What is a PAPI Obstacle Protection Surface (OPS)?

A PAPI obstacle protection surface (OPS) is a surface set for a PAPI above which new objects are not permitted, except in defined cases.

What the obstacle protection surface is

The EU rules define an obstacle protection surface as a surface established for a visual approach slope indicator system. Objects and extensions of existing objects are not permitted above it, unless an existing immovable object shields them (Regulation (EU) No 139/2014, Article 2(15)). The surface is established when an aerodrome provides such a system. The precision approach path indicator (PAPI) and the abbreviated PAPI (APAPI) are both visual approach slope indicator systems (CS ADR-DSN.M.655(a)). Annex 14 states the same rule in 5.3.5.42.

The FAA says that its equivalent surface gives the pilot a minimum clearance over obstacles during the approach (AC 150/5340-30J, 7.5.4.4.1). The elevation settings of the PAPI units give the same safe margin over objects in the approach area (CS ADR-DSN.M.650(b)).

It is a different surface from the obstacle limitation surfaces (OLS) . The OLS limit how high objects may project into the airspace around the aerodrome. The OPS belongs to one lighting system. Its start is fixed to the PAPI, so the surface moves when the PAPI moves (Table M-2, note 2).

Dimensions and slope

CS ADR-DSN Table M-2 and Annex 14 Table 5-3 give the dimensions by runway type and code number. D1 is the distance from the threshold to the PAPI, before any displacement to remedy a penetration. The table below is the EASA version for the PAPI and the APAPI.

Annex 14 gives the same values for the PAPI and the APAPI. “None given” means that the standard sets no slope. Annex 14 gives the reason: the system is unlikely to be used on that runway type (Table 5-3, note c). Annex 14 also lists a slope of 1.9 degrees for the T-VASIS and the AT-VASIS.

“A” is the setting angle of unit A of the wing bar (CS ADR-DSN Figure M-5). For a 3 degree PAPI, unit A is set at 2°30′, so the surface slopes at 2.5 minus 0.57, which is 1.93 degrees. For a 3 degree APAPI, unit A is set at 2°45′, so the slope is 2.75 minus 0.9, which is 1.85 degrees. A steeper approach slope raises the surface with it. See PAPI setting angles for how the unit angles are set.

In Figure M-6, the inner edge of the surface lies beyond the threshold on the approach side. The surface then rises along the approach and spreads out on both sides.

The OPS can also change the approach surface. If its slope is lower than the one in the Annex 14 tables, the approach surface slope decreases to match it (Annex 14, 4.2.1.9). See approach surface .

New objects and existing objects

For a new object, or an extension of an existing object, EASA gives one rule and two exceptions. The object is not permitted above the surface unless:

• an existing immovable object would shield it
• a safety assessment shows that it would not adversely affect the safety of aeroplane operations (CS ADR-DSN.M.655(c))

Annex 14 allows only the shielding exception for new objects, in the opinion of the appropriate authority (5.3.5.44). For existing objects, Annex 14 requires removal. It makes an exception for a shielded object, or an object that an aeronautical study shows has no adverse effect (5.3.5.45).

What to do when an object penetrates the surface

If a safety assessment shows that an existing object could adversely affect the safety of operations, EASA lists these measures (CS ADR-DSN.M.655(d)). The operator takes one or more of them:

• remove the object
• raise the approach slope of the system
• reduce the azimuth spread of the system, so that the object is outside the beam
• displace the axis of the system and its surface by no more than 5 degrees
• displace the threshold
• where that is impracticable, move the system upwind of the threshold, so that the object no longer penetrates the surface

Moving the system upwind reduces the operational landing distance (GM1 ADR-DSN.M.655(a)). See displaced threshold .

Annex 14 lists the same measures, with the displacement of the threshold left out (5.3.5.46). It uses an aeronautical study where EASA uses a safety assessment.

An object can also stand outside the surface but inside the lateral limits of the beam. Suppose it rises above the plane of the surface, and a safety assessment shows an effect on safety. The operator then restricts the azimuth spread of the beam. The object must stay outside the confines of the beam (CS ADR-DSN.M.650(b)(3), Annex 14, 5.3.5.40). Doc 9157 gives an alternative. The operator can turn the light units from the centreline by an azimuth angle of 5 degrees or less (8.3.61).

Marking and lighting of an object above the surface

A fixed object that extends above an obstacle protection surface is marked. If the runway is used at night, it is also lighted. The marking and lighting may be omitted when another fixed obstacle shields the object (CS ADR-DSN.Q.840(g)). Annex 14 states the first part in 6.1.1.8 without the shielding exception. See obstacle light .

The FAA obstacle clearance surface

The FAA calls the equivalent surface the PAPI obstacle clearance surface (OCS). Its dimensions differ from the EASA and ICAO surface (AC 150/5340-30J, 7.5.4.4).

For an L-881 two-box system, the lowest on-course signal is for the unit farthest from the runway. The FAA asks for a site survey to check that no obstacle penetrates the surface. If an obstruction cannot be removed, the FAA raises the glide slope angle or moves the PAPI farther from the threshold. That gives an increased threshold crossing height that equals the penetration height (7.5.4.4.7 and 7.5.4.4.8). See threshold crossing height .

How the surface is checked

The EU rules put the duty on the aerodrome operator. The operator monitors the obstacle limitation and protection surfaces and has procedures to mitigate the risk of a penetration (ADR.OPS.B.075(a)(1) and (b)). The operator also has procedures to monitor changes in the obstacle environment (AMC1 ADR.OPS.B.075(a)).

The maintenance programme for the PAPI also covers the surface, so that it is clear of all obstacles (AMC1 ADR.OPS.C.015(b) and (f)). ICAO Doc 9157 says that the surface must be examined to check that nothing infringes it (8.3.57). It adds that the data from the obstacle survey of the runway approach surface helps, because the OPS follows the pattern of that surface (8.3.59). The EASA text gives no interval for this check. It says that the alignment of the system is checked at defined intervals, and after an aircraft occurrence.

The FAA flight check looks at obstacle clearance within the lateral limits of the visible light beam. That can be wider than the standard surface, because the beam can spill more than 10 degrees on either side of the centreline. The FAA has no periodic flight inspection requirement for these systems. It asks for a check of safe operation with other flight inspections of the runway (AC 150/5340-30J, 7.7.6.6.4, items 5 to 7). See flight inspection .

TarmacView inspects PAPI units by drone. The check of the PAPI protection surface against surveyed obstacles is in development. See the PAPI inspection page.