AGL Fundamentals
From basic concepts to airport engineering — how Airfield Ground Lighting guides aircraft during approach, landing, taxiing and ground movement, and how the CCR, series circuit, isolating transformers, cables, connectors, signs, PAPI and monitoring systems all fit together.
What Is Airfield Ground Lighting?
Airfield Ground Lighting (AGL) is the system of visual aids and associated equipment used to guide aircraft during approach, landing, takeoff, and movement on the airport surface — especially at night or in reduced visibility.
A complete AGL system can include approach lighting, runway lighting, taxiway lighting, PAPI and other visual approach aids, illuminated airfield signs, stop bars and runway guard lights, Constant Current Regulators, series-circuit cabling, isolating transformers, connectors, and control, monitoring, testing and maintenance systems.
Guides Landing
Approach and runway lighting gives pilots continuous visual reference from several miles out through touchdown.
Guides Ground Movement
Taxiway lighting and signage prevent runway incursions and guide aircraft safely between runway, apron, and gate.
Supports Low-Visibility Operations
AGL provides visual guidance during night operations and, where applicable, supports reduced-visibility operations in conjunction with the airport's procedures, navigation systems and other required facilities.
Understanding the Airport Layout
Before AGL makes sense, the basic movement areas need to. Different areas require different visual guidance systems — that's the entire logic behind which lighting goes where.
Runway
Used primarily for aircraft takeoff and landing.
Taxiway
Provides aircraft movement between runways, aprons and other airport areas.
Apron
Where aircraft are parked, loaded, unloaded, refuelled and serviced.
Holding Position
A designated position where an aircraft may be required to hold before entering a runway or protected area.
Why Layout Matters to AGL
The Basic AGL Power & Lighting Chain
Every AGL fixture, no matter which category it belongs to, traces back through the same chain of equipment. This is the backbone of the whole system.
TRANSFORMER
CABLE
TRANSFORMER
CABLE
TRANSFORMER
CABLE
The CCR supplies controlled current to the airport lighting series circuit. That circuit connects individual isolating transformers, each feeding a light fixture over its own secondary cable. The isolating transformer provides electrical separation between the shared primary series circuit and its own fixture circuit; the secondary cable then carries that isolated, lower-current supply the short distance to the fixture itself.
Primary & Secondary Circuits
Primary Circuit
The high-voltage series circuit supplied by the CCR. Typical AGL systems use a constant-current series arrangement rather than a conventional constant-voltage distribution setup — that's the key difference from ordinary building wiring.
Secondary Circuit
The secondary side of an isolating transformer supplies the individual lighting fixture: CCR → primary AGL cable → isolating transformer → secondary cable → light fixture.
Why Use a Series Circuit?
The series arrangement lets multiple lighting loads operate as part of one controlled airport lighting circuit. Because the CCR controls circuit current directly, it also controls the lighting system's intensity steps for the entire circuit at once.
Constant Current Regulator (CCR)
A CCR is the major component of an AGL system that supplies controlled current to the series lighting circuit — think of it as the heart of the whole chain.
Main Functions
Intensity control: a typical AGL system provides multiple selectable brightness levels. The actual number of steps, and the current value at each step, depends on the applicable system design and specification.
The AGL Series Circuit
An AGL series circuit connects multiple lighting units through series-connected primary cable and isolating transformers — each fixture taps off the same loop of controlled current.
Key Engineering Concepts
Isolating Transformer
Sits between the primary series circuit and one individual airfield lighting fixture — the component that makes a series circuit of independent fixtures possible at all.
Electrical Isolation
Separates the fixture circuit from the primary series circuit.
Individual Fixture Connection
Allows one lighting unit to be connected onto the shared series circuit.
System Reliability
Supports continued operation of the remaining series circuit under appropriate fault conditions.
Typical Construction
Primary and secondary windings, magnetic core, insulation system, encapsulation/moulding, cable connections, and a protective housing.
Important Transformer Parameters
How to Select an AGL Isolation Transformer
Selection starts from the fixture's rated burden and the series circuit's rated current, then checks that against the transformer's rated power, turns ratio and insulation level for the applicable circuit voltage class. Environmental rating, mounting method (e.g. in a light base or pit) and compliance with the applicable ICAO/FAA/IS specification round out the decision.
Open the Isolation Transformer Selection CalculatorAGL Primary Cable
The primary cable forms the main electrical path between AGL series-circuit equipment.
Important Characteristics
CCR
Primary Cable
Isolating Transformer
Primary Cable (continues)
Next Isolating Transformer
AGL Connectors
Connectors provide reliable electrical connections between AGL cables and equipment — small parts with an outsized effect on circuit reliability.
A Poor Connection Can Cause
AGL Cable
Primary Connector
Isolating Transformer
Secondary Connection
Light Fixture
The Main Lighting Systems
Runway Lighting
Provides the visual references that define the runway during landing and takeoff. Colour, location, spacing, intensity and configuration each communicate something different to the pilot.
- Runway Edge Lights
- Runway Centreline Lights
- Threshold & Runway End Lights
- Touchdown Zone Lights
- Runway Guard Lights
Taxiway Lighting
Guides aircraft movement between runways, aprons and other airport areas, defining the intended route and cueing intersections and holding positions.
- Taxiway Edge & Centreline Lights
- Stop Bars
- Runway Guard Lights
- Clearance Bar Lights
- Associated Taxiway Signs
Approach Lighting
Extends outward from the runway threshold, establishing alignment with the runway and supporting the transition from instrument approach to visual reference.
- Approach Light Bars & Crossbars
- Centreline Lighting
- Sequenced Flashing Lights
- Threshold Lighting
Signage
Illuminated mandatory, location, direction and destination signs identifying taxiways, runways and holding points.
- Mandatory Signs
- Location Signs
- Direction Signs
- Destination Signs
Reading the Colours
| System | Typical Visual Characteristic |
|---|---|
| Runway edge | White, with specified variations depending on location/system |
| Runway centreline | White, with specified colour changes near the runway end |
| Runway end | Red |
| Threshold | Green |
| Taxiway edge | Blue |
| Taxiway centreline | Green |
| Stop bar | Red |
PAPI — Precision Approach Path Indicator
PAPI tells a pilot where the aircraft sits relative to the desired approach slope, using four light units in a row. The white-to-red balance across the bar shifts in single-lamp steps as the aircraft moves through the slope — click a position below to see it change.
AGL Control & Monitoring
Modern airport lighting systems require more than turning lights ON and OFF. A control and monitoring layer sits above the CCR and circuit.
Photometry & Colour
An AGL fixture isn't acceptable merely because it illuminates — its optical performance has to meet the applicable requirements.
Photometric Parameters
Intensity, distribution, beam characteristics, colour, horizontal and vertical coverage, alignment, and uniformity where applicable — tested in the lab for controlled evaluation and in the field to verify installed performance.
Colour
Colour is an important part of airport visual communication — different systems use specified colours for different operational purposes. Evaluation covers chromaticity coordinates, colour boundaries, luminous intensity and colour consistency.
Open CIE Colour ToolTesting, Maintenance & Common Faults
A reliable AGL system requires systematic testing and maintenance across four areas — click a fault card to see its likely causes.
Electrical
Insulation resistance, continuity, transformer and cable testing, connector inspection.
Photometric
Intensity, distribution, colour and alignment.
Mechanical
Fixture, transformer and connector condition, mounting, housing, sealing.
Operational
CCR operation, intensity steps, control system, monitoring, fault indications.
| Fault | Possible Cause | First Check / Test | Corrective Action |
|---|---|---|---|
| Open circuit | Cable break, connector failure, transformer connection failure, loose connection | Circuit continuity test | Locate the break by sectional continuity/loop testing; repair or replace the cable, connector or connection |
| High leakage / insulation failure | Cable damage, moisture ingress, connector deterioration, transformer insulation failure | Insulation resistance (megger) test | Identify the failed section; dry out, reseal or replace the affected cable, connector or transformer |
| Fixture OFF (individual) | Transformer fault, secondary cable fault, or fixture/lamp-driver fault | Secondary voltage/current check at the transformer | Replace the transformer, secondary cable or fixture as isolated by the test |
| CCR trip / circuit shutdown | Earth fault, short circuit, or overcurrent condition on the series circuit | Insulation/earth-fault check and circuit sectioning | Isolate the faulty section, clear the fault, and confirm circuit current is restored before re-energising |
| Transformer problem | Winding issue, insulation failure, overheating, incorrect rating, mechanical/environmental damage | Insulation resistance, turns ratio and visual/thermal inspection | Replace the transformer with a correctly rated, tested unit |
| Fixture failure | Lamp/LED failure, driver failure, optical damage, water ingress, mechanical damage | Secondary current draw and visual/photometric inspection | Replace the lamp/LED module, driver, or complete fixture as required; reseal housing |
Understanding the Standards
AGL engineering should always be considered against the applicable requirements, specifications and project standards. A standard isn't a list of numbers to memorise — the engineer should understand what requirement each number is trying to achieve.
International aviation standards and recommended practices.
Detailed airfield lighting equipment and system guidance, widely used as a technical reference.
Applicable Indian aviation regulatory and airport requirements.
Relevant Indian Standards for applicable AGL equipment.
AGL Engineering Design
Once the fundamentals are understood, the next step is seeing how the components work together as a complete, designed system.
Airfield Lighting Basics, Overview, Purpose & Principles
Airfield Lighting Basics
Airfield lighting basics: lights along runways, taxiways and aprons give pilots visual guidance in darkness and poor visibility. They run on series circuits fed by constant current regulators, with each light connected through an isolating transformer.
Airfield Lighting Overview
An airfield lighting overview covers approach lights, runway lights, taxiway lights, PAPI, signs and beacons, plus the power, regulators, cables and control behind them. Together they form one visual guidance system.
Purpose Of Airfield Lighting
The purpose of airfield lighting is to let pilots find the runway, line up, land, taxi and park safely at night and in low visibility, by showing position, direction and distance through light colour and pattern.
Airfield Lighting For Beginners
Airfield lighting for beginners is easiest to learn in order: first what each light shows, then the series circuit that powers it, then how it is controlled, tested and maintained. This page follows that sequence.
Airfield Lighting Principles
Airfield lighting principles are consistency, reliability and clarity: patterns and colours follow international standards so pilots read them the same everywhere, and circuits are designed so failures do not leave large dark areas.
Airfield Lighting Functions
Airfield lighting functions include marking runway edges and centrelines, identifying thresholds and ends, guiding aircraft along taxiways, giving glide path information and signalling hazards such as obstacles or runway holding positions.
Airfield Lighting Types
Airfield lighting types are grouped by where they are used: approach lighting, runway lighting, taxiway lighting, glide path indicators such as PAPI, apron and stand lighting, beacons and obstruction lights. Fixtures are elevated or inset.
Airfield Lighting Working Principle
Airfield lighting working principle: a constant current regulator forces one set current through a series loop of cable, and each light draws power through an isolating transformer. Changing the current changes brightness for every light on the circuit.
Airfield Lighting Visual Aids
Airfield lighting visual aids are part of the wider group of visual aids in ICAO Annex 14, which also includes markings, signs and markers. Lights, markings and signs must be designed to work together.
Airport Lighting Signals
Airport lighting signals are the information carried by light colour, steadiness, flashing and arrangement, for example green threshold, red runway end and flashing sequences on approach. They differ from light gun signals sent from the tower.
Airfield Ground Lighting Working Principle, Types, Functions & Specification
Airfield Ground Lighting Working Principle
Airfield ground lighting working principle is series-circuit power: each circuit has one regulator, one cable loop and many lights on isolating transformers, so a single lamp failure does not stop the others.
Airfield Ground Lighting Types
Airfield ground lighting types cover elevated and inset fixtures, unidirectional, bidirectional and omnidirectional beams, and white, green, red, blue, yellow and other colours chosen for the function of each light.
Airfield Ground Lighting Functions
Airfield ground lighting functions are guidance and information: where the runway is, where its ends are, where to turn or stop on taxiways, and whether the aircraft is on the correct approach path.
Airfield Ground Lighting Specification
An airfield ground lighting specification sets photometric output, colour, mechanical strength, ingress protection and electrical data for each product. It cites standards such as ICAO Annex 14, FAA advisory circulars and IEC standards.
Airfield Ground Lighting Selection Guide
Airfield ground lighting selection guide: start from the runway and taxiway category, then choose fixture type, light source, colour and intensity, and check compatibility with the existing regulators and transformers.
Airfield Ground Lighting Technical Guide
An airfield ground lighting technical guide ties together design, installation, testing and maintenance. Use it with the manufacturer data and the standards that apply to your aerodrome.
Airport Lighting Infrastructure, Types, Functions & Engineering Guide
Airport Lighting Infrastructure
Airport lighting infrastructure includes substations and CCRs, series cable and ducts, light fixtures, control and monitoring, standby power and earthing. It is long-life infrastructure, so design for maintainability from the start.
Airport Lighting Working Principle
Airport lighting working principle for airfield lights is series current control, while apron floodlights and building lighting normally use ordinary parallel supply. Do not confuse the two systems.
Airport Lighting Types
Airport lighting types include airfield ground lighting for aircraft guidance, apron floodlighting for ground handling, obstruction lights and beacons, and terminal and road lighting, each with different standards.
Airport Lighting Functions
Airport lighting functions range from aircraft guidance on the airfield to safe working light on aprons and security and safety lighting around the airport.
Airport Lighting Selection Guide
Airport lighting selection guide: separate airfield lighting from general lighting, select each by its standard, and weigh energy use, maintenance access, life and spare parts availability.
Airport Lighting Technical Guide
Airport lighting technical guide: this site's AGL pages cover the technical side of airfield lighting, from basics to design, testing and troubleshooting, and link to the relevant standards.
Airport Lighting Engineering Guide
An airport lighting engineering guide covers load calculation, circuit design, cable sizing, regulator selection, control philosophy and standby power. See the AGL Engineering Design section above.
Airfield Lighting Engineering
Airfield lighting engineering combines electrical design, optics and photometry, civil installation, control systems and maintenance planning, all applied to meet aviation standards.
AGL Technical Guide
An AGL technical guide should explain the system in layers: field fixtures, series circuits, regulators, control and monitoring, and the standards and tests that prove compliance.
Airfield Lighting Requirements, Operational Requirements & Selection
Airfield Lighting System Requirements
Airfield lighting system requirements follow from the runway type, approach category and operations. Annex 14 and national rules state what lights are needed, their performance, and the supply and monitoring they require.
Airfield Lighting Requirements
Airfield lighting requirements are set by ICAO Annex 14, national regulators such as DGCA in India, and the airport operator, and cover which lights are needed, their intensity and colour, and serviceability limits.
Airfield Lighting Operational Requirements
Airfield lighting operational requirements cover availability, response to failures, switching time for standby power, and procedures with air traffic control, especially for low-visibility operations.
Airfield Lighting Selection Guide
Airfield lighting selection guide: define the operational need, pick fixtures and light sources that meet the standard, then size circuits, regulators and control to suit, with spares and maintenance in mind.
Airfield Lighting Technical Guide
Airfield lighting technical guide: use the manufacturer's data with the design standard to check layout, photometry, electrical ratings and installation details before purchase and again at commissioning.
The AGL Engineer's Checklist
Tick items off as you work through a project — this resets on reload, so treat it as a working reference, not a saved record.
Before Design
During Design
Before Commissioning
After Commissioning
Your Learning Path
Understand the Language
AGL terminology, airport layout, runway, taxiway, apron and basic visual aids.
Browse the Glossary →Understand the System
CCR → Series Circuit → Transformer → Fixture → Control.
Revisit the Architecture ↑Understand the Hardware
CCR, transformers, cables, connectors, fixtures, signs and PAPI.
Open the Component Library →Understand the Design
Circuit design, equipment selection, loading, installation and testing.
Explore the Visual Encyclopedia →Understand the Requirements
ICAO, FAA, DGCA and applicable Indian standards.
Open the Standards Companion →Apply the Knowledge
Testing, troubleshooting, inspection, FAT, SAT, commissioning and maintenance.
Take a Quiz →Quick Knowledge Check
Q1. What is the primary function of a CCR?
Q2. What is the primary purpose of an AGL isolating transformer?
Q3. Which system provides visual approach-slope information?
Ready to go beyond the fundamentals?
You've learned the basic architecture of an AGL system — now explore the components, standards, and testing methods used in real airport projects.
Engineering & Expert Notes
Trace One Light Back to the Source
Learning MethodPick a single fixture and follow it through transformer, cable, CCR and supply. Once you can walk that path on a drawing, faults stop looking random and start pointing to a place.
Constant Current Changes How You Read Faults
Series CircuitOpen circuits, earth faults and high-resistance joints behave differently from a constant-voltage system. Learn the symptom of each before you pick up a meter, and treat every series circuit as live until proved dead.
Most Field Faults Sit at Connections, Not in the Big Equipment
Fault FindingConnectors, joints and secondary terminations fail far more often than CCRs or transformers. Check and test them first, and look for the water path that caused the failure.
A Light That Is On May Still Be Wrong
PhotometryElectrical health does not prove the right intensity, colour or aim. Verify optical performance and alignment, and keep commissioning data as a baseline.
Learn Which Document Answers Which Question
StandardsDesign standards, equipment specifications and national rules serve different purposes. Cite the current edition, and say which framework governs when a project mixes them.
Records Are Part of the System
DocumentationAccurate as-builts, test results and fault logs make every later job faster and safer. Update them when the work is closed, not weeks later.
Frequently Asked Questions
Quick answers about Airfield Ground Lighting basics.
Q1What is Airfield Ground Lighting (AGL)?
Airfield Ground Lighting is the system of visual aids and associated equipment used to guide aircraft during approach, landing, takeoff and movement on the airport surface, especially at night or in reduced visibility. It includes approach, runway and taxiway lighting, PAPI, illuminated signs, stop bars and runway guard lights, plus the CCRs, series-circuit cabling, isolating transformers, connectors and control and monitoring systems behind them.
Q2How does the AGL power and lighting chain work?
Airport power feeds a Constant Current Regulator (CCR), which supplies a primary series circuit. Along that circuit, each light fixture is fed by its own isolating transformer and secondary cable. Every fixture, whatever its category, traces back through this same chain.
Q3What is a Constant Current Regulator (CCR)?
A CCR is the major component that supplies controlled current to the series lighting circuit. It maintains the selected current level, provides multiple intensity steps, monitors operating conditions and gives protection and fault indication. A typical circuit is a 6.6 A series circuit.
Q4Why does AGL use a series circuit?
A series arrangement lets many lighting loads operate as part of one controlled circuit. Because the CCR controls circuit current directly, it also sets the brightness step for the whole circuit at once. This differs from ordinary constant-voltage building wiring.
Q5What does an isolating transformer do in an AGL circuit?
It sits between the primary series circuit and one light fixture, electrically separating the fixture circuit from the shared primary. It lets each fixture connect to the series circuit and supports continued operation of the rest of the circuit under appropriate fault conditions.
Q6What is the difference between the primary and secondary circuit?
The primary circuit is the high-voltage series circuit supplied by the CCR. The secondary circuit is the isolated supply from the secondary side of an isolating transformer to an individual fixture: CCR, primary cable, isolating transformer, secondary cable, then light fixture.
Q7What do the colours of airfield lights mean?
As a general orientation, runway edge lights are white, runway ends are red, thresholds are green, taxiway edges are blue, taxiway centrelines are green and stop bars are red. Exact colour, location, spacing and configuration depend on the applicable standard and the specific runway or taxiway layout.
Q8How does PAPI show whether an aircraft is on the approach slope?
PAPI uses four light units in a row. The balance of white and red changes in single-unit steps as the aircraft moves through the slope, and two white and two red means the aircraft is on the correct approach slope.
Q9What are common faults in an AGL series circuit?
Typical faults include open circuits (cable, connector or transformer connection failure), insulation or high-leakage failures, an individual fixture being off, CCR trips from earth faults or overcurrent, transformer problems and fixture failures. Continuity and insulation resistance tests are the usual first checks.
Q10Is a fixture acceptable just because it lights up?
No. A fixture's optical performance must meet the applicable requirements, including intensity, distribution, beam characteristics, colour, coverage, alignment and uniformity. Testing covers both the laboratory and the installed field condition.
Don't Just Learn the Basics. Build the Complete Picture.
Fundamentals is where AGL understanding begins, not where it ends.
Continue with structured knowledge across components, standards and real testing scenarios to turn fundamentals into practical, job-ready AGL capability.
Learn the basics. Build the depth. Apply it on the airfield.
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Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page or any result produced by it.
- Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel, and for people learning the subject. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables and connectors can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock.
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