.

Minimum eye height over threshold

The number that actually sites the system is not the slope, it is the minimum eye height over the threshold. Take the eye-to-wheel height of the most demanding aeroplane that uses the runway regularly — regularly is the operative word — add the wheel clearance for its group, and you have the eye height that must be protected.

The siting distance then follows from angle B minus two arcminutes. Two arcminutes, not two degrees: the raw text drops the symbol, and settling it took two national transpositions.

What changes it on a real aerodrome

The calculation assumes the units, the adjoining runway centre line and the threshold are at the same elevation. If unit B's ground differs from the threshold by more than 0.3 m, the system displaces. On an ILS runway the siting distance comes from the glide path origin instead, corrected for eye-to-antenna height, and it may never reduce wheel clearance below the tabulated minimum.

Every figure on this page is cited to the paragraph it comes from. Where a value is a Recommendation rather than a Standard, the row says so. Verified against ICAO Annex 14 Volume I, with EASA CS-ADR-DSN and two national transpositions used as cross-checks.