"What is the purpose of an Approach Lighting System (ALS)?"

"An Approach Lighting System provides visual cues to pilots during the final approach phase of landing, especially in low visibility conditions. It helps them align with the runway, judge distance, and safely transition from instrument-guided to visual navigation, as required for landing."

"How does ALS affect approach minimums?"

"The presence of a functioning ALS can lower the required visibility minimums for instrument approaches. If the ALS is partially or fully inoperative, approach minimums (such as visibility requirements) must be increased as regulated by ICAO and FAA standards."

"What are the main types of ALS?"

"The main types include Simple ALS (SALS), Precision Approach CAT I ALS, CAT II/III ALS, and various FAA-specific systems like ALSF-1, ALSF-2, MALSR, and ODALS. Each type is designed for specific visibility and operational needs."

"What is the difference between ALS and runway edge lights?"

"ALS is a dedicated system for approach guidance before the runway threshold, while runway edge lights mark the edges of the runway itself. ALS extends into the approach area and is critical for aiding pilots during instrument approaches."

"Can pilots control ALS intensity?"

"At many airports, especially those without control towers, pilots can adjust the intensity of ALS using pilot-controlled lighting systems by keying their microphone on a specific radio frequency."

Approach Lighting

Approach Lighting Systems (ALS) are precision-arranged lights at runway approaches, enabling pilots to transition safely from instrument to visual references, particularly in poor visibility. ALS configurations vary for different approach categories and are vital for instrument landings.

Aviation Airports Runway Lighting Safety

Approach Lighting – Comprehensive Guide to ALS at Airports

Definition

Approach Lighting System (ALS):
An Approach Lighting System (ALS) is a structured configuration of high-intensity lights installed symmetrically along the extended centerline of a runway, beginning at the threshold and extending into the approach area. ALS provides essential visual guidance to pilots during the critical transition from instrument flight (using navigation instruments) to visual flight during landing, especially under low visibility conditions such as fog, rain, or at night.

ALS is internationally regulated and standardized by the International Civil Aviation Organization (ICAO) Annex 14 and nationally by the Federal Aviation Administration (FAA) in the United States. It is a crucial safety aid for runways supporting instrument approach procedures (IAP), including all categories of Instrument Landing System (ILS) approaches.

Key functions:

Provides directional, distance, and orientation cues.
Enhances runway and threshold identification.
Facilitates safe pilot transition from instrument references to visual cues.

References: ICAO Annex 14 , FAA AIM 2-1-3 , Wikipedia .

Purpose and Operational Role

Purpose

The ALS is engineered to penetrate atmospheric obscurants (fog, precipitation, haze) and provide visual guidance far enough from the runway to allow pilots to align, descend, and orient properly for a safe landing. The system is mandatory on runways with precision instrument approaches and directly affects the minimum visibility required for landing.

Directional Guidance: Assists pilots in aligning with the runway centerline.
Distance Indication: Provides cues to judge distance from the runway threshold.
Descent Path Awareness: Helps maintain the correct glidepath.
Orientation: Allows pilots to perceive lateral and vertical position relative to the runway.

Operational Role

Instrument-to-Visual Transition: ALS is typically the first airport lighting system pilots see during approach, marking the point where they transition from instrument scanning to visual cues. This is vital for low-visibility and precision approaches.
Runway Recognition: The arrangement and color coding of ALS components help distinguish the runway, its axis, and critical points such as the threshold and touchdown zones.
Regulatory Compliance: The existence and operational status of ALS determine approach minimums and are regulated by ICAO and FAA.

Example: According to FAR §91.175 , pilots may descend below decision altitude if the approach lights are visible, but not below 100 feet above the touchdown zone unless specific runway features are also visible.

System Configuration and Components

ALS systems follow precise international and national standards for uniformity and safety. The configuration depends on runway category, visibility requirements, and operational needs.

Main Components:

Centerline Lights: White lights along the runway’s extended centerline, typically spaced at 30 meters for precision approaches.
Crossbars: Rows of lights perpendicular to the centerline at specified intervals (e.g., 150, 300, 450 meters), providing lateral orientation.
Side Row Lights: Red lights parallel to the centerline for high-category (CAT II/III) approaches, extending up to 270 meters from the threshold.
Sequenced Flashing Lights (SFL): High-intensity white strobe lights flashing in sequence toward the threshold (“the rabbit”), aiding orientation and depth perception.
Runway Alignment Indicator Lights (RAIL): Subset of SFL, terminating at the first steady-burning ALS lights.
Barettes: Closely spaced lights forming bars for enhanced visibility.
Decision Bar: Prominent crossbar, usually 300 meters from the threshold, acting as a visual decision point.

References: ICAO Annex 14 , FAA Order JO 6850.2C .

Types and Configurations

ALS designs vary based on required precision, visibility, and runway operations. The main types as defined by ICAO and FAA include:

1. Simple Approach Lighting System (SALS)

Application: Non-precision approaches.
Length: Minimum 420 meters from threshold.
Arrangement: Single row of white centerline lights at 60-meter intervals, one crossbar at 300 meters.
Reference: ICAO Annex 14 .

2. Precision Approach Category I (CAT I) ALS

Application: ILS CAT I approaches.
Length: 900 meters from threshold.
Arrangement: Centerline lights at 30-meter intervals, crossbars at 150, 300, 450, 600, and 750 meters.
Reference: ICAO Annex 14 .

3. Precision Approach Category II/III (CAT II/III) ALS

Application: ILS CAT II/III approaches (lowest visibility).
Length: 900 meters from threshold.
Features: White centerline and crossbar lights; red side row bars up to 270 meters from threshold; SFLs; barrettes.
Reference: ICAO Annex 14 .

4. FAA Standard ALS Types

System	Length	Features	Application
ALSF-1	2,400 ft/732 m	White centerline/crossbar lights, SFL	CAT I precision
ALSF-2	3,000 ft/914 m	White & red lights, SFL, red side rows	CAT II/III precision
MALSR	2,400 ft/732 m	Medium-intensity, RAIL, SFL optional	Precision/non-precision
ODALS	1,500 ft/457 m	Omnidirectional flashing lights	Non-precision approaches

References: FAA AC 150/5340-30J , Wikipedia .

Technical Specifications and Visual Features

Light Colors and Functions

White: Centerline and crossbars.
Red: Side row bars and terminating bars (CAT II/III).
Green: Threshold lights (not part of ALS but vital for landing).
SFL: Sequenced white strobe lights for depth and direction.

Spacing and Arrangement

Centerline: 30 meters for precision, 60 meters for simpler systems.
Crossbars: Perpendicular to centerline at 150, 300, 450 meters, etc.
Side Rows: Red, up to 270 meters from threshold in CAT II/III.

Intensity and Control

High-Intensity: Used for CAT I/II/III approaches; intensity is adjustable.
Medium/Low Intensity: Used for non-precision runways.
Pilot-Controlled Lighting: At many airports, pilots can activate ALS via radio (PCL).

References: FAA AIM 2-1-3 , Wikipedia .

Visual Examples

Operational Guidance and Regulatory Context

How ALS Aids Pilots

Visual Acquisition: ALS is often the first airport visual aid seen by pilots in IMC, providing early alignment and orientation cues.
Descent Below Minimums: Under FAR §91.175 , pilots may descend below minimums if the ALS is in sight (excluding SFLs), but not below 100 feet above the touchdown zone unless further runway features are visible.
Missed Approach: If ALS is not visible at minimums, a missed approach is required.

System Outages

NOTAMs: ALS outages must be published as NOTAMs, specifying type and operational impact.
Minimums Adjustment: Inoperative ALS may require increased visibility minimums as per regulatory tables.

Regulatory Standards

ICAO Annex 14: International ALS standards.
FAA AC 150/5340-30J: U.S. ALS design, installation, and maintenance.

References: ICAO Annex 14 , FAA AIM 2-1-3 .

Historical Background and Evolution

Early Development

ALS originated during World War II at RAF Drem, Scotland, and Arcata-Eureka Airport, California. Early systems were basic but established the importance of structured lighting for safe instrument landings.

Technological Advances

Postwar Standardization: Adoption of strobe lighting, standardized arrays, and frangibility (breakaway structures for safety).
Modernization: Integration of LEDs, computer-based control, and solar power for efficiency and reliability.

References: Wikipedia .

ALS works in conjunction with other airport visual aids:

Runway Edge Lights: Define lateral runway limits.
Threshold Lights: Green lights marking the beginning of the runway.
Runway End Identifier Lights (REIL): Flashing lights for runway end identification.
Visual Approach Slope Indicator (VASI) and Precision Approach Path Indicator (PAPI): Vertical guidance for approach.
Touchdown Zone Lights (TDZL): Illuminate landing area.
Taxiway Lighting: Blue or green lights for taxi guidance.

References: Wikipedia , ICAO Annex 14 .

Conclusion

Approach Lighting Systems are critical visual aids enabling safe landings under all weather conditions. Standardized internationally, ALS technology continues to evolve, integrating advanced lighting, control, and safety features to support modern aviation operations. For airports, compliance with ICAO and FAA ALS requirements is essential for operational safety and regulatory approval.

References: ICAO Annex 14 , FAA AIM 2-1-3 , Wikipedia .

Frequently Asked Questions

What is the purpose of an Approach Lighting System (ALS)?: An Approach Lighting System provides visual cues to pilots during the final approach phase of landing, especially in low visibility conditions. It helps them align with the runway, judge distance, and safely transition from instrument-guided to visual navigation, as required for landing.
How does ALS affect approach minimums?: The presence of a functioning ALS can lower the required visibility minimums for instrument approaches. If the ALS is partially or fully inoperative, approach minimums (such as visibility requirements) must be increased as regulated by ICAO and FAA standards.
What are the main types of ALS?: The main types include Simple ALS (SALS), Precision Approach CAT I ALS, CAT II/III ALS, and various FAA-specific systems like ALSF-1, ALSF-2, MALSR, and ODALS. Each type is designed for specific visibility and operational needs.
What is the difference between ALS and runway edge lights?: ALS is a dedicated system for approach guidance before the runway threshold, while runway edge lights mark the edges of the runway itself. ALS extends into the approach area and is critical for aiding pilots during instrument approaches.
Can pilots control ALS intensity?: At many airports, especially those without control towers, pilots can adjust the intensity of ALS using pilot-controlled lighting systems by keying their microphone on a specific radio frequency.

Enhance Airfield Safety

Ensure your airport meets international safety and operational standards with modern approach lighting systems. Reliable ALS reduces risk and enables safe landings in all conditions.

Learn more