What is a Barrette in Aviation?

Barrette

Definition

A barrette is a transverse row of three or more close aeronautical ground lights that looks like one short bar from a distance.

ICAO Annex 14 and EASA CS-ADR-DSN give the same definition. A barrette is not a type of light. It is a way to arrange lights. Each light in it is a normal approach light or runway light, and the group acts as one light position.

Where barrettes appear

The standards use barrettes in these places.

The rapid exit taxiway indicator lights also use one. ICAO and EASA call the first set of three lights a three-light barrette.

Length and spacing

The values depend on the system.

• Simple approach lighting. A barrette is at least 3 m long. ICAO notes that a spacing of 1.5 m between lights is satisfactory. Use 4 m if the system may grow into a precision approach system later (ICAO 5.3.4.7, Notes 1 and 2).
• CAT I, II and III approach lighting. A barrette is at least 4 m long. When it uses point sources, the lights are evenly spaced at most 1.5 m apart (ICAO 5.3.4.16 and 5.3.4.33).
• Touchdown zone lights. A barrette has at least three lights, at most 1.5 m apart. ICAO recommends a length of 3 to 4.5 m (5.3.13.3 and 5.3.13.4).

In ICAO, the 4 m length, the 1.5 m spacing and the three-light minimum are Standards. The 3 to 4.5 m length of the touchdown zone barrette is a Recommendation. CS ADR-DSN words the same values as “should”.

Barrettes in the approach lighting system

A barrette lets one position on the centre line carry more light than a single source. The approach lighting system page has the full patterns. These are the points that concern barrettes.

• CAT I centre line. Each position has either a barrette, or one, two or three single lights that give distance information. Where the operator can show the serviceability level in the maintenance objective, a single light at each position is also allowed (ICAO 5.3.4.14 and 5.3.4.15).
• Flashing lights. ICAO recommends that each barrette has a flashing light, unless it is unnecessary for the system and the weather. The flashing lights fire twice a second, in sequence from the outermost light toward the threshold (5.3.4.17 and 5.3.4.18). See sequenced flashing lights .
• CAT II and III centre line. The first 300 m consist of barrettes showing variable white. Single lights alone may replace them only where the threshold is displaced 300 m or more. Where the operator can show the serviceability level, single lights may alternate with barrettes (5.3.4.30).
• Red side rows. The side rows consist of barrettes showing red. In the full pattern a row starts 30 m from the threshold and reaches 270 m. The lateral spacing between the two innermost lights is 18 to 22.5 m, preferably 18 m, and equals that of the touchdown zone lights. The red intensity must suit the white lights (5.3.4.22, 5.3.4.24, 5.3.4.36 and 5.3.4.38).
• Installation. EASA guidance says that barrettes stand at right angles to the centre line of the system. The tolerance is 30 minutes of arc for the pattern in Figure M-2(A), and 2 degrees for Figure M-2(B) (GM1 ADR-DSN.M.625(b)(4)).

Touchdown zone lights

ICAO requires touchdown zone lights on a CAT II or III precision approach runway (5.3.13.1).

The pattern is pairs of barrettes placed symmetrically about the runway centre line. The lateral spacing between the innermost lights of a pair equals the spacing of the touchdown zone marking. The pairs stand 30 m or 60 m apart along the runway. ICAO notes that 30 m may suit lower visibility minima.

The lights extend 900 m from the threshold. On a runway shorter than 1,800 m, they stop at the midpoint of the runway. The lights are fixed and unidirectional, and they show variable white (ICAO 5.3.13).

Barrette, crossbar and light bar

A crossbar is a longer row across the centre line. In the CAT I system the crossbar is 30 m long and stands 300 m from the threshold. In the CAT II and III system, crossbar lights stand at most 2.7 m apart. The crossbar at 300 m extends 15 m on each side of the centre line (ICAO 5.3.4.10, 5.3.4.26 and 5.3.4.37). A barrette is shorter, and it forms a position of the centre line or a side row.

The FAA does not use the word barrette in its definitions. FAA Order JO 6850.2C defines a light bar as a cluster of signal lights in a row perpendicular to the runway centerline. In the ALSF-2 system, the side row bars have three red lights each, in the inner 900 ft.

Serviceability

A light is unserviceable when its main beam average intensity is below 50 percent of the specified value, (ICAO 10.5.1). EASA also counts a missing or damaged filter, or a wrong beam colour, as unserviceable (ADR.OPS.C.015(c)).

For CAT II and III operations, no unserviceable light may stand next to another. The exception is a barrette or a crossbar, where two adjacent unserviceable lights may be permitted (ICAO 10.5.7, ADR.OPS.C.015(b)(3)). EASA guidance says that lights are adjacent when they are consecutive in the same barrette or crossbar, or in the same row of barrettes. It also says that in a barrette or crossbar, guidance is not lost with two adjacent unserviceable lights (GM3 ADR.OPS.C.015(b)).

How to check a barrette

For a CAT II or III runway, ICAO recommends visual inspection and in-field measurement of intensity, beam spread and orientation. This applies to the approach and runway lights. ICAO also recommends that the operator measures all lights, as far as practicable, with a mobile measuring unit (ICAO 10.5.3 to 10.5.5).

For a barrette, check three things. Count the lights in each one. Check the colour of each light. Check that the group stands at right angles to the centre line. A drone inspection can record each light of a barrette as its own unit. TarmacView’s approach lights inspection does this for the approach lighting system.