Approach Lighting Systems
The visual transition between the approach path and the runway. An Approach Lighting System (ALS) is an arrangement of aerodrome lights installed in the area leading toward a runway threshold, giving pilots visual information about approach alignment, runway direction, threshold location and the transition from instrument approach to visual reference. It's more than a row of lights — it's a coordinated system of light units, supporting structures, electrical circuits, CCRs, isolation transformers, connectors, control, alignment and photometric performance.
What Is an Approach Lighting System?
An Approach Lighting System provides visual guidance to an aircraft approaching a runway. It extends outward from the runway threshold into the approach area and creates a visual reference that helps the pilot establish the relationship between the aircraft's approach path, the approach lighting, the runway threshold and the runway itself.
The exact configuration depends on the runway, approach procedure, visibility requirements, aerodrome category and applicable standards.
Not Just a Row of Lights
An ALS is a carefully arranged system of light units, structures, circuits, CCRs, transformers, connectors, control and photometric performance.
Progressive Visual Reference
During an instrument approach the pilot may initially have little or no visual reference — the ALS builds progressively stronger visual information as the runway nears.
Alignment Is Everything
Even a correctly functioning light can provide incorrect visual information if it is installed in the wrong position.
Why Approach Lighting Is Important
The approach lighting system helps provide progressively stronger visual information as the aircraft approaches the runway, establishing visual continuity between the approach and the runway.
Runway alignment
Centreline and crossbar arrangement establish the visual axis toward the runway.
Runway direction
Approach lighting confirms the correct heading and orientation on final approach.
Approach centreline
Provides the primary reference line the pilot tracks toward the threshold.
Threshold location
Marks precisely where the runway begins.
Distance from the runway
Progressive lighting elements give the pilot a sense of remaining distance.
Transition to the runway
Creates continuity from the approach environment into the runway lighting system.
Approach Lighting System at a Glance
A simplified system connects the airport's electrical supply all the way through to the pilot's visual reference.
In more complex installations, the system may also incorporate sequenced flashing lights, dedicated control equipment, monitoring, multiple circuits, approach-lighting structures, and elevated and inset fixtures.
Main Elements of an Approach Lighting System
An approach lighting system can include combinations of the following elements. The exact arrangement is determined by the applicable ALS configuration.
Approach Centreline Lighting
Establishes the visual axis leading toward the runway and provides an important reference for aircraft alignment. Can incorporate individual light units, light bars, supporting structures and sequenced flashing elements where applicable. Correct positioning and alignment are critical.
Approach Crossbars
Provide lateral visual references across the approach centreline, helping the pilot recognize the approach lighting geometry and maintain visual orientation relative to the runway. Arrangement, spacing and dimensions depend on the applicable ALS configuration.
Sequenced Flashing Lights
Create a visual sequence that leads the pilot's attention toward the runway threshold. Must operate with the correct timing, order and visual relationship to the rest of the approach lighting system.
High-Intensity Approach Lights
Used where enhanced conspicuity is required for the operational category and visibility conditions applicable to the runway.
Supporting Structures
Elevated structures or other approved mounting arrangements that carry approach light fixtures, cabling and, where applicable, frangible elements.
Control & Monitoring Equipment
Coordinates circuit switching, intensity, sequencing and fault monitoring across the complete ALS installation.
Approach Lighting Configurations
Approach lighting is not one universal system — different configurations exist for different operational requirements, including simple approach lighting systems, precision approach lighting systems, systems incorporating sequenced flashing lights, high-intensity approach lighting systems, and category-specific approach lighting configurations.
The terminology and detailed configuration should always be checked against the applicable ICAO, FAA, national and project requirements.
Approach Lighting System Types: CAT I, CAT II, CAT III & Non-Precision
An ALS airport selects its approach lighting system from the runway type and the approach category. Approach lighting requirements, pattern and intensity come from the applicable standard, so confirm them against the approved design.
| Approach lighting system types | Typical use | Character |
|---|---|---|
| Non precision approach lighting | Non-precision and simpler runways | Simpler pattern, shorter extent and lower intensity than precision systems |
| CAT I ALS | Precision approach, Category I | Centreline lights with crossbar, giving the pilot a visual alignment reference |
| CAT II ALS | Precision approach, Category II | Adds side-row lighting near the threshold for lower-visibility operations |
| CAT III ALS | Precision approach, Category III | The most complete pattern and highest intensity for the lowest visibility |
Approach Lighting Working Principle & Approach Lighting Pilot Guidance
The approach lighting working principle is pattern recognition. The centreline row shows the runway axis, crossbars give a roll and distance reference, and sequenced flashing, where fitted, draws the eye along the approach. This is approach lighting pilot guidance: it lets the pilot transition from instruments to visual references.
Approach Lighting Visibility
Approach lighting visibility depends on intensity, colour, correct aiming, clean optics and the prevailing weather. A light that is on but misaimed or dirty can still fail the pilot.
ICAO Annex 14 requirementsCAT II ALS & CAT III ALS
CAT II ALS adds side-row lighting near the threshold for lower-visibility operations. CAT III ALS is the most complete pattern, with the highest intensity, for the lowest visibility. Pattern and intensity come from the applicable standard, so confirm them against the approved design.
Sequenced Flashing Concept
Sequenced flashing lights create a visual sequence leading the pilot's attention toward the runway threshold — outer approach, flash, flash, flash, threshold. Watch the simplified sequence below.
Approach Lighting Colour
Colour is part of the visual coding of an approach lighting system. Approach lighting commonly uses white light for the principal approach-light elements, while specific associated lighting functions can have different colour requirements. Colour requirements should always be evaluated according to the particular ALS configuration and applicable standard.
Approach Lighting Geometry & Structures
Design Geometry
Approach lighting is highly dependent on geometry: runway centreline, threshold location, approach centreline, longitudinal spacing, crossbar position, lateral positioning, approach direction, supporting structure location, terrain, obstacles and aircraft visual perspective.
Supporting Structures
Structural design may need to consider fixture mounting, structural stability, height, alignment, wind loading, foundation, frangibility requirements where applicable, cable routing, maintenance access and environmental conditions. The structure is part of the overall ALS engineering system.
Centreline barrettes establish the approach axis; a crossbar provides lateral reference; sequenced flashers lead the eye in from the outer approach toward the threshold
Approach Lighting Layout, Elevation, Azimuth, Aiming & Support Structures
The approach lighting layout and approach lighting drawing fix every light position relative to the runway centreline and threshold. Elevation and azimuth then define how each fixture is set.
Approach lighting system diagram — illustrative plan view of a precision pattern (not to scale)
| Topic | What it covers |
|---|---|
| Approach lighting elevation | The vertical aiming angle of each fixture and the installed height of the light relative to the approach profile |
| Approach lighting azimuth | The horizontal direction of the beam relative to the runway centreline |
| Approach lighting aiming | Setting and checking elevation and azimuth against the design values with the proper alignment tools |
| Approach lighting support structure | Frangible supports and foundations that hold each light at the design height |
| Approach lighting tower / approach lighting mast | Taller structures used where lights must stand above terrain or obstructions, subject to the obstruction and frangibility rules |
Related Guides
See how alignment and photometry confirm the aiming of each light.
AGL photometry AGL design engineeringApproach Lighting Drawing
The approach lighting drawing fixes every light position relative to the runway centreline and threshold. Keep it updated as built, because maintenance and fault finding depend on it.
Approach Lighting Elevation
Approach lighting elevation is the vertical aiming angle of each fixture, together with the installed height of the light relative to the approach profile.
Approach Lighting Azimuth
Approach lighting azimuth is the horizontal direction of the beam relative to the runway centreline.
Approach Lighting Aiming
Approach lighting aiming is setting and checking elevation and azimuth against the design values with the proper alignment tools, then confirming by measurement.
The Approach Lighting Series Circuit
Many approach lighting installations use series circuits. The exact circuit architecture depends on the particular ALS design.
The CCR drives a single constant-current loop out through each isolating transformer and fixture, then back — every fixture on the loop carries the same current
CCR, Isolation Transformers & Connectors
Constant Current Regulator
Supplies controlled current to the approach lighting circuit. Design considers number of lights, fixture ratings, transformer ratings, circuit length, total load, required intensity levels, circuit configuration, and control & monitoring requirements. Different ALS circuits may use separate CCRs depending on the system design.
Isolation Transformers
Provide the electrical interface between the primary series circuit and individual approach lights (Series Circuit → Transformer Primary → Transformer Secondary → Approach Light). Selection must be coordinated with the fixture rating and complete circuit design.
Connectors
Provide the electrical connection between the series circuit, isolation transformers and fixtures — primary connectors, secondary connectors, plugs, receptacles, connector kits. Connector condition can directly affect system reliability: loose connections, high resistance, water ingress, insulation deterioration and intermittent contact are common problems.
Approach Lighting Control
Approach lighting may be controlled through the airport's AGL control system. Sequenced flashing systems require particular attention to the relationship between lighting sequence, timing and control.
Control Functions
What Intensity Level Affects
Command flows down from the control source to the circuit; status and fault data flow back up so the control system knows the circuit obeyed the command
Approach Circuit Load Estimator
CCR selection should be based on the actual system design. Adjust the sliders to see how fixture count, unit load and estimated circuit losses combine into a total circuit requirement.
Photometry & Alignment
Approach lighting requires careful photometric verification because the lights are viewed from a specific approach direction. A light can be electrically operational while still having low intensity, incorrect beam direction, incorrect colour or abnormal distribution.
Photometry
Evaluates intensity, distribution, colour, alignment, individual fixture performance and system-level performance — helping identify optical faults objectively.
Alignment
Considers horizontal direction, vertical direction, centreline relationship, crossbar relationship, fixture orientation and structure position. An alignment error can create a visual error even when the electrical system is operating normally.
Electrical Testing
Cable continuity, insulation resistance, transformer testing, connector inspection, series-circuit testing, CCR testing, current verification, control verification, flashing sequence verification, monitoring/fault verification and earthing checks where applicable.
Field Inspection Areas
Fixtures
Housing, lens, optical assembly, mounting and alignment.
Structures
Structural condition, fasteners, corrosion, foundation and alignment.
Cables
Cable entry, physical damage, connections and identification.
Environment
Water, vegetation, dirt, obstructions and foreign objects.
Approach lighting is exposed to weather and environmental conditions, so physical inspection is an important part of maintaining system reliability.
Common Approach Lighting Faults
Tap a symptom to see possible causes.
One approach light OFF
Possible Cause
- Fixture fault
- Transformer fault
- Connector or secondary connection
Several lights OFF
Possible Cause
- Series circuit problem
- Cable problem
Complete ALS circuit OFF
Possible Cause
- CCR fault
- Cable fault
- Protection or control fault
Low intensity
Possible Cause
- Fixture degradation
- Optical contamination
- Electrical issue
Incorrect beam direction
Possible Cause
- Fixture misalignment
Flashing sequence incorrect
Possible Cause
- Control/sequence system fault
Flashing light not operating
Possible Cause
- Fixture fault
- Transformer fault
- Circuit or controller fault
Uneven lighting
Possible Cause
- Fixture degradation
- Alignment issue
- Component issue
A Practical Troubleshooting Approach
This approach separates electrical failure from optical and alignment failure. Work through the checks in sequence.
Is the circuit energized?
Confirm the ALS circuit is receiving power.
Is CCR output correct?
Verify the regulator is delivering the correct current.
Is the series circuit healthy?
Check continuity and insulation resistance along the circuit.
Is the transformer healthy?
Test the isolation transformer's primary and secondary performance.
Is the secondary supply correct?
Verify voltage/current reaching the fixture.
Is the fixture operating?
Confirm the fixture itself is functioning electrically.
Is the fixture correctly aligned?
Check horizontal and vertical alignment against the approach geometry.
Is the optical output correct?
Assess beam direction, distribution and colour.
Verify through photometry
Confirm the fix with objective photometric measurement.
Approach Lighting Maintenance
A complete maintenance program should consider six connected areas.
Electrical
CCR, circuits, transformers, connectors and cables.
Mechanical
Fixtures, structures, fasteners and foundations.
Optical
Lenses, LEDs, lamps and optical assemblies.
Alignment
Fixture orientation and approach geometry.
Photometric
Intensity, colour and distribution.
Control
ON/OFF operation, intensity settings and sequencing.
Approach Lighting Commissioning Sequence
Approach Lighting LED, Halogen, Flashers, Wiring & Acceptance Testing
Technology, flashing equipment and wiring decisions affect both performance and maintenance. The acceptance test then proves the finished system.
Approach Lighting Halogen vs LED
Approach lighting halogen fixtures are proven and easy to source; approach lighting LED fixtures typically offer longer life and lower power. Whichever is chosen, it must still meet the photometric requirement.
Approach Lighting Flashers & Flash Sequence
Approach lighting flashers produce the sequenced flash that runs along the approach. The approach lighting flash sequence depends on timing and synchronisation equipment, which needs its own commissioning checks.
Approach Lighting Wiring
Approach lighting wiring follows the series circuit from the CCR through isolation transformers and connectors to each fixture. Correct connector installation and earthing are essential.
Troubleshooting EngineApproach Lighting Acceptance Test & Test Report
The approach lighting acceptance test covers visual, electrical, alignment, photometric and functional checks. The approach lighting test report records measured against required values for the commissioning file.
AGL testing & inspectionApproach Lighting, Runway Lighting & PAPI
One Visual Guidance Chain
Approach lighting and runway lighting should be treated as one visual guidance chain: Approach → Approach Lighting → Threshold → Runway → Runway Lighting. The transition between these systems is particularly important for the pilot's visual acquisition and alignment.
Approach Lighting vs PAPI
Approach lighting and PAPI perform different functions. Approach lighting provides visual references for approach alignment, runway direction, threshold identification and transition toward the runway. PAPI provides visual information about the vertical approach path / glide path. They complement one another rather than perform the same function.
Approach Lighting for CAT I, CAT II & CAT III
How the approach lighting pattern changes with the category of operation.
Approach lighting for CAT I
Approach lighting for CAT I supports precision approaches with the lowest category of visibility minima. It gives the pilot a clear centreline and distance cues leading to the threshold. Pattern, length and intensity must follow ICAO Annex 14 or the national standard the aerodrome uses.
Approach lighting for CAT II
Approach lighting for CAT II adds to the CAT I pattern, notably with side-row lighting near the threshold, to give better roll and lateral guidance in lower visibility. Intensity, serviceability and power supply requirements are tighter than for CAT I.
Approach lighting for CAT III
Approach lighting for CAT III supports operations in the lowest visibility. It builds on the CAT II pattern and carries the most demanding requirements for intensity, serviceability and secondary power. Confirm the exact pattern and performance limits against the standard you follow. See the Approach Lighting System Types section above.
Approach Lighting Types, Functions & LED Approach Lights
What the main kinds of approach lighting do and how LED fits in.
Approach lighting types
Approach lighting types are grouped by the kind of approach they serve, such as non-precision, CAT I and CAT II/III patterns, and by the layout and light units used, including centreline barrettes, crossbars, side rows and sequenced flashing lights. Naming varies between ICAO, FAA and national systems.
Approach lighting functions
Approach lighting gives the pilot visual guidance in the final approach: alignment with the runway centreline, a sense of roll, and distance cues to the threshold. It helps the pilot transition from instruments to visual references and decide whether to continue the landing.
LED approach lights
LED approach lights replace halogen light sources with LED units. Typical attractions are lower power use, long life and fewer lamp changes. They must still meet the required photometric performance and work correctly on the series circuit and with the CCR, so compatibility and approval need to be checked. See the Approach Lighting LED, Halogen, Flashers, Wiring & Acceptance Testing section above.
Power Supply, ICAO Standards, Selection Guide & Technical Guide
The supply, the rules and the practical way to choose and understand the system.
Approach lighting power supply
Approach lighting is normally fed from a CCR on a constant-current series circuit, with isolation transformers at each light. Higher categories of operation also require secondary power and defined changeover arrangements, so the supply design should be checked against the category served and the standard followed.
Approach lighting ICAO standards
The main ICAO references are Annex 14 Volume I for the approach lighting specifications and the ICAO Aerodrome Design Manual for guidance on visual aids and electrical systems. FAA advisory circulars are often used alongside, and DGCA requirements apply in India. Confirm the editions that apply to your project.
Approach lighting selection guide
To select an approach lighting system, start with the runway and approach category, then check the standard, the available space and structures, the power supply, the light technology and the maintenance plan. Compare the LED and halogen options on photometry, compatibility and total life cost.
Approach lighting technical guide
As a technical guide, an approach lighting system is a set of light units on structures, fed by series circuits, controlled from the CCR and control system and checked by alignment and photometry. Each section on this page covers one link in that chain.
Approach Lighting Testing Procedure, Troubleshooting & Preventive Maintenance
Proving the system, finding faults and keeping it serviceable.
Approach lighting testing procedure
A testing procedure normally covers visual inspection, insulation and continuity checks, circuit current and brightness-step checks, flasher operation, control checks, and alignment and photometric verification. Test values and acceptance criteria come from the project specification and standard, so none are given here. See Photometry & Alignment above.
Approach lighting troubleshooting
Troubleshoot by symptom: one light out, a section out, flashers not sequencing or a whole circuit tripped. Check connectors, lamps or LED units, transformers and circuit insulation before replacing equipment. See Common Approach Lighting Faults and A Practical Troubleshooting Approach above.
Approach lighting preventive maintenance
Preventive maintenance covers regular inspection of lights and structures, cleaning, alignment checks, insulation trending, connector checks and records of faults, so repeated problems become visible. The interval should follow your maintenance plan and the aerodrome's requirements.
Engineering Checklist
A practical way to track an ALS project from design through commissioning. Click any item to mark it complete.
Design
Installation
Testing
Photometry & Commissioning
Approach Lighting Project Documentation
Quick Knowledge Check
Q1. What is the fundamental purpose of an Approach Lighting System?
Q2. How do Approach Lighting and PAPI differ?
Q3. Why can a correctly functioning approach light still provide incorrect visual information?
Explore More AGL Engineering
AGL Fundamentals
Build your understanding of the complete Airfield Ground Lighting system.
Open AGL Fundamentals →AGL Design & Electrical Engineering
Layout, series circuits, CCR sizing, transformers, cable design and control.
Open AGL Design →AGL Photometry
Verify the optical performance of the installed system.
Open AGL Photometry →AGL Troubleshooting Engine
Diagnose approach lighting electrical, optical, alignment and control faults.
Open the Troubleshooting Engine →Runway Lighting Systems
The runway environment approach lighting leads into.
Open Runway Lighting →PAPI Engineering Guide
The precision approach path indicator alongside the ALS.
Open PAPI Guide →AGL Installation & Commissioning
Field installation and commissioning of approach lighting.
Open Installation & Commissioning →AGL Testing & Inspection
Verification of the installed approach lighting system.
Open Testing & Inspection →ICAO Annex 14 AGL Requirements
The SARPs governing approach lighting configurations.
Open ICAO Annex 14 →Bridge the Approach. Guide the Threshold.
From approach geometry to energized circuit, from alignment to photometric verification — explore the complete AGL knowledge base for approach lighting engineering.
Explore the AGL Engineering Knowledge Hub
Fundamentals
Standards & Compliance
Runway, Taxiway & Approach Lighting
Surface Movement & Safety
PAPI
Electrical Engineering & Power
Photometry & Testing
Maintenance, Inspection & Safety
Resources, Documentation & Careers
Frequently Asked Questions
Q1What is an Approach Lighting System (ALS)?
An Approach Lighting System is an arrangement of aerodrome lights installed in the area leading toward a runway threshold. It gives pilots visual information about approach alignment, runway direction, threshold location and the transition from instrument approach to visual reference. The exact configuration depends on the runway, approach procedure, visibility requirements, aerodrome category and applicable standards.
Q2Why is approach lighting important?
It provides progressively stronger visual information as the aircraft nears the runway, establishing visual continuity between the approach and the runway. It helps the pilot judge runway alignment, runway direction, the approach centreline, threshold location, distance from the runway and the transition into the runway lighting system.
Q3What are the main elements of an approach lighting system?
Depending on the configuration, an ALS can include approach centreline lighting, approach crossbars, sequenced flashing lights, high-intensity approach lights, supporting structures, and control and monitoring equipment. The exact arrangement is determined by the applicable ALS configuration.
Q4What are sequenced flashing lights in an ALS?
Sequenced flashing lights create a visual sequence that leads the pilot's attention toward the runway threshold, flashing in order from the outer approach toward the threshold. They must operate with the correct timing, order and visual relationship to the rest of the approach lighting system.
Q5Is there one universal approach lighting configuration?
No. Different configurations exist for different operational requirements, including simple approach lighting systems, precision approach lighting systems, systems with sequenced flashing lights, high-intensity systems and category-specific configurations. Terminology and detailed configuration should always be checked against the applicable ICAO, FAA, national and project requirements.
Q6What colour is approach lighting?
Approach lighting commonly uses white light for the principal approach-light elements, while specific associated lighting functions can have different colour requirements. Colour should always be evaluated against the particular ALS configuration and applicable standard.
Q7Why does geometry and alignment matter so much in approach lighting?
Even a correctly functioning light can give incorrect visual information if it is installed in the wrong position. Approach lighting depends on the runway centreline, threshold location, approach centreline, longitudinal spacing, crossbar position, lateral positioning, approach direction, supporting structure location, terrain, obstacles and the aircraft's visual perspective.
Q8What do the supporting structures for approach lights do?
Supporting structures are elevated structures or other approved mounting arrangements that carry approach light fixtures, cabling and, where applicable, frangible elements. Structural design may need to consider fixture mounting, stability, height, alignment and wind loading.
Q9What is the electrical chain behind an approach lighting system?
A simplified chain runs from the airport electrical supply and AGL control system to the CCR, series circuit and isolation transformers, then to the approach light fixtures, their optical output and finally the pilot's visual reference. More complex installations can add sequenced flashing, dedicated control equipment, monitoring and multiple circuits.
Q10How does an ALS relate to the rest of runway lighting?
The approach lighting system creates the transition from the approach environment into the runway lighting system, so it should be considered as part of the overall runway visual guidance rather than in isolation.
Q11What are the approach lighting system types?
The main approach lighting system types are non precision approach lighting and the precision CAT I ALS, CAT II ALS and CAT III ALS patterns, with the choice set by runway type and approach category.
Q12What is the approach lighting working principle?
Approach lighting gives pilots a recognisable pattern of lights leading to the threshold, so they can align with the runway and judge distance when they transition from instruments to visual flight.
Q13What does an approach lighting acceptance test include?
Visual, electrical, alignment, photometric and functional checks, followed by an approach lighting test report recording measured against required values.
Engineering & Expert Notes
Configuration Follows the Operational Need
SelectionChoose the system according to approach category and applicable requirements, and document the basis.
Geometry Drives Performance
LayoutSpacing, elevation and the line of the centreline determine how the pattern appears to the pilot.
Check Flashing and Sequencing
SequenceVerify that timing and synchronisation function after maintenance and after power events.
Monitor Brightness Steps
IntensityConfirm each step is set and works as intended, including interaction with control systems.
Inspect Supports and Frangible Parts
StructuresCheck for corrosion, damage and correct installation of structures.
Document the Baseline
RecordsKeep photometric and alignment data from commissioning to compare later.
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