BARRETTE CROSSBAR
In an approach lighting system, a centreline barrette and a crossbar are both transverse groups of lights, but they have different locations, sizes, and functions. A centreline barrette is the repeated light bar forming the approach centreline; a crossbar is a wider transverse reference bar used at a specified distance from the threshold.
At a Glance
| Feature | Centreline Barrette | Crossbar |
|---|---|---|
| Primary purpose | Establish approach centreline | Provide transverse distance/reference cue |
| Location | Along extended runway centreline | At specified station across/around centreline |
| Orientation | Perpendicular to centreline | Perpendicular to centreline |
| Size | Relatively short | Wider / longer |
| Repetition | Multiple stations | Usually specific designated stations |
| Main visual function | Alignment | Distance/geometry reference |
| Typical example | 5-light centreline barrette | 500-ft / 1,000-ft crossbar |
| Example system | ALSF-2, MALSR | ALSF-2 |
| Flashers mounted with it | May be | May be associated depending on ALS |
FAA documentation defines a barrette as three or more closely spaced lights arranged transversely so that they appear as one short illuminated bar.
What Are They?
Centreline Barrette
A barrette is a group of closely spaced lights arranged transversely. In an ALSF-2 system, the centreline light bars are installed at regular intervals along the extended runway centreline. FAA specifies five equally spaced lights in the standard ALSF-2 centreline light bar arrangement.
APPROACH
↓
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│
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│
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│
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│
THRESHOLDThe repeated barrettes create the visual centreline of the approach.
Crossbar
A crossbar is a substantially wider transverse lighting arrangement installed at a specified distance from the runway threshold. It provides a strong horizontal reference and helps the pilot perceive distance along the approach.
│
│
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│The centreline passes through the middle of the crossbar.
Alignment Reference vs Distance Reference
Centreline Barrette
Short + repeated + centreline.
"This is the runway centreline."
Crossbar
Wide + specific station + horizontal reference.
"This is a significant distance/reference point along the approach."
Where the Difference Becomes Clear
The difference is especially clear in an ALSF-2. FAA specifies centreline light bars at 100-ft intervals, beginning 100 ft from the threshold and extending outward.
At the 1,000-ft station, the centreline bar is extended with additional lights on both sides to form a much wider 1,000-ft distance-marker crossbar. The ALSF-2 also has a 500-ft crossbar.
APPROACH
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│
──────────┼──────────
500-ft bar
│
│
│
──────────────┼──────────────
1,000-ft bar
│
│
│
THRESHOLDThe repeated small bars are the centreline barrettes. The large transverse bars are the crossbars.
What Each One Does for the Pilot
Centreline Barrette Function
- Runway alignment
- Extended runway centreline reference
- Approach direction
- Lateral visual reference
- Continuity of the approach-lighting pattern
Because multiple barrettes are positioned along the centreline, they form a continuous visual path toward the runway.
Crossbar Function
- Horizontal visual reference
- Distance reference
- Enhanced conspicuity
- Additional approach geometry
- Assistance in judging the aircraft's position along the approach
The 1,000-ft bar is particularly important because its much greater width makes it visually distinctive.
A Useful Mental Picture
Centreline Barrette
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│
│Crossbar
─────────────────
│
│The crossbar is therefore much more visually prominent. In ALSF-2, FAA describes the normal centreline light bar as approximately 13½ ft long with five equally spaced lights, while the 1,000-ft distance-marker crossbar is approximately 100 ft wide and contains 21 lights.
Centreline Barrette vs Crossbar in ALSF-2
| Item | Centreline Barrette | 500-ft Crossbar | 1,000-ft Crossbar |
|---|---|---|---|
| Basic function | Centreline | Distance/reference | Distance/reference |
| Approx. location | Repeated stations | 500 ft | 1,000 ft |
| Typical appearance | Short bar | Wider bar | Very wide bar |
| Lights | 5 in standard centreline bar | 4 each side of centreline | 21 total |
| Orientation | Transverse | Transverse | Transverse |
| Alignment | On centreline | Around centreline | Across centreline |
| Main cue | Alignment | Distance/geometry | Major distance marker |
Why Are Crossbars Needed?
Imagine the approach lighting as only a straight line:
● ● ● ● ● ● RUNWAY
It gives excellent alignment information but relatively little information about distance along the approach. Now add crossbars:
●
│
───────┼───────
│
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│
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●The crossbars break up the pattern and provide additional horizontal and distance references.
Are All Barrettes Centreline Barrettes?
No. Barrette describes the physical arrangement of closely spaced lights. A barrette can be used for different purposes, including:
- Centreline bar
- Side-row bar
- Other defined ALS elements
FAA's ALSF-2 description specifically identifies centreline light barrettes and side-row barrettes.
Barrette = physical light arrangement. Centreline barrette = barrette used on the approach centreline. Crossbar = larger transverse visual reference at a defined station.
Crossbar Is Not Simply a "Large Barrette"
They may look physically similar, but engineering terminology should be kept separate. A barrette describes the lighting element itself. A crossbar describes the functional/configurational element within the ALS.
For example, FAA specifically calls the 1,000-ft element the "1,000-foot distance marker crossbar."
A Useful Contrast
MALSR provides a useful contrast. FAA describes MALSR as having nine sets of five steady-burning light sources called light bars. Seven are located along the extended runway centreline at 200-ft intervals, while two are offset at the 1,000-ft bar.
Do not transfer the ALSF-2 500-ft/1,000-ft crossbar configuration to every ALS. Each ALS has its own defined configuration.
Electrical Considerations
Both centreline barrettes and crossbars normally contain multiple individual light fixtures. A simplified series-circuit arrangement may be:
CCR ↓ Primary Series Circuit ↓ Isolation Transformer ↓ Light Fixture
For a barrette: several fixtures → several isolation transformers → one defined lighting element. The crossbar may contain substantially more fixtures and therefore has a larger electrical load than a normal centreline barrette.
Maintenance Difference
Centreline Barrette
Inspect:
- Individual lamps/LEDs
- Lens/prisms
- Fixture alignment
- Transformer
- Wiring
- Structural support
Crossbar
All of the above, plus:
- Complete transverse alignment
- Uniformity of the crossbar
- Correct distance/station
- Individual fixture orientation
- Overall visual geometry
A missing fixture on a crossbar can be particularly noticeable because it disrupts the horizontal reference.
How Each One Appears
Centreline Barrette
──── ──── ──── ────
This creates a repeated centreline pattern.
Crossbar
──────────────
This creates a strong horizontal reference.
Combined:
│
│
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│
│
───────────┼───────────
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THRESHOLDThe result is a three-dimensional visual impression of the approach geometry.
Myths vs Facts
No. Both are transverse light arrangements, but their purpose and configuration are different.
No. Those are configuration-specific. ALSF-2 explicitly has these crossbars.
No. ALSF-2 also uses side-row barrettes.
The centreline barrettes/light bars establish the longitudinal centreline; the crossbar provides a transverse reference.
Simple Memory Diagram
Centreline Barrette
● ● ● ● ●
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│
│
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● ● ● ● ●Purpose: ALIGNMENT
Crossbar
│
│
● ● ● ● ●●● ● ● ● ●
│
│Purpose: DISTANCE / HORIZONTAL REFERENCE
Barrette vs Crossbar — The Bottom Line
A centreline barrette is a short transverse group of closely spaced lights repeated along the extended runway centreline to provide a stable alignment reference. A crossbar is a wider transverse lighting element installed at a defined distance from the threshold to provide a prominent horizontal and distance reference.
For ALSF-2, the FAA specifically identifies the 1,000-ft distance-marker crossbar and 500-ft crossbar, while the repeated centreline elements are the centreline light bars/barrettes.
Engineering & Expert Notes
Position Is Part of the Meaning
LayoutBarrettes and crossbars give different cues by where they sit. Verify survey positions.
Check Aiming and Level of Each Unit
AlignmentSmall errors in a long line of lights are visible to the pilot.
Walk the Pattern at Night
InspectionLook for gaps or dim units that break the pattern.
Record As-Built Positions
RecordsKeep measurements for future checks.
Frequently Asked Questions
No. Both are transverse light arrangements, but their purpose and configuration are different — the barrette establishes alignment, the crossbar provides a distance/horizontal reference.
No. Those are configuration-specific — ALSF-2 explicitly has these crossbars, but this should not be assumed for every ALS.
No. ALSF-2 also uses side-row barrettes in addition to centreline barrettes.
No. The centreline barrettes/light bars establish the longitudinal centreline; the crossbar provides a transverse reference.
The standard centreline light bar is approximately 13½ ft long with five equally spaced lights. The 1,000-ft distance-marker crossbar is approximately 100 ft wide and contains 21 lights.
No. Each ALS has its own defined configuration — for example, MALSR uses nine sets of five-light bars at different intervals rather than ALSF-2's 500-ft/1,000-ft crossbar arrangement.
Governing Documents
For an actual airport project, verify the current applicable editions of the relevant standards before using this comparison for project design.
FAA Order 6850.2C / 6850.2B
Defines ALSF-2 centreline light bar spacing, the 500-ft and 1,000-ft distance-marker crossbars, and side-row barrette configuration.
AC 150/5300-18B, Part 2
General guidance defining a barrette as three or more closely spaced lights arranged transversely so that they appear as one short illuminated bar.
FAA VGLS — MALSR Reference
Describes MALSR's nine sets of five-light steady-burning light bars, spaced at 200-ft intervals with two offset at the 1,000-ft bar.
Final Engineering Note
Barrette and crossbar dimensions, light counts and spacing are configuration-specific to each ALS type. For final design, use the configuration drawings and dimensional criteria from the applicable current standard rather than a generic comparison page.
Barrette → short + repeated + centreline. Crossbar → wide + specific distance + horizontal reference.
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