What is the adjacent-lamp-out rule?
The percentage says how many lights may be out. The adjacent-lamp-out rule says where. Unserviceable lights must not sit side by side, because a row can be inside its percentage and still have a break in the line of lights the pilot is following.
The Annex 14 serviceability objectives for low-visibility operations have two parts: a minimum share of lights that must work (Entry 27), and a statement that, to provide continuity of guidance, unserviceable lights are not permitted next to each other. The second part is the adjacent-lamp-out rule. It applies on top of the percentage, not instead of it.
1Why position matters as much as count
Take a row of lights at specified spacing. One light out leaves a gap of two spacings between lit lights. Two adjacent lights out leave a gap of three spacings. Same number of outages in the row, but the second is a visible break. That is why a row can pass the percentage test and still fail this one.
2How the wording differs by operation
The rule is worded differently for each objective. For precision approach runways it applies except in a barrette or crossbar, where two adjacent unserviceable lights may be permitted. For take-off in RVR below 550 m and for stop bars it applies unless the spacing is significantly less than specified. For take-off in RVR of 550 m or more and for taxiway centreline lights (RVR below 350 m) no exception is stated.
With spacing at half the specified value, two adjacent lights out leave a gap of 1.5 normal spacings, which is why closer-spaced lighting can be treated differently.
Where alternate lights are fed from different circuits, one circuit down puts out every second light: no adjacent pair, but check the percentage at once.
3Reading “adjacent” in practice
Adjacent means neighbouring lights along the same line, such as two consecutive runway edge or centreline lights. For approach lighting, a barrette or crossbar is treated differently from a line of lights; follow the wording and the guidance in Doc 9137. Where you cannot tell whether a gap lapses guidance, restore one of the pair first.
- Check adjacency as well as the percentage
- Restore one of an adjacent pair first
- Log where each outage is, not just the total
- Treat a row as fine because it is inside its percentage
- Assume the spacing exception applies without checking the wording
- Leave a pair out until the next routine visit
| Operation | Light type | Adjacent outages |
|---|---|---|
| Precision approach (CAT I, II/III) | Approach and runway lights | Not adjacent to another unserviceable light, except in a barrette or crossbar, where two adjacent lights may be out |
| Take-off, RVR below 550 m | Runway lights | Not adjacent to another unserviceable light, unless spacing is significantly less than specified |
| Take-off, RVR 550 m or more | Runway lights | Not adjacent to another unserviceable light |
| Stop bar, RVR below 350 m | Stop bar | Not two adjacent (and no more than two out), unless spacing is significantly less than specified |
| Taxiway, RVR below 350 m | Taxiway centreline | No two adjacent lights unserviceable |
Frequently Asked Questions
Q1What is the adjacent-lamp-out rule?
It is the part of the ICAO maintenance objectives that says where unserviceable lights may sit. To keep continuity of guidance, unserviceable lights are not permitted next to each other, with limited exceptions that depend on the operation and the lighting pattern.
Q2Why is the percentage not enough on its own?
Because the same number of outages can be harmless or harmful depending on position. One light out leaves a gap of two light spacings between lit lights; two adjacent lights out leave a gap of three. A row can be inside its percentage and still have a break in guidance.
Q3Does the rule ever allow adjacent unserviceable lights?
In some wording, yes. For precision approach runways it applies except in a barrette or crossbar, where two adjacent unserviceable lights may be permitted. For take-off in RVR below 550 m and for stop bars it applies unless the light spacing is significantly less than that specified. For take-off in RVR of 550 m or more, and for taxiway centreline lights in RVR below 350 m, no exception is stated.
Q4What counts as adjacent?
Neighbouring lights along the same line of lights, such as two consecutive runway edge or centreline lights. For approach lighting, a barrette or crossbar is treated differently from a line of lights; follow the wording and the guidance in Doc 9137, and check how your regulator reads it.
Q5What should I fix first when two adjacent lights are out?
Restore one of the pair first, so the outages are no longer side by side, then deal with the rest. Treat it as a defect to report and correct promptly rather than waiting for a routine visit.
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