What is a centreline barrette?
A short row of closely spaced white lights across the extended runway centreline, so each station reads as one bar and the row of bars leads the pilot to the threshold.
A centreline barrette is a short row of closely spaced white lights set across the runway centreline that reads from the air as a single bar. Barrettes are repeated along the centreline at regular intervals. In the FAA ALSF-1 and ALSF-2 systems each barrette is about 13½ ft long with five lights and they stand every 100 ft out to the end of the system. ICAO precision systems use barrettes at least 4 m long, with lights not more than 1.5 m apart, set 30 m apart along the centreline. The line of bars gives the pilot the line to follow and, as the bars pass, a sense of how far out the aircraft is.
A bar is brighter and easier to see than a single light in fog, haze or rain and against background lights, and its width helps the pilot judge alignment. In the ICAO pattern a CAT I centreline may use barrettes in place of single lights, and in a CAT II/III system the centreline over the first 300 m from the threshold is made of barrettes. ICAO recommends that barrettes be supplemented with flashing lights that flash in sequence toward the threshold, twice a second. In the FAA ALSF systems these are bluish-white flashers, one on the centreline at each bar from 1,000 ft out to the end of the system.
The pattern only works if every bar is complete and square. A dead light shortens a bar, and a barrette that is tilted or shifted changes how it is seen from the approach, so after work check that every light is lit, the bar is level and square to the centreline, the lenses are clean and the frangible supports are intact.
Frequently Asked Questions
Q1What is a centreline barrette?
A centreline barrette is a short row of closely spaced white lights set across the extended runway centreline. From the air the lights read as a single bar. A row of barrettes along the centreline gives the pilot the line to follow to the runway and, by the bars passing, a sense of how far out the aircraft is.
Q2Why use barrettes instead of single lights?
A bar of light is brighter and easier to pick out than one light, especially in fog, haze or rain and against bright lights on the ground. Its width across the centreline also helps the pilot judge alignment and bank. That is why barrettes are used where the approach is flown in poor visibility, and why ICAO recommends supplementing them with sequenced flashing lights.
Q3How many lights are in a barrette, and how far apart are the barrettes?
In the FAA ALSF-1 and ALSF-2 systems each barrette is about 13 and a half feet long with five white lights 40 and a half inches apart, and the barrettes are 100 ft apart along the centreline; the decision bar at 1,000 ft is a wider bar of 21 lights. ICAO precision systems use barrettes at least 4 m long with lights not more than 1.5 m apart, 30 m apart along the centreline; the values in the edition of Annex 14 in force are the ones to use.
Q4Which systems use centreline barrettes?
ALSF-1 and ALSF-2 are built on them. In the ICAO pattern a CAT I centreline may use barrettes instead of single lights, and in a CAT II/III system the centreline over the first 300 m from the threshold is made of barrettes. Simple systems for non-precision approaches use single lights.
Q5What happens if a light in a barrette fails?
The bar gets shorter or gappy, and if a whole barrette is lost there is a break in the line of the centreline. Authorities set limits on how many lights, and how many neighbouring barrettes, may be out of service, so failures are logged and repaired within the airport's maintenance plan.
Q6What should be checked after work on a barrette?
That every light is lit, that the bar is level and square to the centreline, that the lights are aimed as the standard and the manufacturer require, that the lenses are clean and that the frangible supports and couplings are intact. A tilted or shifted bar changes how it is seen from the approach.
Q7What is hazardous when working on one?
The lights are normally on a series circuit, which can carry a dangerous voltage and current, and opening a live series circuit can produce a lethal voltage. Isolate the circuit at the CCR, lock it out, prove it dead and earth it as your procedure requires, under the permit-to-work, before touching any fitting or cable, and isolate any separate supply such as the flasher control. The barrettes stand in the approach area close to the runway end, so access, aircraft-movement control and keeping the supports frangible all apply.
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