What is the Allowable Percentage of Serviceable Lights?

The allowable percentage of serviceable lights is the minimum share of lights in a lighting system that the maintenance programme aims to keep serviceable.

What the allowable percentage is

EASA and ICAO set a maintenance objective for the lights of a runway. The operator keeps a share of the lights in each system serviceable, and the share depends on the system and on the operation. The EU rule is ADR.OPS.C.015(b). The ICAO text is Annex 14, Volume I, 10.5.7 to 10.5.12. EASA calls the figures “allowable percentages of serviceable lights” in GM1 ADR.OPS.C.015(b).

The percentage is a maintenance performance objective. Annex 14 says that these specifications are not intended to define whether the lighting system is operationally out of service (10.5, Note 1). EASA guidance gives the same purpose (GM3 ADR.OPS.C.015(b)). The section on the maintenance objective and the closing limit gives the detail.

The EASA rule also makes a general duty. The operator establishes a preventive and corrective maintenance programme for the individual lights and the reliability of the lighting systems. The programme includes checks at defined intervals, and the operator keeps records (ADR.OPS.C.015(a), (b), (f) and (g)). See preventive maintenance .

When a light counts as unserviceable

A light is unserviceable if:

• its main beam average intensity is less than 50 % of the value in the certification specifications. If the design value is higher, the 50 % is related to that design value.
• its filter is missing or damaged.
• it does not produce the correct colour light beam (ADR.OPS.C.015(c), Annex 14, 10.5.1).

A light below 50 % is a failed light from an operational view. EASA guidance asks the operator to define two levels in the maintenance programme. The maintenance level is always above 50 % and warns the staff that a light gives an output well below the required value. The unserviceability level is the 50 % value (GM1 ADR.OPS.C.015(b) and (c)). See lamp failure .

LED lights lose output and shift colour over time. EASA says that an LED light still operates, but it can fall outside the specification, so preventive maintenance is necessary (GM2 ADR.OPS.C.015(b)).

The percentages by system

The table gives the figures of ADR.OPS.C.015(b)(1), (6), (7) and (8). Annex 14 gives the same figures in 10.5.7, 10.5.10, 10.5.11 and 10.5.12. A dash means that the rule gives no percentage for that case.

For Category I, the rule gives 85 % for the approach lighting system as a whole. GM1 Table 1 splits it into the inner and outer 450 m. The GM table is guidance, and it differs from the rule text in two places. It shows 85 % for the runway centre line on a Category I runway and for a take-off in RVR of 550 m or more. It also shows 85 % for the touchdown zone on a Category I runway, if touchdown zone lights are available. The rule text lists none of these. Use the rule text as the requirement. See Category I , Category II and runway visual range .

The objective of the rule is that all approach and runway lights are serviceable during any period of the operation. The percentage is the minimum in any event.

See runway edge light , approach lighting , threshold light and runway end light for what each system is.

Pattern and adjacency

A share of unserviceable lights must not change the basic pattern of the system. That is the rule for Category II and III (ADR.OPS.C.015(b)(2)). Annex 14 gives the reason: continuity of guidance (10.5.7).

The adjacency rules are these:

• Category II or III. An unserviceable light must not be next to another. The exception is a barrette or a crossbar, where two adjacent unserviceable lights may be permitted.
• Category I. An unserviceable light must not be next to another, unless the light spacing is significantly less than required.
• Take-off. An unserviceable light must not be next to another.

Lights are adjacent when they are consecutive and in the same barrette or crossbar. They are also adjacent in the same row of edge lights or barrettes (GM3 ADR.OPS.C.015(b), Annex 14, 10.5.7, Note). In a barrette or a crossbar, guidance is not lost by two adjacent unserviceable lights. See barrette .

Stop bars, taxiway lights and PAPI

Three more rules in the same article do not use a percentage:

• Stop bar. At a runway holding position used with a runway for RVR below 550 m, no more than two lights stay unserviceable. Two adjacent lights do not stay unserviceable, unless the light spacing is significantly less than required. See stop bar .
• Taxiway. On a taxiway for RVR below 550 m, no two adjacent centre line lights are unserviceable.
• Visual approach slope indicator. On a runway with these systems, all units should be serviceable. A unit is unserviceable if the number of unserviceable lights means that the unit does not provide the intended guidance (ADR.OPS.C.015(b)(4), (5) and (9)).

Annex 14 has the same stop bar and taxiway objectives. It uses an RVR of less than 300 m, where EASA uses 550 m (Annex 14, 10.5.8 and 10.5.9). Annex 14 has no PAPI objective in 10.5.

EASA guidance gives the count for a PAPI or APAPI unit. A unit of three or more lights is unserviceable when more than one light is unserviceable. A unit of fewer than three lights is unserviceable when one light is. If a red filter is missing, damaged or the wrong colour, the operator extinguishes all lights behind it. They stay off until the filter is rectified (AMC1 ADR.OPS.C.015(b) and (f)). See precision approach path indicator .

A maintenance objective, not a closing limit

The percentages say how the operator maintains the lights. They do not say when a runway is closed or downgraded. Three sources show where that decision sits:

• Annex 14 says that the objectives are not intended to define whether the lighting system is operationally out of service (10.5, Note 1).
• EASA lists the equipment failures that the operator reports for low-visibility procedures, with their effect on flight operations. The list covers approach, runway centre line, runway edge, touchdown zone and taxiway lighting. For each, the effect is a restriction that depends on the flight operations rules (AMC1 ADR.OPS.B.045(c)).
• The operator informs the aeronautical information services and the air traffic services of a change in equipment status that affects low-visibility operations (ADR.OPS.B.045(c)).

The operator originates a NOTAM for significant changes to visual aids. It also originates one when major components of aerodrome lighting systems stop or return to operation (ADR.OPS.A.057(b)(5) and (6)). The rule gives no light count. See NOTAM .

Annex 14 also recommends monitoring for a runway for RVR below 550 m. The system should indicate when the serviceability level of any element falls below the minimum level in 10.5.7 to 10.5.11. A second indication should show when the level falls below the minimum that the authority sets, below which operations should not continue (8.3.4 and 8.3.5).

How the percentage is checked

The operator needs evidence for the percentage. For a Category II or III runway, EASA guidance gives these checks (AMC1 ADR.OPS.C.015(b) and (f)):

• visual inspection and in-field measurement of the intensity, beam spread and orientation of the approach and runway lights
• measurement of all lights, as far as practicable, against the figures in CS ADR-DSN.U.940
• an instrument of sufficient accuracy to analyse the individual lights
• a frequency that follows traffic density, pollution, equipment reliability and the earlier results
• at least twice a year for in-pavement lights and once a year for other lights

Annex 14 recommends the same measurements and the same two minimum frequencies (10.5.3 to 10.5.6). The EASA text also asks for photometric measurements at appropriate intervals on all runway types. It asks for a flight check of the lighting systems at least once a year. See photometric testing and in-pavement light .

The guide on runway light serviceability limits takes the figures further, system by system. TarmacView inspects airfield lighting by drone and records each light by itself, which gives a count and the relative intensity of each unit. See the airfield lighting inspection page.