Yellow Clearance Bar
A critical taxiway lighting system that provides a visual indication of a holding position where clearance from Air Traffic Control (ATC) is required before proceeding. Typically installed at taxiway/taxiway holding positions on taxiways, these yellow lights help pilots and vehicle operators identify points where they must stop and await further instructions, enhancing airfield safety.
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What Is It?
A Yellow Clearance Bar is a row of steady-burning yellow lights installed across a taxiway at a taxiway/taxiway holding position — a point where one taxiway crosses another and traffic must hold short until cleared by ATC. It is part of the low-visibility taxiway lighting system standardised under FAA AC 150/5340-28, historically also referred to as a "hold bar." At runway holding positions, flashing yellow Runway Guard Lights have replaced the clearance bar as the primary conspicuity aid, so today's clearance bars are installed specifically at taxiway/taxiway intersections, positioned to agree with the taxiway/taxiway intersection marking. The bar increases the conspicuity of that marking in low visibility; it does not itself issue a stop or proceed instruction the way a red stop bar does — the pilot's authority to cross comes from the ATC clearance, and the marking plus the bar make the holding position itself hard to miss.
Purpose & Function
The yellow clearance bar exists to mark a taxiway/taxiway holding position that isn't obvious from the pavement layout alone. In good visibility the painted holding-position marking is usually enough, but in fog, heavy rain, or at night a pilot following a taxi clearance could roll past the marking without a clear visual cue that a hold is required there. By placing a row of steady yellow lights directly across the pavement at exactly that point, the fixture makes the marking itself conspicuous, so a pilot who is holding short — or who needs to confirm exactly where the holding position is before proceeding on an ATC clearance — has an unmissable visual reference regardless of visibility conditions.
Where Is It Used?
Yellow clearance bars are installed on taxiways at taxiway/taxiway holding positions, typically at:
- Taxiway/taxiway intersections where one taxiway crosses another and traffic must hold short
- Complex taxiway junctions where the holding position isn't obvious from the pavement layout alone
- Airports operating low-visibility procedures (LVP) where the wider low-visibility taxiway lighting system (centreline lights, stop bars, clearance bars) is in use
- Busy airports with complex taxiway networks and high traffic density
Note: at runway holding positions specifically, flashing yellow Runway Guard Lights have generally replaced the clearance bar as the primary conspicuity aid, so a clearance bar of this type is not typically found there.
Installation & Location
Fixtures are set out in a row across the taxiway at the taxiway/taxiway intersection they mark, using the standard load-rated inset base can recessed into the pavement, spaced tightly enough to read as a continuous yellow line, positioned to agree with the location of the taxiway/taxiway intersection marking. The bar is powered and switched together with the rest of the low-visibility taxiway lighting circuit it belongs to — typically brought on with the LVP taxiway lighting system rather than commanded fixture-by-fixture — so wiring generally follows the same series-circuit/CCR channel as the other lights in that circuit. Setting-out is tied to the taxiway/taxiway intersection marking survey, so the bar sits exactly on the line it marks.
How Does It Work?
Yellow clearance bars are powered from the series circuit via isolating transformers. They produce a steady (not flashing) yellow light — flashing yellow is the signature of a different fixture, the Runway Guard Light, which serves a related but distinct role at runway holding positions. The lights are arranged in a row across the taxiway, forming a continuous bar that is highly visible to pilots and vehicle operators in low visibility. The bar is switched together with the rest of the low-visibility taxiway lighting system it belongs to, typically via ALCMS, rather than being commanded individually per clearance event:
- Yellow (steady): Marks a taxiway/taxiway holding position, increasing the conspicuity of the pavement marking in low visibility. The bar itself does not instruct stop or proceed — that comes from ATC clearance and the holding-position marking.
- Red (stop bar): A separate fixture type; consists of steady-burning red in-pavement lights switched on to mean stop and off to mean proceed, typically at a runway holding position.
- Relationship: The clearance bar works alongside taxiway centreline lights, stop bars (where fitted), and runway guard lights as part of the wider low-visibility taxiway guidance system, but each fixture type has a distinct role and is not interlocked with the others as a stop/proceed pair.
System Architecture & Control Logic
The bar is commanded as a single block from ALCMS, not fixture by fixture — one command switches every lamp in the row together, as part of bringing the low-visibility taxiway lighting system on or off (typically tied to LVP activation rather than a per-aircraft clearance event). Unlike a stop bar, the clearance bar has no stop/proceed interlock logic of its own: it does not switch off to authorise an aircraft to proceed, and where a stop bar is also present nearby, the two are separate fixture types serving separate functions rather than a linked pair. The clearance bar's state changes when the operational condition governing the whole low-visibility lighting circuit changes — for example, LVP being activated or deactivated — not in response to individual ATC clearances.
Bar Continuity Under Single Fixture Fault
CCR
Series circuit, 45W / 65W loop
Fixture 1
Transformer + LED
Fixture 2
Transformer + LED
Fixture 3
LED driver fails, this lamp goes dark
Fixture 4
Transformer + LED
Series Circuit Remains Intact
Fixtures 1, 2 & 4 keep operating — only the failed lamp is dark
Physical Construction
A yellow clearance bar is built using the same flush or elevated fixture construction as other taxiway intersection lights: an L-852 base can housing the light source and isolating transformer for inset installations, with a hardened, gasket-sealed lens achieving an IP67/IP68 rating, or a frangible-mounted elevated unit where flush mounting is impractical. Because the bar functions as a single controlled unit that illuminates steadily together across the full width of the taxiway, the individual fixtures making up the bar are wired on a common series circuit or CCR channel, with sealed L-823 connectors linking each fixture back to its transformer and onward to the shared control circuit so all lights in the bar switch in unison under ALCMS control. The fixture body is kept low-profile and load-rated where inset, built to withstand repeated aircraft and vehicle wheel loading at the holding position, and stainless-steel fasteners and corrosion-resistant wiring are used throughout given the fixture's direct exposure to surface traffic and weather at this safety-critical location.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Light Source | LED array (yellow/amber) |
| Colour | Yellow (steady-burning, not flashing) |
| Intensity Steps | Controlled by CCR (typically 5-step) |
| Beam Pattern | Uni-directional towards approaching traffic or omnidirectional |
| Mounting Type | Flush-mounted (inset) or elevated |
| Load Capacity | 100,000 kg (220,000 lbs) wheel load |
| Isolating Transformer | 45W / 65W (6.6A series circuit) |
| Protection Class | IP67 / IP68 (submersible) |
| Integration | Part of the wider ALCMS-controlled low-visibility taxiway lighting system (alongside, but not interlocked with, taxiway centreline lights, stop bars, and runway guard lights) |
Electrical Requirements
| Requirement | Typical Value |
|---|---|
| Circuit Type | Series circuit, constant current (CCR fed), switched together with the low-visibility taxiway lighting circuit it belongs to via ALCMS |
| Rated Series Current | 6.6A (5-step: 2.8A / 3.4A / 4.1A / 5.2A / 6.6A) |
| Isolation | Via dedicated isolating transformer (45W/65W), not directly connected to series loop |
| Cable Type | L-824 single-conductor, direct-burial rated |
| Connector | L-823 primary connector, submersible-rated splice |
| Insulation Resistance | ≥ 50 MΩ (megger test, cable-to-ground) |
| Earthing/Grounding | Fixture body and base can bonded to airfield earth grid |
| Fault Protection | CCR-level ground fault/open-circuit detection plus ALCMS fault indication |
| Power Source | CCR output via ALCMS-controlled switching, regulated per ICAO/FAA intensity step schedule |
Photometric Requirements
| Parameter | Typical Value |
|---|---|
| Main Beam Intensity | ≥ 100 cd (yellow) per ICAO Annex 14 / FAA AC 150/5345-46 |
| Intensity Steps | 100% / 30% / 10% / 3% / 1% of rated peak intensity |
| Vertical Beam Spread | Main beam 2°–10° above horizontal (peak ~3°–4°) |
| Horizontal Beam Spread | ± 5° either side of taxiway centreline at the clearance bar position |
| Isocandela Distribution | Per ICAO Annex 14 Attachment A / FAA AC 150/5345-46 clearance bar figures |
| Chromaticity (Yellow) | Within ICAO Annex 14 yellow boundary (CIE coordinates) |
| Maintenance Factor | ≥ 50% of new-fixture intensity before replacement/cleaning is due |
| Test Method | Mobile photometric van/goniophotometer or fixed test bench per fixture |
Applicable Standards
ICAO Annex 14
Taxiway holding position light requirements (yellow clearance bars)
FAA AC 150/5345-46
Specification for Runway, Taxiway, Heliport, and Vertiport Light Fixtures (includes clearance bar fixtures)
Installation Requirements
Fixture Setting-Out Across Taxiway
CivilSet out fixtures in a row across the taxiway at the taxiway/taxiway intersection marking, spaced tightly enough to read as a continuous yellow line.
L-852 Base Can Installation
CivilInstall the standard load-rated inset base can recessed into the pavement at each fixture position, tied to the taxiway/taxiway intersection marking survey.
Common Series Circuit/CCR Channel Wiring
ElectricalLink each fixture back to its transformer and onward to the shared low-visibility taxiway lighting circuit so all lights in the bar switch in unison under ALCMS control.
Survey Line Alignment
MechanicalConfirm the bar sits exactly on the taxiway/taxiway intersection marking it is protecting.
Pre-Energisation Sign-off
DocumentationRecord fixture setting-out, control wiring, and survey alignment as-built before backfill or reinstatement of the base cans.
Testing & Commissioning
Insulation Resistance (Megger) Test
Pre-EnergisationMegger-test the isolating transformer and series loop before the circuit is energised, confirming insulation resistance clears the acceptable minimum ahead of backfill.
Loop Continuity & Resistance Test
Pre-EnergisationVerify series loop continuity end to end across every fixture in the row and record total loop resistance before first energisation.
Primary/Secondary Voltage Verification
EnergisationWith the CCR energised, confirm secondary voltage at each fixture's connector matches the transformer's rated ratio at the commissioned current step.
CCR Brightness Step Response
EnergisationCycle the CCR through every intensity step and confirm every lamp in the row responds together as a single commanded block.
ALCMS Block-Switching Verification
EnergisationConfirm the bar switches on and off as a single block under ALCMS command together with the rest of the low-visibility taxiway lighting circuit it belongs to, with no lamp lagging or failing to respond.
Chromaticity & Photometric Test
Final AcceptanceMeasure colour chromaticity and beam intensity across the row with a calibrated photometer against ICAO/FAA commissioning tolerances.
Documentation & As-Built Sign-off
Final AcceptanceRecord insulation, continuity, voltage, block-switching, and photometric readings alongside as-built position in the commissioning dossier for handover.
Inspection & Maintenance
Lens Cleaning
QuarterlyClean the lens of rubber deposits, oil, and de-icing fluid with approved solvents to maintain light output and colour fidelity.
LED Driver Inspection
AnnualCheck LED drivers for thermal stress damage that could cause individual lights to dim or fail.
Chromaticity & Photometric Test
AnnualTest light output against ICAO yellow chromaticity requirements to catch colour shift from thermal stress before it affects compliance.
Communication Link Test
Semi-annualVerify the bar's ALCMS communication link switches the lights on and off correctly with the rest of its circuit, checking redundant paths if fitted.
Housing & Seal Inspection
AnnualCheck the housing and seals for moisture ingress that could affect the LED array or communication electronics.
Common Field Issues
Lens Contamination
Yellow clearance bars are exposed to rubber deposits, oil, and de-icing fluids. Regular cleaning with approved solvents and soft brushes is essential to maintain light output and colour fidelity.
LED Driver Failure
In LED-based units, driver failure can cause individual lights to go out or dim. Regular inspection and replacement of faulty drivers is critical to maintain the visual integrity of the bar.
Colour Shift
Over time, LED colour can shift toward orange or become less saturated due to thermal stress. Regular photometric testing ensures compliance with ICAO yellow chromaticity requirements.
Communication Failure
Clearance bars rely on communication with the ALCMS to switch on and off with the rest of the low-visibility taxiway lighting circuit. Network interruptions can cause the bar to fail to switch at the correct time along with that circuit. Redundant communication paths are recommended.
Troubleshooting Guide
Individual LED Segment Not Illuminating
Bar Fails to Switch With Its Lighting Circuit
Colour Appears Washed Out or Shifted from Yellow
Entire Bar Dark
Failure Modes & Root Causes
Individual LED Segment Failure
Command Interface Failure
Lens Filter Fade / Contamination
Power Supply / Main Circuit Failure
Safety & Operational Notes
Series Circuit Isolation
Electrical SafetyDe-energise and lock out the CCR feeding the clearance bar circuit and verify zero energy at the fixtures before opening a base can or handling connectors.
ATC Coordination for a Dark Clearance Bar
OperationalA clearance bar out of service removes the visual reminder to hold for ATC permission at an taxiway/taxiway holding position; notify ATC before starting work, arrange an alternate procedure, and raise a NOTAM for the outage.
FOD Control at the Base Can
OperationalKeep tools, fasteners, and gaskets accounted for while the base can is open, and secure or remove loose material before reinstating the fixture.
Live-Traffic Exposure
Personnel SafetyIntermediate holding positions carry ongoing taxiway traffic; keep personnel and open base cans clear of the through path and use spotters or a closure when working during active movement.
Insulation Test Before Re-Energisation
Electrical SafetyMegger test the affected circuit after any cable, connector, or transformer work before restoring power, since a residual fault at re-energisation can trip the CCR or darken the row.
PPE & Grounding
Personnel SafetyUse insulated tools and appropriate PPE on series-circuit connections, and confirm each base can is bonded/grounded before reconnecting.
Restore to Compliant State
ComplianceConfirm all lights in the row are yellow, correctly aimed, and at full intensity before closing out the work and reopening the holding position.
Engineering & Expert Notes
Clearance Bar Colour Is the Whole of Its Function
ComplianceBecause a clearance bar's job is entirely conveyed by its distinct yellow colour, any drift toward an amber-white cast or uneven colour across the row degrades its function well before intensity alone would flag it as a fault; check chromaticity, not just lumens, when assessing a "dim-looking" row.
Flush Fit Matters as Much as Photometrics
MechanicalAn inset fixture sitting even slightly proud of grade becomes a snag or FOD risk at a holding position with continuous traffic well before it fails photometrically; verify flush seating against the surrounding pavement grade at every reinstall, not just light output.
Trend Loop Readings, Don't Just Spot-Check
TroubleshootingMoisture ingress at an L-823 connector usually shows up first as a slow drift in loop insulation resistance, not a hard fault. Logging megger readings over successive maintenance cycles catches a failing splice months before it takes the row down.
Size the Transformer to the Load, Not the Legacy Lamp
ElectricalReusing an isolating transformer sized for the old incandescent wattage on a much lower-draw LED fixture is a common source of premature transformer failure and nuisance loop trips; re-select VA rating against the actual LED load rather than assuming the existing transformer is "close enough."
Balance Loops Sharing One CCR
DesignWhere a clearance bar's fixtures are split across two separate series circuit loops for redundancy, each fed by its own CCR, unequal loop lengths or lamp counts between the two loops can produce visibly uneven brightness across the row even though each individual fixture tests within photometric tolerance. Balance loop impedance at the design stage, not after commissioning complaints.
De-Icing Fluid Ages Seals Faster Than UV Alone
DurabilityRepeated exposure to glycol-based de-icing fluid degrades gaskets and lens seals faster than sun and weather alone at an taxiway/taxiway holding position that sees regular de-icing traffic. Specify seal materials rated for glycol contact and inspect gasket condition at overhaul, not only at initial install.
Frequently Asked Questions
Short answers about yellow clearance bars: where they are used, colour, intensity, control, standards, maintenance and common faults.
Q1What is a yellow clearance bar?
A yellow clearance bar is a row of steady-burning yellow lights across a taxiway at a taxiway/taxiway holding position, where one taxiway crosses another and traffic must hold short until cleared by ATC. It is part of the low-visibility taxiway lighting system in FAA AC 150/5340-28 and was historically called a hold bar.
Q2Where are yellow clearance bars installed today?
At taxiway/taxiway intersections, positioned to agree with the intersection marking. At runway holding positions, flashing yellow Runway Guard Lights have replaced the clearance bar as the primary conspicuity aid.
Q3Why is a clearance bar needed?
In good visibility the painted holding-position marking is usually enough, but in fog, heavy rain or at night a pilot could roll past it without a clear cue to hold. A row of steady yellow lights directly across the pavement makes the hold point unmistakable.
Q4Is a clearance bar steady or flashing?
Steady-burning. Flashing yellow is the signature of a different fixture, the Runway Guard Light.
Q5How bright is a yellow clearance bar?
Main beam intensity of at least 100 cd (yellow) per ICAO Annex 14 and FAA AC 150/5345-46, in steps of 100%, 30%, 10%, 3% and 1% of rated peak. Chromaticity must fall within the ICAO Annex 14 yellow boundary.
Q6How is a clearance bar controlled?
It is powered from the constant-current series circuit (5-step, 2.8 A to 6.6 A) through a 45 W or 65 W isolating transformer, and switched together with the rest of the low-visibility taxiway lighting circuit via the ALCMS rather than individually.
Q7How is a yellow clearance bar installed?
Fixtures are set in a row across the taxiway using the standard load-rated inset base can, spaced closely enough to read as a continuous yellow line. Units are typically inset (100,000 kg wheel load, IP67/IP68) or elevated, and can be uni-directional toward approaching traffic or omnidirectional.
Q8Which standards apply to yellow clearance bars?
The main references are ICAO Annex 14 (taxiway holding position light requirements) and FAA AC 150/5345-46 (which includes clearance bar fixtures). Always verify the current edition and your approved requirements.
Q9How often should a yellow clearance bar be maintained?
Typical intervals are quarterly for lens cleaning, semi-annual for communication link tests, and annual for LED driver inspection, chromaticity and photometric tests and housing and seal inspection. Your approved maintenance programme takes precedence.
Q10Why is one segment of the bar dark?
Failure of a single LED driver channel or a loose connection within the fixture. Check segments during an ALCMS-commanded activation, then test driver output and connector integrity and replace the failed channel or LED module.
Q11Why doesn't the bar switch with its lighting circuit?
A communication interruption between the ALCMS and the bar's control interface. Compare the ALCMS command log with the observed state, test the network or hardwired link and restore the communication path.
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