Approach Lighting
Approach Threshold Light for Airfield Ground Lighting
Green threshold identification at the approach-to-runway transition
Approach threshold lights provide green threshold identification and visual transition between the approach light field and the runway threshold, configured as part of the complete approach lighting system.
Supplied by Precision AGL Solutions
Product Overview
Approach Threshold Light
What It Is
Approach threshold lights form the threshold-light element of an airport approach lighting system, giving a clear visual indication of the runway threshold where the approach-light field meets the runway.
The threshold is where the aircraft transitions from the approach lighting environment to the runway visual environment, so the array is an important reference alongside approach centreline, crossbar, side-row and runway lighting.
Selected According To
- Runway width and threshold configuration
- Approach category and lighting system type
- Centreline, crossbar and side-row arrangement
- Threshold geometry and fixture spacing
- Light colour, optical direction, intensity
- Mounting arrangement
- Electrical circuit and control arrangement
- Project technical specification
The final configuration should be based on the approved approach-lighting system design and applicable aerodrome requirements.
Applications
Where Approach Threshold Lights Are Used
Simple approach systemsThreshold element.
Precision Category IApproach lighting.
Precision Category II / IIIIntegrated arrays.
MALS / MALSF / MALSRFAA medium-intensity.
ALSF-type systemsHigh-intensity.
Approach rehabilitationReplacement arrays.
Runway upgradesAirport projects.
New installationsNew approach lighting.
Array replacementExisting threshold arrays.
FAA treats threshold lights as an integral element of systems such as MALSR, with aviation-green steady-burning lights at and parallel to the threshold. FAA ALSF-2 likewise has a dedicated green threshold array with white centreline lighting, red side rows and sequenced flashers.
Function
The Approach-to-Runway Transition
Approach lighting fieldReference
Threshold light arrayIdentification
Runway thresholdTransition
Runway lightingContinuation
Main Functions
- Identify the threshold
- Provide a clear visual transition
- Support runway alignment and reinforce landing direction
- Provide a strong transverse visual reference
- Complement approach centreline lighting
- Connect the approach-light pattern with runway lighting
Evaluate As a System
Evaluate the threshold-light array together with the complete approach-lighting geometry.
System Context
Approach Threshold vs Runway Threshold Light
Approach Threshold Light
The threshold-light element associated with a configured approach-lighting system.
Runway Threshold Light
The runway visual-aid component identifying the threshold within the runway lighting system.
On many airports these functions are physically integrated and use the same green array — FAA MALSR places the threshold array within the approach light field. The product specification should therefore identify the system in which the threshold lights are used.
Light Colour
Aviation Green
Colour
The threshold array is associated with aviation green. FAA MALSR and ALSF-2 both use green threshold lights.
Evaluate The Fixture For
- Green colour and chromaticity
- Intensity and beam distribution
- Visibility and uniformity across the threshold
- Compatibility with the approach-lighting system
Typical Arrangement
Typical Threshold Arrangement
APPROACH CENTRELINE LIGHTS
GREEN THRESHOLD LIGHTS
RUNWAY
Schematic only. The actual array width, number of light sources and relation to runway-edge lights depend on the selected approach-lighting system and runway geometry.
FAA MALSR
FAA MALSR Threshold Light
MALSR Incorporates
- Threshold lights
- Steady-burning white light bars
- Sequenced flashers
The threshold array consists of 18 to 33 aviation-green steady-burning lights, depending on runway width, in a line at and parallel to the threshold.
Place in the MALSR
The threshold array and the seven centreline light bars together form the approach light field. The five sequenced flashers continue outward at 200 ft intervals, starting at 1,600 ft from the threshold.
Runway width and system configuration are key selection parameters. Take the exact requirement from the current approved FAA system drawing and specifications.
FAA ALSF-2
FAA ALSF-2 Threshold Light
ALSF-2
A high-intensity approach lighting system for Category II/III instrument approaches. The threshold array is integrated with green threshold lights, white steady-burning centreline elements, red side-row lights and sequenced flashers.
Typical System
FAA describes a typical ALSF-2 system with 49 green threshold lights in the complete array.
Select the threshold array as part of the approved ALSF-2 configuration, not as a standalone product.
Position & Quantity
Threshold Position, Runway Width and Light Quantity
Coordinate The Array With
- Runway threshold line and runway centreline
- Runway edges and runway edge lights
- Approach centreline and side rows
- TDZ lights
The geometry must give a clear transition from approach lighting to runway lighting.
Quantity Varies
MALSR: 18 to 33 green lights by runway width. ALSF-2: 49 in a typical system. There is no single universal number.
Runway Width + Approach System + Approved Layout = Threshold Light Quantity
Optics & Photometry
Optical Configuration & Photometric Performance
Technical Characteristics
- Fixed green output
- Appropriate horizontal and vertical coverage
- Uniform visual appearance
- Optical direction consistent with the approach
- Controlled intensity and stable colour
Obtain actual beam characteristics from the approved fixture’s photometric documentation.
Intensity
Intensity must be coordinated with the complete system — approach-lighting system, visibility, ambient conditions, operating procedures, CCR settings and architecture. The product must be compatible with the system’s specified intensity control.
| Parameter | Requirement |
|---|---|
| Light source | LED / approved technology |
| Colour | Aviation green / system specific |
| Operation | Steady-burning |
| Intensity | System / project specific |
| Horizontal / vertical coverage | Applicable requirement |
| Beam distribution | Fixture specific |
| Chromaticity | Applicable standard |
| Uniformity | Across threshold array |
| Alignment | Critical |
| Photometric verification | Project dependent |
Assess a threshold array as a complete visual pattern, not only by individual fixture output.
LED Light Source
LED Technology in Threshold Arrays
Potential Advantages
- Long operating life
- Reduced lamp replacement and maintenance
- Stable green output and consistent colour
- Controlled optical distribution
- Reduced energy consumption
- Compatibility with modern AGL control systems
Compliance
Where a legacy lamp-based system is retained, replacement equipment must match its optical and electrical characteristics. LED technology alone does not establish compliance.
Electrical & Control
Electrical Characteristics and Control
Potential Configurations
- Dedicated approach-lighting circuit or series circuit
- CCR-fed circuit and local power supply
- Isolating transformers and LED drivers
- Control cabinet, intensity control, ALCMS interface
Verify operating current, power consumption, circuit architecture, driver, transformer, cable, connector, control interface and monitoring.
Control Functions
- ON/OFF and intensity selection
- Approach-system activation
- Remote control and centralized monitoring
- Fault indication and maintenance mode
FAA approach systems use coordinated approach-lighting controls, not an unrelated runway circuit.
Series: CCR → Primary Approach-Lighting Circuit → Isolating Transformer → Secondary Connection → Threshold Light
Cabinet: Approach Lighting Control Cabinet → Power / Driver → Threshold Light Array
The actual electrical architecture must follow the approved system design.
Mechanical & Installation
Configuration, Mounting and Civil Coordination
May Be Provided As
- Individual fixtures or threshold light bar
- Modular LED assembly
- Low-profile or elevated assembly
- System-specific threshold array
Select based on runway width, threshold geometry, approach system, support requirements, frangibility and maintenance access.
Mounting & Support — Verify
- Fixture elevation and runway threshold reference
- Horizontal alignment and support arrangement
- Frangibility where applicable
- Runway-edge relationship, cable routing, maintenance access
The array should form a clean, consistent transverse reference.
Alignment
- Threshold line and centreline
- Fixture spacing and runway width
- Outer-light position
- Relationship to runway edge lights and approach centreline
- Optical axis
Civil Coordination
- Runway pavement and threshold marking
- Approach area and drainage
- Cable trenches, junction boxes, foundations
- Light supports and frangible structures
Approved civil and electrical drawings control all installation dimensions.
System Relationships
Centreline, Crossbar, Side Rows and Runway Threshold
Approach Centreline
The centreline gives longitudinal alignment; the threshold array gives threshold identification and transition. Keep them visually balanced at all applicable intensity settings.
Approach Crossbar
Approach Crossbar → Centreline → Threshold Light → Runway. The threshold array is the final major transverse reference before the runway visual environment.
Side-Row Lighting
In Category II/III systems the side rows and threshold lights form one visual pattern (ALSF-2: green threshold, red side rows, white centreline, sequenced flashers). Coordinate with side-row and centreline geometry.
Runway Threshold Light
Where one physical fixture serves both approach and runway threshold functions, the specification should identify the applicable system and standard.
Environmental Exposure
- Rain, dust, humidity
- Temperature variation, UV, corrosion
- Wind, vibration, wildlife
- Aircraft operating environment
- Cleaning activities
Commissioning & Testing
Test as Part of the Complete System
Visual & Electrical
- Fixture, lens, green output, array uniformity
- Position, spacing, alignment, supports, cables, connectors
- Continuity, insulation resistance, CCR output
- Driver and transformer verification, cable integrity
- Intensity-step operation, control system, monitoring
Photometric & System-Level
- Green colour, intensity, beam distribution, uniformity
- Optical direction and threshold alignment
- Verify the transition: Approach Centreline → Crossbar → Side Rows → Threshold → Runway Lighting
Inspection & Maintenance
Optical, Mechanical, Electrical and Geometry
Optical & Mechanical
- Lens cleanliness and damage, green colour, light output
- Beam distribution and uniformity
- Mounting, alignment, fasteners
- Support condition, frangibility, corrosion
Electrical & Geometry
- Circuit, cable, connector, transformer/driver, CCR
- Control panel and monitoring
- Threshold position and array alignment
- Outer-light, centreline and runway-edge relationship
Common Failure Modes
Symptoms and Possible Causes
| Symptom | Possible Causes |
|---|---|
| Threshold array completely OFF | Circuit failure; CCR/power failure; control-system problem; cable fault; transformer/driver failure |
| Individual light OFF | LED/lamp failure; connector issue; secondary cable fault; internal fixture failure |
| Uneven threshold appearance | Unequal intensity; mixed fixture types; contamination; LED/lamp degradation; incorrect intensity settings |
| Wrong colour | Incorrect fixture; optical component failure; light-source degradation; contamination |
| Misaligned threshold array | Installation error; pavement/threshold change; fixture displacement; structural movement; incorrect survey reference |
Engineering Troubleshooting Sequence
Use a Systematic Diagnostic Sequence
Approach control systemCommand
CCR / powerVerify
Primary circuitCheck
Transformer / driverCheck
Secondary connectionCheck
Threshold fixtureCheck
Light sourceInspect
Optical assemblyInspect
AlignmentVerify
PhotometryVerify
For a system-level problem also verify: Threshold line → Runway centreline → Runway edge → Approach centreline → Crossbar → Side rows.
Product Selection Matrix
Key Selection Parameters
| Selection Parameter | Requirement |
|---|---|
| Application | Approach-lighting threshold |
| Approach system | MALS / MALSF / MALSR / ALSF / ICAO system |
| Approach category / Runway width | Project specific |
| Installation | System specific |
| Light source | LED / approved technology |
| Colour | Aviation green |
| Operation | Steady-burning |
| Light quantity | System / runway width dependent |
| Spacing | Approved system layout |
| Intensity / Beam | System specific |
| Threshold position | Approved survey/layout |
| Centreline / Crossbar / Side rows | Coordinated |
| CCR / power | System specific |
| Transformer | Where applicable |
| Control / Monitoring | Project specific |
| Mounting / Frangibility | System specific; applicable |
| Environment | Airport specific |
| Documentation | TDS / photometry / certificates |
Product Selection Checklist
Before Ordering, Confirm
- Airport / project
- Runway designation and width
- Threshold location
- Approach category
- Approach-lighting system
- MALS / MALSF / MALSR / ALSF / ICAO configuration
- Number of threshold lights and light spacing
- Threshold-light array length
- Green colour and light-source technology
- Intensity and intensity steps
- Optical distribution
- Centreline, crossbar and side-row relationship
- Runway-edge relationship
- Mounting arrangement and frangibility
- CCR / power supply
- Transformer/driver and cable arrangement
- Control interface and monitoring
- Environmental requirements
- Applicable standard
- Manufacturer TDS
- Photometric documentation
- Type-test/compliance documentation
- Project approval requirements
Technical Documents
Technical Documents to Request
For technical approval and procurement, request the applicable manufacturer documentation.
- Product datasheet
- Technical specification
- Photometric report
- Beam distribution diagram
- Green colour/chromaticity data
- Threshold-array drawing
- Fixture quantity and spacing
- Light-bar/barrette dimensions
- Runway-width compatibility
- Electrical specification
- Operating current
- Power consumption
- CCR compatibility
- Driver details
- Transformer requirement
- Connector details
- Mounting drawing
- Support details
- Frangibility information
- Installation dimensions
- Cable-routing requirements
- Control/interface diagram
- Monitoring information
- Intensity-step information
- Environmental rating
- Installation manual
- Maintenance manual
- Compliance certificates
- Type-test reports
- Warranty information
Standards & References
Governing Standards
ICAO
The ICAO Store lists Annex 14, Volume I, 9th Edition, July 2022, including Amendment 18 (effective 4 August 2025; aerodrome-design and visual-aids provisions applicable from 27 November 2025, ground handling from 26 November 2026, obstacle limitation surfaces from 21 November 2030). Annex 14 specifies threshold lights as fixed unidirectional green in the approach direction. Evaluate the threshold-light arrangement as part of the applicable approach-lighting system, coordinated with centreline, crossbar, side-row and runway threshold provisions. ICAO also sets minimum threshold-light quantities and spacing by runway category; take these from Annex 14 Chapter 5 rather than from this page.
FAA
MALSR: threshold array of 18 to 33 aviation-green steady-burning lights, depending on runway width, at and parallel to the threshold.
ALSF-2: green threshold lights, red side-row lights, white high-intensity approach lights and sequenced flashers.
AC 150/5345-46F is the active FAA specification for runway and taxiway light fixtures; its L-850E is the in-pavement unidirectional threshold light for use with a medium-intensity approach lighting system. FAA Order JO 6850.2C (Visual Guidance Lighting Systems, August 2022) gives the installation criteria for approach lighting systems such as MALSR and ALSF-2. Derive the exact fixture, electrical configuration, spacing, photometry and approvals from the specified approach system and current FAA project documentation.
India — Also Verify
- DGCA requirements
- Airport-specific AGL specification
- Approved approach-lighting layout and runway threshold layout
- Approach category and ILS requirements
- Project BOQ and Employer’s requirements
- Applicable Indian Standards
- Airport operator requirements
Always verify the governing document edition and project-specific requirements before technical approval, procurement or installation.
Comparison
Approach Threshold Light vs Runway Threshold Light
| Feature | Approach Threshold Light | Runway Threshold Light |
|---|---|---|
| Primary context | Approach-lighting system | Runway visual-aid system |
| Main function | Transition/reference at approach-to-runway interface | Threshold identification |
| Typical colour | Green | Green |
| Typical operation | Steady | Steady |
| System relationship | Centreline/crossbar/side rows | Edge/end/centreline |
| Quantity | System/runway-width dependent | Runway configuration dependent |
| Main design driver | Approach system | Runway configuration |
In many installations the physical threshold-light array performs both functions.
Comparison
Approach Threshold vs Approach Centreline Light
| Feature | Threshold Light | Centreline Light |
|---|---|---|
| Main function | Threshold identification | Longitudinal alignment |
| Location | At threshold | Extended centreline |
| Typical colour | Green | White |
| Main reference | Transverse | Longitudinal |
| System position | Runway interface | Approach field |
Comparison
Approach Threshold vs Crossbar Light
| Feature | Threshold Light | Crossbar |
|---|---|---|
| Location | At runway threshold | Approach field |
| Main function | Threshold indication | Transverse approach reference |
| Typical colour | Green | White (ICAO); system-specific (FAA) |
| Geometry | Along threshold | Perpendicular to centreline |
| System role | Transition | Horizontal/roll reference |
FAQ
Approach Threshold Light — Frequently Asked Questions
What is an approach threshold light?
Approach threshold lights form the threshold-light element of an approach lighting system, giving a clear visual indication of the runway threshold where the approach-light field meets the runway. The threshold is where the aircraft transitions from the approach lighting environment to the runway visual environment, so the array is an important reference alongside the approach centreline, crossbar, side-row and runway lighting.
What colour are approach threshold lights?
Approach threshold lights are aviation green, and FAA MALSR and ALSF-2 both use green threshold lights. Annex 14 specifies threshold lights as fixed unidirectional green in the approach direction. Evaluate the fixture for green colour and chromaticity, intensity and beam distribution, visibility and uniformity across the threshold, and compatibility with the approach lighting system.
What is the FAA MALSR threshold arrangement?
The threshold array consists of 18 to 33 aviation-green steady-burning lights, depending on runway width, in a line at and parallel to the threshold. Together with the seven centreline light bars it forms the approach light field, and the five sequenced flashers continue outward at 200 ft intervals, starting at 1,600 ft from the threshold. Take the exact requirement from the current approved FAA system drawing and specifications.
What is the FAA ALSF-2 threshold arrangement?
ALSF-2 is a high-intensity approach lighting system for Category II/III instrument approaches. Its threshold array is integrated with green threshold lights, white steady-burning centreline elements, red side-row lights and sequenced flashers, and FAA describes a typical ALSF-2 system with 49 green threshold lights in the complete array. Select the threshold array as part of the approved ALSF-2 configuration, not as a standalone product.
How many threshold lights are needed?
There is no single universal number: runway width plus approach system plus approved layout determines the threshold light quantity. FAA MALSR uses 18 to 33 green lights depending on runway width, and ALSF-2 uses 49 in a typical system. ICAO sets minimum threshold-light quantities and spacing by runway category, which should be taken from Annex 14 Chapter 5.
How does an approach threshold light differ from a runway threshold light?
An approach threshold light is the threshold-light element associated with a configured approach lighting system, while a runway threshold light is the runway visual-aid component identifying the threshold within the runway lighting system. On many airports these functions are physically integrated and use the same green array, since FAA MALSR places the threshold array within the approach light field, so the specification should identify the system in which the threshold lights are used.
Which FAA fixture type is used for approach threshold lights?
AC 150/5345-46F is the active FAA specification for runway and taxiway light fixtures, and its L-850E is the in-pavement unidirectional threshold light for use with a medium-intensity approach lighting system. FAA Order JO 6850.2C (August 2022) gives installation criteria for approach lighting systems such as MALSR and ALSF-2. Derive the exact fixture, electrical configuration, spacing, photometry and approvals from the specified approach system and current FAA project documentation.
How are approach threshold lights powered and controlled?
Potential configurations include a dedicated approach-lighting circuit or series circuit, a CCR-fed circuit with a local power supply, isolating transformers and LED drivers, and a control cabinet with intensity control and an ALCMS interface. Control functions include on/off and intensity selection, approach-system activation, remote control and centralized monitoring, and fault indication. FAA approach systems use coordinated approach-lighting controls, not an unrelated runway circuit.
How should the threshold array be installed and aligned?
The array should form a clean, consistent transverse reference. Verify alignment to the threshold line and centreline, fixture spacing and runway width, outer-light position, the relationship to the runway edge lights and approach centreline, and the optical axis, together with fixture elevation, frangibility where applicable and cable routing. Coordinate with runway pavement and threshold marking, drainage, cable trenches, junction boxes and foundations; approved civil and electrical drawings control all installation dimensions.
What are the common faults in approach threshold lighting?
Typical symptoms are the threshold array completely off, an individual light off, an uneven threshold appearance, the wrong colour and a misaligned array. Troubleshoot in sequence: approach control system, CCR or power, primary circuit, transformer or driver, secondary connection, threshold fixture, light source, optical assembly, alignment and photometry. For a system-level problem also verify the threshold line, runway centreline, runway edge, approach centreline, crossbar and side rows.
What should be confirmed before ordering approach threshold lights?
Confirm the runway designation and width, threshold location, approach category, approach lighting system and configuration (MALS, MALSF, MALSR, ALSF or ICAO), number of threshold lights, spacing and array length, green colour and light-source technology, intensity steps, optical distribution, the relationship to centreline, crossbar, side rows and runway edge, mounting and frangibility, and electrical and control details. Request the threshold-array drawing, photometric report, green chromaticity data and compliance documents.
Why Precision AGL Solutions
Approach System and Runway Threshold Interface
We treat the approach threshold light as part of the complete Approach Lighting System and runway threshold interface. Our approach considers:
Approach Category → System Type → Threshold → Centreline → Crossbars → Side Rows → Runway Edge → TDZ → Optics → Supports → Power → CCR → Control → Photometry → Maintenance
This helps keep the threshold array correctly coordinated with the entire runway approach visual system.
Procurement Information for Indian Projects
Indian public tenders commonly ask for the documents below. State which ones your tender requires when you send the enquiry, so they are covered in the quotation.
- Manufacturer / OEM authorisation
- Country of origin and local-content declaration (for Make in India purchase preference)
- Type-test and routine-test certificates, and the FAA / ICAO / IEC compliance documents named in the specification
- Datasheets, drawings and installation instructions
- Warranty, delivery period and spares support
- GeM catalogue or tender-portal requirements, where applicable
Request Threshold Lighting Selection
Planning a new approach lighting system, runway upgrade or approach-lighting rehabilitation project? Submit your runway designation and width, threshold details, approach category, existing approach system, approved layout, centreline, crossbar, side-row and TDZ arrangements, BOQ, technical specification, fixture quantity, electrical/control details and existing fixture information. Precision AGL Solutions can review the approach/threshold configuration and support selection of the appropriate threshold-light arrangement, optical characteristics, mounting system, electrical interface and technical documentation.
- Emailprecisionaglsolutions@gmail.com
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- Phone+91 98214 96412
Engineering Note
Approach threshold lights should be selected as part of the complete approach-lighting system, not simply as a generic green light.
For FAA systems the threshold arrangement differs materially between configurations such as MALSR and ALSF-2: MALSR uses an aviation-green steady-burning array whose quantity varies with runway width, while ALSF-2 uses a larger green array integrated with white centreline lighting, red side rows and sequenced flashers. For ICAO-based projects, the applicable approach category and Annex 14 configuration should control the final arrangement.
Select the final product from the approved approach-lighting layout, applicable governing standard, manufacturer technical data and project-specific requirements.
Product Disclaimer
Product availability, configuration, dimensions, electrical characteristics, photometric performance, certifications and compliance status depend on the selected manufacturer and project specification. The technical values published on this page should not be treated as a substitute for the manufacturer's current approved Technical Data Sheet.
Final selection shall be based on the approved project specification, applicable aviation standards, authority requirements, manufacturer documentation and site conditions. Use of this page is also subject to the Legal Notice, Disclaimer & Terms of Use.
Standards and editions referenced on this site are listed on the Standards Reference page.