How are critical lights kept serviceable?
Stop bars and CAT II/III runway centreline lights are held to the tightest maintenance objectives, because a missing light there costs the most. A stop bar is judged by a count, a CAT II/III centreline by a percentage, and both by adjacency.
Most lights can tolerate a few outages. These two cannot: the objectives are among the tightest in Annex 14, because the stop bar is the red signal at a runway-holding position and the CAT II/III centreline is the main guidance along the runway when visibility is low. The general figures are in Entry 27, and the adjacency rule is in Entry 28.
1Stop bars, RVR below 300 m
For a stop bar used with a runway intended for operations in RVR below 300 m, no more than two lights should be unserviceable, and two adjacent lights should not be out unless the spacing is significantly less than specified. A stop bar is a short row, so the objective is stated as a count, not a percentage. Where your site monitors stop bars, use that status to keep the count current.
2CAT II/III runway centreline
During CAT II or III operations the aim is that all approach and runway lights are serviceable. The floor, “in any event”, is 95 per cent of the centreline lights, with unserviceable lights not adjacent (the barrette and crossbar exception does not apply to a centreline row). Both limits apply together: a row can be inside the percentage and still fail on adjacency.
No more than two out, and not two neighbours unless the spacing is significantly less than specified. Easy to check on the spot, and it does not depend on how long the bar is.
At least 95% serviceable, with no adjacent outages. On 100 lights that is at most 5 out.
3When the objective cannot be held
Tell airfield control straight away, so operations can be adjusted under the site procedure. Restricting operations or using alternative procedures is a decision for the aerodrome operator and air traffic services, not for the maintenance team. Then restore the lights, starting with any adjacent pair, and recheck the count and positions.
- Check these lights first on each inspection round
- Keep spares for them on site
- Recheck count and position after every repair
- Wait for the next routine visit to fix a failure
- Count a lit but dim light as serviceable
- Judge a stop bar by a percentage
| Light | Operation | Objective |
|---|---|---|
| Stop bar | RVR below 300 m | No more than 2 out; two adjacent not out unless spacing is significantly less than specified |
| Runway centreline | CAT II/III | At least 95% serviceable; no two adjacent |
| Runway centreline (where provided) | Take-off, RVR below 550 m | At least 95% serviceable; not adjacent |
| Taxiway centreline | RVR below 300 m | No two adjacent lights unserviceable |
Frequently Asked Questions
Q1Why are stop bars and CAT II/III centreline lights called critical lights?
Because the maintenance objectives are tightest where a missing light costs the most. A stop bar is the red signal at a runway-holding position when visibility is low, and CAT II/III centreline lights are the main guidance along the runway in low visibility.
Q2What is the objective for a stop bar?
For a stop bar used with RVR below 300 m, no more than two lights should be unserviceable, and two adjacent lights should not be out unless the light spacing is significantly less than specified. It is a count, not a percentage.
Q3What is the objective for CAT II/III runway centreline lights?
During CAT II or III operations the aim is that all approach and runway lights are serviceable and, in any event, at least 95 per cent of the centreline lights are, with no two unserviceable lights adjacent (the barrette and crossbar exception does not apply to a centreline row).
Q4What should I do if the objective cannot be met?
Report it to airfield control straight away, so operations can be adjusted under the site procedure. Restricting operations or using alternative procedures is a decision for the aerodrome operator and air traffic services, not the maintenance team.
Q5How do I know how many lights are out?
Use the lighting control and monitoring system where it reports lamp status, and confirm by inspection. Check counts and positions after every repair, because both the number and the adjacency matter.
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