How PAPI Transition Angles Are Verified in the Field

You verify a PAPI transition angle in the field by finding the height at which each unit changes from red to white, then turning that height and the distance to the unit into an angle above the horizontal. Ground aiming checks, flight checks and drone measurements all do this, and each sees something different.

What is being verified

A PAPI is a wing bar of four units. Each light box throws a beam that is red in its lower part and white in its upper part, and the setting angle is the elevation of the line between them. Annex 14 requires each unit to be adjustable so that the lower limit of the white part can be fixed anywhere from 1°30′ to at least 4°30′ above the horizontal (5.3.5.34, a Standard). For a 3 degree approach the four units are set at 2°30′, 2°50′, 3°10′ and 3°30′, from the unit farthest from the runway to the nearest. CS-ADR-DSN Figure M-5 shows those angles and Doc 9157 Table 8-1 lists the same four. A note on Figure M-5 gives 2°25′, 2°45′, 3°15′ and 3°35′ where the on-course sector is widened from 20′ to 30′ to bring the PAPI closer to the ILS glide path.

A pilot on the slope sees the two nearest units red and the two farthest white (Annex 14 5.3.5.26). So the angle that counts is where the colour changes in the air in front of the unit, not where the housing points.

The three minutes that are not a tolerance

The figure people quote for a PAPI is 3 minutes of arc. In the sources it limits something else. Annex 14 5.3.5.30 says the change from red to white must appear, to an observer at not less than 300 m, to occur within a vertical angle of not more than 3 minutes of arc. CS-ADR-DSN.M.645 carries the same wording. That is the depth of the transition. It does not say how far a unit may be from its design angle.

The text of Annex 14 and CS-ADR-DSN.M.650 that we searched gives no tolerance on the setting angle. The limits that exist sit in lower-ranking material, and they differ.

The EU rule itself, ADR.OPS.C.015(b), requires a maintenance programme that keeps the individual lights serviceable. For a runway with visual approach slope indicators its objective is that all units are serviceable, and it sets no setting angle tolerance. The one minute of arc sits in the AMC, which speaks of visual approach slope indicator systems and names the PAPI in the same point.

Ground aiming checks

The first check is at the unit. A PAPI is typically supplied with a clinometer, and the technician reads each box’s aiming angle against its own structure (Doc 9157 8.3.21 and 8.3.23). The inclinometer is also the tool that resets a box. The FAA asks for a monthly check of aiming with the PAPI aiming instrument, with the angle and the date written in a maintenance log. It adds that aiming should be checked more often when the soil freezes, thaws or changes moisture, especially clay (AC 150/5340-26C, 5.7.1.2 point h).

A clinometer has a blind spot. It measures aim against the housing, not the projected beam, and the optical axis can differ from the mechanical one after damage in transport or a blow from snow-clearing exhaust (8.3.32). The manual therefore asks for a ground check by a method independent of the clinometer, and says that check is not a calibration technique. If it finds a discrepancy, the box is reset with the clinometer (8.3.24).

Doc 9157 describes three ground methods.

• Ground survey (8.3.25 to 8.3.30). A theodolite is levelled on top of the unit and an observer slides an engineer’s square up a surveyor’s rod until the signal turns white, then down until it turns red. The midpoint of the two heights gives the transition, and the angle is read from the theodolite. The manual puts the accuracy at about 3 minutes of arc and warns that the observer and too intense a setting both bias the readings.
• Image analysis (8.3.32 to 8.3.35). A camera on a tripod 10 to 15 m from the unit finds the centre of the transition, referenced to an electronic gravity platform. The manual gives an accuracy better than 1 minute of arc and says some States allow it as a substitute for flight checks.
• Checkboard (8.3.36 and 8.3.37). A screen on a rod stands 20 m ahead of the unit, marked in 1 minute steps over 5 minutes either side of a zero line. The transition should fall on the zero line.

Doc 9157 is guidance. Annex 14 points to Part 4 only in Notes, such as the one after 5.3.5.32, and Notes are not part of the Standards. Each of these is therefore a method a State may adopt, not one it must.

Flight checks

A flight check sees the system the way the pilot does. The EU AMC asks for a flight check of the approach, runway and taxiway lighting, the slope indicators, the aerodrome beacon and the intensity control at regular intervals, at least yearly. It also asks for special flight checks after major maintenance or changes and before new systems are commissioned (AMC1 to ADR.OPS.C.015(b) and (f), point (d)). The guidance material adds that on precision approach runways the slope indicators are also checked for correct approach slope and for compatibility with the non-visual aids.

Doc 9157 8.3.42 says the competent authority should flight-inspect a new installation. The inspection covers range, brilliancy control, setting angles and compatibility with the precision instrument glide path. For the angles the manual’s own words are “to ensure that there is no gross misalignment”. The United States works differently. AC 150/5340-30J asks for a commissioning flight inspection of a new visual glideslope indicator with an instrument procedure on the runway, and says there is no periodic inspection requirement for these facilities. Confirmation of safe operation should be done alongside other flight inspections of the same runway (7.7.6.6.4).

Drone measurement

Doc 9157 8.3.43 says an unmanned aircraft system may be used as an alternative method for measuring PAPI settings. The aircraft is positioned at least 300 m downwind of the system. Vertical scanning finds h1 and h2, the upper and lower limits of the transition zone, and the setting angle follows from the arctangent of (h1 + h2) divided by twice the horizontal distance d (8.3.44). Horizontal scanning finds the lateral limits of the beam by noting when units D and A disappear (8.3.45). The manual says a real time kinematic base station is necessary for high dimensional precision, so the positioning rests on RTK .

The reason the position matters is plain geometry, not a figure from the manual. At 300 m, 1 minute of arc is a height difference of about 8.7 cm and 3 minutes is about 26 cm.

Two cautions come with the method. The manual asks for a visible spectrum camera or sensor that is operationally validated against perceived total image quality, and says the method may need special authorization from the State (8.3.46). It also gives no accuracy figure for the unmanned method, so the number has to come from the provider’s own measurement uncertainty statement and the evidence behind it. In 2025 Kazakhstan told the ICAO Assembly that existing ICAO material has no procedures, performance criteria or safety parameters specific to unmanned systems (A42-WP/343, 2.2.1). That is one State’s view in an information paper, not an ICAO position. Whether regulators accept drone PAPI checks is a separate question, answered authority by authority.

Putting the methods together

The methods answer different questions. The clinometer says how the box is set. An independent ground method says where its beam goes. A flight check says what the pilot sees. A drone measurement reports the angle of every unit from the approach, with the heights and the distance behind each figure, so a reviewer can recompute it.

Events call for a new check whatever the interval says. The EU AMC and its guidance material name unstable ground, frost, heavy rain, drought and an aircraft occurrence. The FAA adds contact with mowing equipment. TarmacView’s PAPI inspection is a drone measurement of PAPI settings. It reports a measured angle per unit, and the authority decides how that evidence enters the maintenance programme. For what to do when a unit proves out of tolerance, see PAPI out of tolerance: confirm, re-aim, re-check .