What is the PAPI Approach Slope and Elevation Setting?

A PAPI approach slope and elevation setting is the approach angle that the PAPI shows and the elevation angle of each unit.

What the approach slope and the setting angles are

The approach slope is the angle of the path that a PAPI shows as on slope. The designer picks a slope that suits the aeroplanes in the approach. Where the runway has an ILS, the visual slope should agree as closely as possible with the ILS glide path. The siting and the angle of the units achieve this (CS ADR-DSN.M.650(a), Annex 14, 5.3.5.37). See glide slope angle .

The elevation setting is the angle at which one light unit is set above the horizontal. Each unit shows white above that angle and red below it. The red to white transition is the plane that the setting defines. During manufacture, the centre of the plane of transition is aligned with the horizontal axis of the unit, which is the setting angle datum. The setting angle and the beam elevation are therefore the same (Doc 9157, 8.3.14). Each unit can adjust from 1°30′ to at least 4°30′ above the horizontal (CS ADR-DSN.M.645(c)(4), Annex 14, 5.3.5.34).

The units A to D have different setting angles. Unit A is farthest from the runway, and unit D is nearest. For a PAPI, the approach slope is the mean of the setting angles of units B and C. For an APAPI, it is the mean of units A and B (CS ADR-DSN Figure M-5). A runway can have a wing bar on each side. The corresponding units then have the same angle, so the signals of the two bars change together (CS ADR-DSN.M.650(b)(4)).

The search term “PAPI calibration” covers the check and the reset of these angles. Doc 9157 says that a ground check with a method independent of the clinometer is not a calibration technique. If it finds a discrepancy, the operator resets the unit with the clinometer (8.3.24).

Setting angles for a 3 degree slope

The EASA specification and Doc 9157 give the same angles for a 3 degree PAPI and APAPI. The second pair of columns is the wider option, where a closer match with an ILS glide path is needed. The on-course sector then grows from 20 to 30 minutes of arc (CS ADR-DSN Figure M-5, Doc 9157 Table 8-1 and 8.3.54).

Doc 9157 states the PAPI angles as the approach angle plus or minus 10 and 30 minutes (Figure 8-6). If the system is on the right side of the runway, the highest set unit is inboard and the lowest is outboard (8.3.4).

The FAA aims its four-box L-880 system 30′ above, 10′ above, 10′ below and 30′ below the glide path, from the unit nearest the runway. For a height group 4 aircraft on a runway with an ILS, the offsets are 35′, 15′, 15′ and 35′. The two-box L-881 system aims 15′ above and 15′ below (AC 150/5340-30J, Tables 7-2 and 7-3). See precision approach path indicator for the full unit layout and the colour pattern.

Settings for other slopes

Doc 9157 says the normal difference between the setting angles of a PAPI is 20 minutes of arc. It gives wider differences for steeper approaches, so that the pilot can capture the slope (8.3.5, 8.3.6, 8.3.68 and 8.3.69).

The table lists differential settings that are satisfactory (Table 8-2). It is guidance, not a standard.

Choosing the slope

The slope is chosen to suit the aeroplanes that use the approach. The FAA standard visual glide slope for a PAPI is 3°. On a non-jet runway, it may grow to 4° to give obstacle clearance (AC 150/5340-30J, 7.5.4.3.4). A steeper slope is one of the measures that EASA lists for an object that penetrates the obstacle protection surface. See PAPI obstacle protection surface .

The elevation settings also set the margin over objects. The EASA specification covers a pilot who sees one white and three red lights. That pilot should clear all objects in the approach area by a safe margin. For an APAPI, the lowest on-slope signal is one white and one red light (CS ADR-DSN.M.650(b)(1) and (2), Annex 14, 5.3.5.38 and 5.3.5.39).

The setting angle also decides where the unit stands. Doc 9157 uses an angle M. It is 2 minutes of arc less than the setting of unit B for a PAPI, and of unit A for an APAPI. It divides the minimum eye height over the threshold by the tangent of M to find the distance from the threshold (8.3.66 and 8.3.67). See threshold crossing height .

What tolerance applies

EASA states one limit for the angle. The aerodrome operator corrects alignment errors of more than one minute of arc (AMC1 ADR.OPS.C.015(b) and (f)). The same text asks for a check at defined intervals, which take account of rain, drought and other conditions that move the system. It also asks for a check after an aircraft occurrence.

Doc 9157 asks for units aligned as close as possible to the required angles. Errors that stay in place affect the width of the on-slope sector (8.3.21). In the passages on inspection, it gives the accuracy of each method and not a tolerance for the angle. The next section lists those accuracies.

Three other values look like tolerances, and they are not setting tolerances:

• Transition depth. The red to white change appears within 3 minutes of arc to an observer at 300 m or more (Annex 14, 5.3.5.30). Doc 9157 gives 3 minutes up to 8 degrees either side of the beam centre, and 5 minutes at 15 degrees (8.3.10). See colour transition .
• Azimuth aiming. Each unit aims on a line parallel to the runway centreline within ½ degree (Doc 9157, 8.3.56, AC 150/5340-30J, 7.5.4.7.3).
• Siting. The spacing, the height of the lens centres and the front line of the bar have their own tolerances. See Annex 14 Figure 5-19 and Doc 9157 8.3.56.

How the setting angles are measured

Doc 9157 describes these methods in 8.3.20 to 8.3.46. The accuracies are the ones that the manual states.

The clinometer measures against the housing. The optical axis may not match the mechanical axis, for example after damage in transport or by the exhaust of a snow blower. The image analysis method measures the projected beam instead (8.3.32). Doc 9157 says that some States allow image analysis as a substitute for flight checks (8.3.35).

Measuring the angle with a drone

Doc 9157 says that an unmanned aircraft system (UAS) may be used as an alternative method to measure PAPI settings. A real time kinematic (RTK) base station is necessary for high dimensional precision. The data can be viewed on site and recorded for later analysis (8.3.43).

For a typical operation, the unmanned aircraft is at least 300 m from the PAPI, downwind of it. A vertical scan gives the heights h1 and h2, which are the upper and lower limits of the red to white transition zone. The setting angle is the arctangent of (h1 plus h2) divided by 2d. The letter d is the horizontal distance of the aircraft from the PAPI (8.3.44, Figure 8-24). A horizontal scan gives the lateral limits of the system, from the point where the signals of units D and A disappear (8.3.45).

The method needs a visible spectrum camera with operational validation against the perceived total image quality. It may need special authorization from the competent authority of the State (8.3.46). See RTK .

The EASA rules do not name drones. They require the angular measurement of the unit setting angles, to the instructions of the manufacturer (GM1 ADR.OPS.C.015(b) and (f)). Acceptance therefore goes through the maintenance programme and the authority. The guide on regulatory acceptance of drone PAPI calibration goes through the authorities one by one. The guide on PAPI calibration cost covers the price.

Flight inspection and how often to check

Doc 9157 asks for a flight inspection of a new installation by the competent authority. It checks range, brilliancy control, the setting angles and compatibility with the precision instrument glide path. The setting angles are checked only to ensure that there is no gross misalignment (8.3.42). The EASA flight check looks at the approach slope of the visual system and its compatibility with the non-visual aids. It also looks at the balance of light intensity at all settings (GM1 ADR.OPS.C.015(b) and (f)). See flight inspection .

The FAA needs a commissioning flight inspection for a new VGSI with an IFR procedure. It has no periodic flight inspection requirement for VGSI. It asks for a check of safe operation with other flight inspections of the runway (AC 150/5340-30J, 7.7.6.6.4).

Between flight inspections, ground checks keep the angles in place. Doc 9157 says that the competent authority should set a reasonable interval for ground checks. It adds that sites with less stable ground or extreme weather need more frequent checks. In many circumstances, a monthly alignment check is adequate (8.3.20). The monthly check can include realignment (8.3.49). EASA says that frost, heavy rain or drought can cause misalignment, so the operator adjusts the ground checks to the weather (GM1 ADR.OPS.C.015(b) and (f)).

After maintenance or an LED retrofit

The setting angle datum is the horizontal axis of the unit, so the same methods check an LED unit (Doc 9157, 8.3.14). An LED PAPI may have no red filter, because the diodes give the colour (Doc 9157, note to 8.3.7). Doc 9157 says that the cause of any misalignment is found and corrected before any adjustment of the setting angles (8.3.22). It also says that a flight inspection of a new installation should be undertaken (8.3.42). Whether a retrofit counts as a new installation is a question for the competent authority.