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Safety-critical technical reference — read before use. Independent educational guidance for qualified AGL and aerodrome personnel. It is not an approved procedure, design, specification or regulatory instruction and does not replace your aerodrome’s permit-to-work, isolation/LOTO procedures, the approved design or the manufacturer’s instructions. Series-circuit AGL equipment can carry lethal voltages. Read the full disclaimer ↓
THE AGL ENGINEERING CENTRE by Precision AGL Academy · How It Works

How Airfield Ground Lighting Works AGL, Runway, Taxiway & CCR Explained

Follow the chain from airport power through the CCR, series circuit and isolating transformers to the light fixture and its optical output, and see how control, monitoring, secondary power and testing fit around it.

Start Learning CCR Working Principle
Overview

What Is Airfield Ground Lighting and How Does It Work?

Airfield Ground Lighting (AGL) is an interconnected system of electrical, optical, mechanical and control components that gives aircraft visual guidance on and around an airport. Airport power feeds a Constant Current Regulator (CCR), which controls the current in a primary series circuit. Isolating transformers then pass power from that circuit to individual light fixtures, and control equipment such as an ALCMS operates and monitors the whole thing.

Airport power
CCR
Series circuit
Primary cable
Isolating transformer
Secondary connector
AGL light
Optical output

The key idea: the light fixture is only the final part of the chain. The control system decides which circuits operate and at what intensity.

What AGL includes

Runway lightsTaxiway lightsApproach lightingPAPI and APAPIREILRunway guard lightsStop barsAirport signsCCRsIsolation transformersCables and connectorsJunction boxes and light basesControl and monitoring
The exact system depends on airport layout, runway category, instrument approach requirements, visibility conditions, aircraft operations, national authority requirements and airport-specific design.

Related Guides

All about AGL Physics of AGL AGL components

Complete AGL Airport Layout

TERMINALTOWERAPPRWYTWYSTDCCR HOUSELV DISTRIBUTIONAPRON FLOODLIGHTSWINDSOCK (LIT)LOCALISERLANDING DIRECTIONRUNWAYPARALLEL TAXIWAYAPPROACHLIGHTSAPRON123456789SERIES CIRCUITS FROM THE CCR HOUSE (APPROACH, RUNWAY, TAXIWAY, STAND GUIDANCE)LOW-VOLTAGE SUPPLY TO APRON FLOODLIGHTS (NOT A SERIES CIRCUIT)TYPICAL LAYOUT · CONCEPTUAL PLAN VIEW · NOT TO SCALE
Figure 1. Conceptual plan view of a typical airport lighting layout.
AGL Visual Encyclopedia: Complete AGL Airport Layout Airfield Lighting Zoning AGL Circuit Zones
Interactive

Follow the System Through a Scenario

Pick a scenario to walk through what happens. These are simplified sequences; the approved design governs the real behaviour.

Sequence

    Worth knowing

      Tower Control

      TOWER CONTROL · ATC DECIDES, ALCMS EXECUTESCONTROL TOWER · ATC HMIUSER: ATCRWY EDGE / CLSTEP 5APPROACHSTEP 5TAXIWAY CLONSTOP BARSONPAPISTEP 3SELECT · SET · CONFIRMALCMSChecks accessand interlocksSends to CCRsCOMMANDSTATUSRUNWAY LIGHTINGEdge, centreline, TDZCCRAPPROACH LIGHTINGOwn CCR circuitsCCRTAXIWAY LIGHTINGEdge, centrelineCCRSTOP BARSSwitched with clearanceCCRTO CCRsCIRCUIT GROUPS · EACH WITH ITS OWN CCRWHAT THE TOWER DECIDES• Which lighting groups are on• Brightness for visibility and time of day• Taxiway routes and stop bars to match clearances• When to report a fault to maintenanceWHEN THE TOWER CANNOT COMMAND• CCR selector in Local: tower commands ignored• Network link lost: screen may be out of date• ALCMS down: failure procedure applies• Tell ATC; check status in the field
      Figure 2. Diagram of tower control: air traffic control at the tower HMI selects runway, approach, taxiway and stop bar lighting circuits and brightness steps.
      AGL Visual Encyclopedia: Tower Control ALCMS Communication Architecture Stop Bar Control
      Power

      The Series Circuit and the CCR

      Why a series circuit

      A runway has many lights spread over long distances. A series circuit lets them share one controlled current source: CCR, series cable, transformer 1, transformer 2, transformer 3 and so on, back to the return. Each transformer secondary feeds its own fixture. This differs from building lighting, where loads usually sit in parallel across a fixed voltage.

      What a CCR does

      It maintains the selected current (for example 6.6 A) through the series circuit despite changes in load. The CCR controls current; the series-circuit load determines the voltage needed to maintain it.

      If circuit impedance…CCR output voltage may…Series current…
      IncreasesIncreaseStays controlled
      DecreasesDecreaseStays controlled

      Brightness control

      Generally done by changing the CCR's operating current: lower current gives lower fixture output, higher current gives higher output. Systems can have different numbers of intensity steps, and FAA examples include three-step and five-step CCRs.

      Monitoring

      A monitored system can watch selected parameters and raise alarms: CCR status, series-circuit status, selected current, regulator fault, circuit load change, lighting failure and communication status. Actual functionality depends on the CCR, monitoring system and airport architecture.

      Related Guides

      CCR working principle CCR deep dive Physics of AGL

      AGL Series Circuit Principle

      SERIES LOOP · THE PRINCIPLE ONE LOOP · ONE CURRENT · EQUAL LIGHT CCR I FIXED OUT RTN I I I T1 SAME LIGHT T2 SAME LIGHT T3 SAME LIGHT T4 SAME LIGHT 1 THE CCR SENDS ONE CURRENT AROUND THE LOOP 2 THE SAME CURRENT PASSES EVERY LIGHT IN TURN 3 SAME CURRENT = SAME BRIGHTNESS THE PRINCIPLE ONE LOOP, ONE CURRENT LIGHTS CONNECTED END TO END THE RESULT EQUAL BRIGHTNESS WHATEVER THE POSITION OR CABLE LENGTH THE TRADE-OFF ONE BREAK STOPS THE LOOP SO THE LOOP MUST STAY COMPLETE SCHEMATIC · LOOP SHORTENED · NOT TO SCALE
      Figure 3. Diagram of the series circuit principle in airfield ground lighting: the CCR sends one current around a single loop, the same current passes every light in turn, so every light gives the same brightness, but a break in the loop stops the current for the whole circuit.
      AGL Visual Encyclopedia: AGL Series Circuit Principle CCR-to-Series-Circuit Interface CCR System Overview
      Transformer and Cables

      Isolation Transformer, Primary Cable and Secondary Connector

      Isolation transformer

      Primary current produces magnetic flux, which produces secondary voltage for the fixture. Primary and secondary windings are electrically separate, so it isolates the fixture from the high-voltage series circuit.

      Primary cable

      Part of the series circuit: CCR to a transformer, on to the next transformer. Must suit operating voltage, series current, insulation, environment, circuit length and mechanical conditions.

      Secondary connector

      Connects the transformer secondary to the fixture. Must give reliable contact and suit the installation environment.

      Why the transformer is needed

      PurposeWhat it gives
      Electrical isolationSeparates the individual fixture from the series primary circuit
      Power transferMoves the required energy to the fixture
      Series-circuit continuityIn applicable AGL designs, the primary arrangement keeps the series circuit continuous even if an individual lamp circuit opens
      Environmental installationPurpose-designed for airport lighting, including underground or exposed installation depending on product and specification

      Related Guides

      Airport isolating transformer Cables and connectors Junction boxes and housings

      Isolation Transformer Principle

      SERIES LOOP SIDE FITTING SIDE CONSTANT-CURRENT LOOP 6.6 A (EXAMPLE) HIGH-VOLTAGE LOOP MAGNETIC CORE MAGNETIC FLUX CARRIES THE ENERGY PRIMARY SECONDARY LIGHT FITTING EARTH FAULT AT FITTING NO ELECTRICAL CONNECTION BETWEEN THE WINDINGS ENERGY CROSSES THROUGH THE CORE ONLY ILLUSTRATIVE · CORE AND WINDING LAYOUT VARIES BY DESIGN · NOT TO SCALE
      Figure 4. Schematic of the isolation transformer principle.
      AGL Visual Encyclopedia: Isolation Transformer Principle Primary Cable Secondary Connector Transformer Circuit Position
      Visual Aids

      How Runway, Taxiway, Approach and Other Lights Work

      Runway lighting is a coordinated visual pattern, not a collection of independent lamps. Runway fixtures all follow the same electrical chain; what differs is their optical job.

      LightWhat it doesPractical notes
      Runway edgeDefines lateral limitsOutput depends on CCR current, transformer, connections, light source, optics, alignment and contamination
      Threshold and runway endIdentify the threshold and runway end
      Runway centrelineLongitudinal guidance where installedOften in pavement: rubber, water, lens condition, pavement interface, damage, alignment
      Touchdown zoneExtra guidance in the touchdown areaNeeds correct operation, intensity, optical direction, spacing and availability
      Approach lightingA visual path toward the thresholdCentreline, crossbars, elevated and inset lights, sequencing, PAPI, power and control; coordinated with runway geometry and approach procedure
      Taxiway edgeMarks taxiway boundariesElevated or in-pavement; intensity, colour, alignment, coverage, condition
      Taxiway centrelineA continuous visual path while taxiingCommonly embedded: lens, contamination, water ingress, elevation, alignment, electrical connection, photometry
      Stop barsHolding-position indicationDedicated circuits and control per airport procedures: ATC or operations, control system, stop-bar circuit, lights
      Runway guard lightsEnhanced indication at holding positionsMay flash, be elevated or in-pavement, use dedicated circuits and control, and integrate with runway-entry control
      REILIdentifies the runway end areaTwo units, possibly synchronized flashing, with dedicated control and power
      Airport signsRunway designation, taxiway identification, directions, mandatory instructions, locationPower, control, sign circuit, illuminated legend, status feedback
      Taxiway lighting may be controlled independently of runway lighting because different functions can need different operating states and intensity settings.

      Related Guides

      Runway lighting systems Taxiway lighting systems Approach lighting systems Runway guard lights and stop bars Taxiway guidance signs

      Runway Lighting Layout

      LANDING DIRECTIONTHRESHOLD LIGHTS+ WING BARSRUNWAY END LIGHTSEDGE LIGHTS · 60 m MAX SPACINGRUNWAY CENTRELINE LIGHTSTOUCHDOWN ZONE LIGHTSTOUCHDOWN ZONE · FIRST 900 mCENTRELINE LIGHT COLOUR · RED/WHITE 900–300 m, RED LAST 300 mWHITERED / WHITEREDEDGE LIGHT COLOUR (SEEN FROM THE APPROACH)WHITEYELLOW · LAST 600 mPLAN VIEW · ILLUSTRATIVE, NOT TO SCALE · 3000 m RUNWAY SHOWN
      Figure 5. Plan view of a runway lit for landing from the left.
      AGL Visual Encyclopedia: Runway Lighting Layout Wing Bars Elevated Runway Edge Light

      Taxiway Lighting Layout

      RUNWAY · WHITE EDGE LIGHTSSTOP BARHOLDING POSITION MARKINGRUNWAY GUARD LIGHTSGREEN CENTRELINE CURVES ONTO THE RUNWAYLINK TAXIWAYPARALLEL TAXIWAYTO APRON ►◄ FROM APRONBLUE EDGE LIGHTS · GREEN CENTRELINE LIGHTSPLAN VIEW · ILLUSTRATIVE, NOT TO SCALE
      Figure 6. Plan view of a link taxiway joining a parallel taxiway to a runway.
      AGL Visual Encyclopedia: Taxiway Lighting Layout Complete Taxiway Lighting System Taxiway Junction

      AGL Colour Code Summary

      WHAT EACH COLOUR MEANS, AND WHERE YOU SEE IT PLAN VIEW · LANDING LEFT TO RIGHT · ILLUSTRATIVE, NOT TO SCALE PAPI: WHITE ABOVE · RED BELOW OBSTACLE LIGHT: RED BEACON: WHITE / GREEN RUNWAY EDGE: WHITE CAUTION ZONE: YELLOW RUNWAY END: RED APPROACH LIGHTS: WHITE SIDE ROWS: RED (CAT II/III) CENTRELINE: WHITE → RED/WHITE → RED TOUCHDOWN ZONE: WHITE THRESHOLD + WING BARS: GREEN GUARD LIGHTS: YELLOW TAXIWAY EDGE: BLUE RUNWAY-SIDE CENTRELINE: GREEN / YELLOW STOP BAR: RED TAXIWAY CENTRELINE: GREEN WHITE THE RUNWAY ITSELF RUNWAY EDGE CENTRELINE TOUCHDOWN ZONE APPROACH · PAPI YELLOW CAUTION EDGE, FINAL ZONE GUARD LIGHTS INTERMEDIATE HOLDING ILS-AREA CENTRELINE GREEN GO · SAFE PATH RUNWAY THRESHOLD THRESHOLD WING BARS TAXIWAY CENTRELINE LEAD-ON LIGHTS RED STOP · DANGER RUNWAY END STOP BARS CENTRELINE LAST ZONE OBSTACLES · PAPI LOW BLUE TAXIWAY EDGE TAXIWAY EDGE LIGHTS NEVER ON A RUNWAY OMNIDIRECTIONAL WHITE = RUNWAY · YELLOW = CAUTION · GREEN = GO · RED = STOP · BLUE = TAXIWAY EDGE
      Figure 7. AGL colour code summary.
      AGL Visual Encyclopedia: AGL Colour Code Summary Approach Lighting System Overview Different Signs
      PAPI

      How PAPI Works

      A four-unit PAPI uses individually aimed light units, so the pilot sees different white and red combinations. It converts aircraft position into a visual indication through the geometry of its optical beams. Each unit is designed with separate red and white sectors, and the colour transition changes with the aircraft's elevation relative to the selected approach path.

      What the pilot seesAircraft position
      4 whiteHigh
      3 white + 1 redSlightly high
      2 white + 2 redOn the approach path
      1 white + 3 redSlightly low
      4 redToo low
      Important optical characteristics: intensity, colour, beam coverage, red/white transition, aiming angle and horizontal coverage. That is why PAPI testing needs both alignment and photometry.

      Related Guides

      PAPI engineering guide PAPI installation and aiming PAPI alignment and maintenance

      PAPI Glide Path

      THRESHOLD PAPI WING BAR AIMING POINT ≈ TOUCHDOWN ZONE ≈ 300 m CROSSING HEIGHT ABOVE PATHHIGH · 3–4 WHITE ON GLIDE PATH2 RED · 2 WHITE BELOW PATHLOW · 3–4 RED 3° GLIDE PATH NOT TO SCALE · SLOPE EXAGGERATED APPROACH · SIDE VIEW
      Figure 8. Side view of an approach to the runway: the 3 degree glide path passes over the threshold and meets the PAPI aiming point; an aircraft above the path sees mostly white lights, one on the path sees two red and two white, and one below sees mostly red.
      AGL Visual Encyclopedia: PAPI Glide Path PAPI Optical Principle PAPI Beam PAPI System Overview

      PAPI Optical Principle

      OPTICAL AXIS · TRANSITION ANGLE LIGHT SOURCE LAMP OR LED FILTER AT FOCAL PLANE RED HALF ABOVE AXIS · EDGE ON AXIS PROJECTION LENS INVERTS THE IMAGE FOCAL LENGTH f WHITE · ABOVE TRANSITION RED · BELOW TRANSITION SCHEMATIC · NOT TO SCALE PAPI UNIT · SIDE CROSS-SECTION
      Figure 9. Cross-section of a PAPI unit: a light source at the focal plane of a lens, a filter covering half the source image, and the output beam that is white above the optical axis and red below it.
      AGL Visual Encyclopedia: PAPI Optical Principle PAPI Glide Path PAPI Unit Arrangement
      Control

      How ALCMS and Remote Control Work

      ALCMS

      The Airfield Lighting Control and Monitoring System gives centralized control and monitoring of lighting circuits. The operator selects a circuit or intensity state, and the field system returns status or alarm information.

      Feedback matters

      Status travels back so staff can see whether the commanded lighting state was actually achieved: field equipment, monitoring, ALCMS, operator.

      Operator
      ALCMS or control panel
      CCR control interface
      CCR
      AGL circuit
      Light fixtures

      The control system sets the operating command. The CCR provides the electrical regulation that carries out the selected series-circuit condition.

      Related Guides

      ALCMS CCR working principle

      ALCMS Communication Architecture

      ALCMS COMMUNICATION · COMMANDS OUT, STATUS BACKCONTROLPOSITIONSALCMSSERVERSCOMMUNICATIONNETWORKFIELDCONTROLLERSCCRs &CIRCUITSTOWER CONTROLATC OPERATORMAINTENANCEENGINEERINGREPORTS & LOGSSTATUS HISTORYSERVER ALIVESERVER BSTANDBY / DUPLICATEABPLC / FIELD I/OSUBSTATION 1CCRRUNWAY CIRCUITSPLC / FIELD I/OSUBSTATION 2CCRTAXIWAY CIRCUITSPLC / FIELD I/OSUBSTATION 3CCRPAPI & SIGN CIRCUITSTO SERIES CIRCUITSTO SERIES CIRCUITSTO SERIES CIRCUITSKEY POINTIF A SERVER OR NETWORK PATH FAILS, THE CCRS CAN STILL BE SWITCHED TO LOCAL CONTROLSCHEMATIC · LAYOUT, PROTOCOLS AND REDUNDANCY VARY BY SYSTEM · NOT TO SCALE
      Figure 10. Diagram of ALCMS communication architecture: control positions connect to duplicated ALCMS servers, which connect over two redundant network paths to substation field controllers, which connect to the CCRs feeding the series circuits.
      AGL Visual Encyclopedia: ALCMS Communication Architecture ALCMS Failure Flow Tower Control
      Resilience

      Low Visibility and Secondary Power

      Low-visibility operations

      Place extra weight on the reliability and availability of visual aids. Depending on runway and operational category, systems may include approach, edge, centreline and touchdown zone lighting, taxiway centreline lights, stop bars, PAPI, enhanced monitoring, redundant power and rapid or controlled power transfer. This is why AGL commissioning and maintenance are critical at major airports.

      Secondary power

      Standby arrangements keep critical visual aids available after loss of the normal supply, and can involve generators, UPS, automatic transfer equipment, independent feeders and dedicated AGL distribution. Transfer characteristics depend on operational and regulatory requirements.

      Normal or standby power
      AGL power system
      CCR and lighting equipment
      Critical visual aids

      During a power failure

      Normal power lost
      Detection
      Automatic transfer or standby source
      AGL power restored
      CCR re-energised
      Lighting circuit restored
      The sequence depends on the airport power architecture. Verify actual transfer time and behaviour against the approved design and applicable operational requirements.

      Related Guides

      Emergency backup power ICAO aerodrome lighting requirements

      Mode Changeover Sequence (Normal to LVP and Back)

      NORMAL TO LVP AND BACKNORMALStep follows the step tableStop bars as ATC needs themCAT II/III-only lights maybe off or at a low stepPREPARATIONATC declares preparationLamps, circuits, interlockschecked and read backStandby power made readyLVP IN FORCEFull CAT II/III pattern litStop bars on where in useCentreline off beyond a lit barStep follows the RVR tableSTAND-DOWNVisibility better: margin + delayATC cancels LVPStop bars back to normal useSteps return; state confirmedTRIGGERCHECKEDCANCELBACK TO NORMALDOWNGRADE, WHILE IN FORCEIf serviceability falls below the minimum, the category or minima are reduced and ATC is told.
      Figure 11. Sequence of lighting modes.
      AGL Visual Encyclopedia: Mode Changeover Sequence (Normal to LVP and Back) Low-visibility procedures (LVP) lighting configuration CAT II and III approach lighting

      Airfield substation single-line diagram

      AIRFIELD SUBSTATION · SINGLE-LINE DIAGRAM one line stands for the whole three-phase circuit — follow it from the source to the load UTILITY SUPPLY MV incoming INCOMING BREAKER MV / LV TRANSFORMER STANDBY GENERATOR engine-driven GENERATOR BREAKER CHANGEOVER interlocked switch LV MAIN BUS RUNWAY CCRs edge, threshold TAXIWAY CCRs edge, centreline APPROACH CCRs approach lights UPS ALCMS, controls AUX SERVICES building loads PROTECTED FEEDERS TO THE AIRFIELD LOADS HOW TO READ IT One line stands for the whole three-phase circuit. Each symbol is a device: source, breaker, transformer, switch or bus. Follow the line from the source to the load to see what feeds what. Which loads sit on the standby side is set by the design. 1 2 3 4 5 1 INCOMING SUPPLY utility feed, incoming breaker and the transformer that steps it down 2 STANDBY GENERATOR second source with its own breaker, started on loss of the utility 3 CHANGEOVER selects utility or generator; interlocked so both are never joined 4 LV MAIN BUS the common bus every airfield load is fed from 5 FEEDERS protected outgoing circuits to the regulators, UPS and services Voltages and arrangement depend on the utility and the project. SCHEMATIC · NOT TO SCALE · FOLLOW THE SITE ELECTRICAL DESIGN, THE AUTHORITY AND THE EQUIPMENT MANUALS
      Figure 12. Simplified single-line diagram of an airfield substation.
      AGL Visual Encyclopedia: Airfield substation single-line diagram Standby CCR and Changeover Arrangement CCR Power Input
      Electrical and Optical

      Electrical and Optical Systems Together, and Why ON/OFF Is Not Enough

      Electrical power
      CCR
      Series current
      Transformer
      Fixture electrical input
      Light source
      Optics
      Beam
      Pilot's visual information

      A problem anywhere shows up in the output

      Low CCR current can give low fixture output. Correct CCR current with dirty optics can also give low output. So test electrical and photometric performance together.

      How photometry works

      A calibrated photometer in controlled geometry measures the fixture, giving intensity, colour and distribution data. Compare with the applicable requirement, correct, and retest. It can reveal low intensity, poor beam distribution, incorrect alignment, colour issues and degradation.

      TestQuestion it answers
      Functional testDoes the light operate?
      Photometric testDoes the light provide the required optical performance?
      A light can be on, electrically healthy and photometrically deficient: dirty optics, damaged lens, degraded LED or lamp, wrong alignment or wrong colour. Commissioning and maintenance often need both.

      Related Guides

      AGL photometry Runway lighting photometry Taxiway lighting photometry

      Selective Switching of Taxiway Lights

      PLAN VIEW · ONE AIRCRAFT, ONE CLEARED ROUTE RUNWAY APRON S1 · PASSED · OFF S2 · ROUTE · LIT E1 · NOT USED · OFF S3 · ROUTE · LIT STOP BAR · ON S4 · BEYOND BAR · OFF B1 · NOT USED OFF AIRFIELD LIGHTING CONTROL SYSTEM · ROUTE LOGIC · EACH SECTION SWITCHED ON ITS OWN S1 OFF · PASSED S2 ON · ROUTE S3 ON · ROUTE STOP BAR ON · HOLD S4 OFF · BEYOND BAR B1 OFF · NOT USED E1 OFF · NOT USED ROUTE LIT UP TO THE STOP BAR · OTHER SECTIONS OFF · NEXT SECTION LIGHTS WHEN THE BAR IS CLEARED
      Figure 13. Plan view of an apron, a taxiway running to a link taxiway and the runway, with an aircraft heading right.
      AGL Visual Encyclopedia: Selective Switching of Taxiway Lights Follow-the-greens operation Stop Bar Control Runway Guard Lights
      Commissioning and Maintenance

      Commissioning, Maintenance and Troubleshooting

      A typical commissioning sequence

      Pre-commissioning inspection
      Cable and insulation testing
      Continuity
      Transformer and connector checks
      CCR testing
      Circuit energization
      Functional and brightness-step testing
      Photometric, PAPI and sign testing
      Control-system testing
      Final inspection
      Documentation
      The exact test scope depends on the project and applicable requirements.

      Preventive

      Find and correct degradation before failure.

      Corrective

      Repair failed or degraded equipment.

      Condition monitoring

      Use electrical and photometric measurements to track performance.

      Inspect
      Clean
      Measure
      Diagnose
      Repair
      Test
      Retest
      Record

      Runway Lighting Work Permit

      Work on runway lighting is normally carried out under a work permit issued by the airport. The permit sets out where the work is, what it covers and how long it will take, and the work is coordinated with airport operations and air traffic control before anyone goes airside.

      Because a series circuit can carry current even when the lights look off, the circuit should be made safe under the airport's own isolation procedure before work starts. After the work, the lighting is tested, retested and recorded before it is handed back for use. The exact permit and isolation requirements are set by the airport, so always follow its approved procedure.

      Troubleshoot by following the whole system. Click a symptom to see where to look.

      One light fails

      Possible Cause

      • Light fixture
      • Secondary connector
      • Transformer
      • Primary connector
      • Primary cable
      • CCR
      • Control system

      Several lights fail

      Possible Cause

      • CCR fault
      • Circuit open
      • Cable damage
      • Common connector problem
      • Control-system failure
      • Loss of supply

      A light is dim

      Possible Cause

      • Optical contamination
      • Degraded light source
      • Incorrect current
      • Electrical problem
      • Optical misalignment

      Control or monitoring problem

      Possible Cause

      • Operator command
      • ALCMS and communication
      • CCR interface
      • Field equipment status

      By problem type

      • Electrical: CCR, cable, transformer, connector, fixture
      • Optical: fixture, optics, alignment, photometry
      • Mechanical: foundation, base, mounting, fixture, alignment
      • Control: operator, ALCMS, communication, CCR, circuit
      • Monitoring: field equipment, monitoring, communication, ALCMS

      Why this order

      This system-based approach beats troubleshooting each component independently, and stops a lamp being replaced when the real fault is elsewhere.

      Related Guides

      Installation and commissioning AGL testing and inspection Maintenance handbook Troubleshooting Engine

      Fault-Isolation Flowchart

      FAULT-ISOLATION FLOWCHART · SYMPTOM, TYPE, SECTIONFIRSTTELL ATC · SWITCH OFF AND ISOLATE THE CCR · LOCK OFF · AUTHORISED PERSONNEL ONLYALL LIGHTS OUTCCR open-circuit alarm, no currentOPEN CIRCUITa break in the loopTEST CONTINUITY BY SECTIONFault is where it failsCCR TRIPSEarth-fault or overcurrent alarmEARTH FAULTinsulation to earth is lowTEST INSULATION RESISTANCE BY SECTIONLow reading marks the sectionSOME LIGHTS OUTCCR normal, circuit runningLOCAL FAULTfixture, connector or transformerCHECK THE DEVICE ITSELFLight, secondary connector, transformerNARROW DOWN BY HALF-SPLITTEST: GOOD · SET ASIDEBAD · SPLIT AGAINfault sectionTHEN1 Repair the faulty part2 Retest section and circuit3 Restore, run, confirm status4 Record results, tell ATC
      Figure 14. Fault-isolation flowchart for a series circuit.
      AGL Visual Encyclopedia: Fault-Isolation Flowchart CCR Troubleshooting Earth-Fault Troubleshooting Transformer Troubleshooting
      At a Glance

      AGL Components at a Glance, and How AGL Differs from Building Lighting

      ComponentMain function
      CCRControls series-circuit current
      Primary cableCarries series-circuit power
      Isolation transformerIsolates and feeds the individual fixture
      ConnectorProvides the electrical connection
      Light fixtureProduces visual guidance
      Light baseMounts and protects the fixture
      PAPIVisual approach-slope guidance
      REILIdentifies the runway end or threshold
      Stop barHolding-position indication
      Runway guard lightRunway-holding-position awareness
      Airport signMovement-area information
      ALCMSControls and monitors lighting
      MonitoringReports status and faults

      Why AGL is not normal building lighting

      AGL may involve long series circuits, controlled current, isolation transformers, specialized optical fixtures, airside installation, aircraft loading, frangibility, strict visual guidance requirements, multiple intensity steps, centralized control, monitoring and photometric verification. A runway lighting system is an integrated aviation visual-aid system, not a large outdoor installation.

      Related Guides

      AGL components guide LED AGL
      Standards

      Standards and Technical References

      For FAA-based projects, the FAA lists these specifications. They address different parts of the system and are not interchangeable.

      FAA AC 150/5345-10H

      Constant Current Regulators and Regulator Monitors

      FAA AC 150/5345-47C

      Series-to-Series Isolation Transformers for Airport Lighting Systems

      FAA AC 150/5345-7F

      L-824 Underground Electrical Cable for Airport Lighting Circuits

      FAA AC 150/5345-26E

      L-823 Plug and Receptacle Cable Connectors

      FAA AC 150/5345-46F

      Runway, Taxiway, Heliport and Vertiport Light Fixtures

      FAA AC 150/5345-56B

      L-890 Airport Lighting Control and Monitoring System

      For ICAO-based projects, use ICAO Annex 14 Volume I and its guidance together with the responsible national authority's requirements. For an Indian airport also review applicable DGCA requirements, AAI or project specifications and controlled equipment documentation. Confirm the current edition of each reference before relying on it.

      Related Guides

      ICAO aerodrome lighting requirements FAA AC 150/5340 series explained Standards companion
      Final Engineering Perspective

      The aim is not to illuminate the airport. It is to give the pilot the correct visual information for approach, landing, taxiing and runway movement.

      Power controls the circuit. Optics provide the guidance. Control systems manage the operation. Testing proves the performance.

      Test Yourself

      Quick Knowledge Check

      Q1. What does a CCR control in a series circuit?

      A. Voltage only
      B. Current, while the load sets the voltage needed
      C. Light colour
      D. Sign legend

      Q2. How is brightness generally changed in a series-circuit AGL system?

      A. By changing the lamp colour
      B. By changing the CCR output-current level
      C. By moving the fixture
      D. By switching the transformer

      Q3. Several taxiway lights fail at once. Where do you start?

      A. Every fixture
      B. Shared upstream parts: ALCMS, CCR, primary circuit, common connections
      C. Only the lamp
      D. Nowhere

      Q4. A light is on but dim. What is a sensible first approach?

      A. Replace the fixture immediately
      B. Check CCR current, circuit, transformer and connector, then inspect, clean, check alignment and measure
      C. Ignore it
      D. Raise the step permanently

      Q5. What does photometry answer that a functional test does not?

      A. Does the light operate?
      B. Does it provide the required optical performance?
      C. Is the cable buried?
      D. Is the CCR cold?
      Explore More

      Explore More AGL Engineering

      HUB

      AGL Components

      Component-by-component guide.

      Open AGL Components →
      ELECTRICAL

      CCR Working Principle

      Regulation, current steps and faults.

      Open CCR Working Principle →
      ELECTRICAL

      Airport Isolating Transformer

      Ratings, types and testing.

      Open Airport Isolating Transformer →
      HARDWARE

      Junction Boxes & Transformer Housings

      Light bases and housings.

      Open Junction Boxes & Transformer Housings →
      PERFORMANCE

      AGL Photometry

      Photometric testing and reporting.

      Open AGL Photometry →
      FIELD GUIDE

      Runway Lighting Photometry

      Runway fixture testing.

      Open Runway Lighting Photometry →
      FIELD GUIDE

      Taxiway Lighting Photometry

      Taxiway fixture testing.

      Open Taxiway Lighting Photometry →
      STANDARDS

      ICAO Aerodrome Lighting Requirements

      The ICAO AGL framework.

      Open ICAO Aerodrome Lighting Requirements →
      VISUAL AID

      PAPI Engineering Guide

      Working principle and geometry.

      Open PAPI Engineering Guide →
      PROCESS

      Testing & Commissioning

      Electrical, optical and handover testing.

      Open Testing & Commissioning →
      MAINTENANCE

      AGL Maintenance

      Preventive and corrective maintenance.

      Open AGL Maintenance →
      VISUAL AID

      PAPI Alignment, Testing & Maintenance

      Installation, aiming and maintenance.

      Open PAPI Alignment, Testing & Maintenance →
      STANDARDS

      ICAO PAPI Requirements

      Standards-focused PAPI requirements.

      Open ICAO PAPI Requirements →
      INDIA

      Airfield Ground Lighting in India

      Systems, products, testing and services in India.

      Open Airfield Ground Lighting in India →
      CONTROL

      ALCMS

      Lighting control and monitoring.

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      Fundamentals

      All About AGL How Airfield Lighting Works Physics of AGL AGL Components Airfield Lighting Colour Codes LED Airfield Ground Lighting LED Benefits, Lifespan & Energy Savings Solar AGL (SAGL) Portable AGL (PAGL) Advantages & Disadvantages by System Type Sustainability in AGL

      Standards & Compliance

      ICAO Annex 14 AGL Requirements ICAO Aerodrome Lighting Requirements FAA AC 150/5340 Series EASA CS-ADR-DSN Compliance NOTAMs

      Runway, Taxiway & Approach Lighting

      Runway Lighting Systems Taxiway Lighting Systems Airport Approach Lighting Systems Approach Lighting Systems Apron Lighting & Stand Guidance Apron Floodlighting Design & Lux Levels VDGS: Visual Docking Guidance Obstruction Lighting & Marking Airfield Signage & Guidance Signs Markings & Signage Integration

      Surface Movement & Safety

      Follow the Greens Runway Guard Lights & Stop Bars Runway Incursion Prevention & SMGCS A-SMGCS ALCMS

      PAPI

      How Does PAPI Work? PAPI Engineering Guide ICAO PAPI Requirements PAPI Installation & Aiming PAPI Calibration & Alignment PAPI Alignment & Maintenance PAPI Maintenance Checklist

      Electrical Engineering & Power

      AGL Design & Electrical Engineering Diagrams, Schematics & Wiring CCR Working Principle CCR Deep Dive AGL Isolating Transformers Airport Isolating Transformer Junction Box & Transformer Housing Cables & Connector Engineering Electrical Architecture & Redundancy Emergency Backup Power Emergency Power & Lighting Systems Surge Protection & Lightning Mitigation Lightning & Surge Protection for Circuits

      Photometry & Testing

      AGL Photometry Photometric Testing Guide Photometric Testing Procedure Runway Lighting Photometry Taxiway Lighting Photometry AGL Testing & Inspection Installation & Commissioning Testing & Commissioning Guide

      Maintenance, Inspection & Safety

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      Datasheets, Catalogues & PDF Guides Project Documentation: ITP, As-Built, O&M Engineer & Technician Job Description / Salary
      FAQ

      How Airfield Lighting Works FAQ

      Q1How does airfield lighting work?

      Through electrical power, controlled series circuits, isolation transformers, light fixtures, optical systems and centralized control. In a typical series-circuit installation the CCR controls current, transformers feed individual fixtures and the fixtures produce the required visual guidance.

      Q2How does runway lighting work?

      Different visual-aid fixtures (edge, threshold, end, centreline, touchdown-zone and approach lights) give pilots visual guidance during approach and runway operations.

      Q3How does taxiway lighting work?

      Edge lights, centreline lights, stop bars, runway guard lights and associated signs guide aircraft during ground movement.

      Q4How does a CCR work?

      A CCR regulates the current in the airport series circuit. Its output voltage changes as needed according to the circuit's electrical load.

      Q5Why does airport lighting use a CCR?

      It lets many fixtures spread over a long series circuit operate at a controlled current and selectable intensity levels.

      Q6What does an isolation transformer do in airport lighting?

      It electrically isolates the individual fixture from the primary series circuit while transferring power to the fixture.

      Q7Why are airport lights connected through isolation transformers?

      They separate the fixture-side circuit from the airport's high-voltage series circuit and let each light receive the required secondary supply.

      Q8How does airport lighting brightness change?

      In a series-circuit AGL system, brightness is generally controlled by changing the CCR's output-current level.

      Q9How does ALCMS control airport lights?

      The ALCMS sends commands to lighting-control equipment such as CCR interfaces, which operate the appropriate circuit. It can also receive status and fault information from field equipment.

      Q10How does PAPI work?

      Individually aimed optical units give different red/white indications depending on the aircraft's position relative to the desired approach path.

      Q11How does airport lighting work during low visibility?

      Critical visual aids can be supported by dedicated circuits, intensity control, monitoring and secondary power, according to the airport's operational requirements.

      Q12What happens if an airport lighting circuit fails?

      It depends on the fault. One failed light may need fixture, transformer or connector troubleshooting, while multiple failures may need investigation of the CCR or primary series circuit.

      Q13Why does an airport light stay on but look dim?

      Possible causes are optical contamination, light-source degradation, incorrect current, electrical problems or optical misalignment.

      Q14What is the difference between functional and photometric testing?

      Functional testing confirms that equipment operates. Photometric testing quantitatively evaluates optical performance such as intensity, distribution and colour where required.

      Q15Why is AGL photometry important?

      It gives measured evidence of light performance and can find degradation or alignment problems that visual inspection alone may miss.

      Q16How is airfield lighting maintained?

      Visual inspection, cleaning, electrical testing, component inspection, photometry where required, troubleshooting, corrective action and retesting.

      Q17What are the main AGL components?

      CCRs, primary cables, isolation transformers, connectors, light fixtures, light bases, PAPI, signs, control systems and monitoring systems.

      Q18Is airport lighting connected directly to the airport power supply?

      Usually the airport supply feeds the AGL electrical system, often through a CCR for series-circuit lighting. The detailed architecture depends on the airport design.

      Q19Is airport lighting AC or DC?

      It depends on the equipment and system. Many conventional series-circuit systems use AC-powered CCRs and isolation transformers, while specialized systems can differ.

      Q20Can LED lights be used in an AGL series circuit?

      Yes, where the LED fixture is designed and approved for the specific AGL system. Verify compatibility with the CCR, transformer and circuit characteristics.

      Field Insight

      Engineering & Expert Notes

      RS
      By Rahul Sharma
      Airfield Ground Lighting Specialist
      Trace One Light Back to the Source
      Learning

      Following a single fixture through transformer, cable and CCR makes the system easy to understand.

      The Series Circuit Is the Key Idea
      Electrical

      Constant current explains uniform brightness and many fault behaviours.

      Optical Performance Is Separate From Circuit Health
      Optics

      Check light output and alignment as well as electrical operation.

      Control Layers Add Flexibility and Complexity
      Control

      Understand how remote and local control interact, and what happens if one fails.

      Backup Power Maintains Safety
      Power

      Know how and when secondary power takes over and how that is tested.

      Learn the Terms
      Terminology

      Consistent use of terms avoids confusion among design, operations and maintenance.

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