What is the ICAO serviceability level concept?
ICAO does not ask for every light to work every minute. It sets a level of serviceability, the share of lights that must be working for the operation being flown, and expects the maintenance system to be built to hold it. This entry explains the idea; the figures by light type follow in the next entries.
A serviceability level is the minimum share of lights that must be working in a lighting system for the operation it supports. Annex 14 writes it as an objective of the preventive maintenance system: the system is set up so that, during the operations concerned, the lights stay at or above that level. It is a target that maintenance is planned around, not a pass mark applied once at inspection.
1When is a light unserviceable?
A light is unserviceable when its main-beam average intensity falls below 50 per cent of the value specified in Annex 14, Appendix 2. Where a fixture is designed to give more than that value, the 50 per cent is taken from the design value. So a light can be lit and still count as unserviceable, because it no longer gives the guidance the pilot relies on. A dead lamp is not the only way to lose a light: dirt, ageing, water ingress and misalignment can all take it below the line.
2Why the level is not 100 per cent
Lamps fail and fixtures age, so ICAO accepts a limited number of outages as long as guidance is kept. The allowed share depends on the operation: the lower the visibility minima, the higher the level required. For example, Annex 14 sets 95 per cent for the centreline lights of a CAT II/III runway, and 85 per cent for the approach, threshold and edge lights of a CAT I runway. Entry 27 lists the figures by light type.
CAT II/III operations and low-visibility take-off: a high share of lights must be serviceable, and critical lights such as stop bars are held tighter still.
Runways used in better visibility: a lower share is accepted, because pilots have more visual reference to fall back on.
3Where the outages sit matters
To keep continuity of guidance, the allowable share of unserviceable lights should not fall on adjacent lights, unless the lighting pattern does not allow a lapse in guidance. Two systems with the same percentage can therefore differ: scattered outages may be within the level, while a side-by-side pair is not. The adjacent-lamp-out rule has its own entry (28).
- Count serviceability by light type and circuit
- Check where the outages are, not only how many
- Treat a dimmed or dirty light as a defect
- Count a light as serviceable just because it is lit
- Wait until the level is breached before acting
- Leave two adjacent lights out because the total looks fine
| Term | Meaning |
|---|---|
| Serviceable light | Main-beam average intensity at least 50% of the specified (or design) value |
| Unserviceable light | Below 50%, including a lamp that is out |
| Serviceability level | Share of lights in a system that must be serviceable |
| Maintenance objective | What the preventive maintenance system is built to hold during the operations |
| Adjacent lights | Outages should not fall side by side where guidance would lapse |
| Set by | The operation served: approach category, visibility, take-off minima |
Frequently Asked Questions
Q1What is the ICAO serviceability level concept?
It is the idea that a lighting system must keep a minimum share of its lights working for the operation it supports. Annex 14 states it as an objective of the preventive maintenance system, so maintenance is planned to hold the level rather than only to repair failures.
Q2When does ICAO consider a light unserviceable?
When its main-beam average intensity falls below 50 per cent of the value specified in Annex 14, Appendix 2. Where the fixture is designed to give more than the specified value, the 50 per cent is taken from that design value. A light that is still lit can therefore be unserviceable.
Q3Why is the required level not 100 per cent?
Lamps fail and fixtures age, so some outages are unavoidable. ICAO accepts a limited number of them as long as guidance is kept, and sets the allowed share according to the operation: the lower the visibility minima, the higher the level required.
Q4Why do adjacent unserviceable lights matter?
To keep continuity of guidance, the allowable share of unserviceable lights should not fall on adjacent lights, unless the lighting pattern allows no lapse in guidance. Two systems with the same percentage can differ: scattered outages may be acceptable while a side-by-side pair is not.
Q5Where do I find the actual percentages?
In Annex 14, Volume I, Chapter 10 (maintenance), which gives the objectives by type of runway and operation. The next entries in this division list them by light type and explain the adjacent-lamp-out rule. Always check the edition in force and your national regulations.
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