Sequenced Flashing Lights (SFL) and Airport Lighting

Introduction

Sequenced Flashing Lights (SFL) are a critical part of modern airport infrastructure, providing pilots with dynamic, high-visibility visual guidance during the approach and landing phases, especially under low visibility conditions. These specialized lights, often referred to as the “running rabbit” due to their sequential flashing pattern, are a cornerstone of international airfield safety standards.

This glossary entry will explore the technology, purpose, regulatory requirements, and evolution of SFL and related approach lighting systems. Whether you are an airport operator, aviation professional, pilot, or student, understanding SFL is vital to appreciating the complexity and safety considerations of today’s runways.

What Are Sequenced Flashing Lights (SFL)?

Sequenced Flashing Lights (SFL) are high-intensity lighting fixtures installed along the extended runway centerline in airport approach lighting systems. Their primary function is to provide a rapidly moving, directional visual reference—flashing in a timed sequence from the approach end toward the runway threshold. This creates a moving “chase” effect, guiding pilots’ eyes along the optimal descent path.

These lights are especially valuable in conditions of reduced visibility, such as fog, heavy rain, snow, or nighttime operations, bridging the gap between instrument flight and visual cues required for a safe landing. SFLs are found on runways supporting precision instrument approaches (Category I, II, and III), where precise alignment and depth perception are paramount.

History and Standardization

Sequenced Flashing Lights were first standardized in the 1960s, evolving alongside the increased use of instrument landing systems (ILS) and the growing demand for all-weather airport operations. Their adoption was quickly codified in international and national regulations:

• ICAO Annex 14, Volume I: Sets the global baseline for SFL placement, intensity, sequencing, and frangibility.
• FAA Advisory Circulars (e.g., AC 150/5340-30J): Define the U.S. standards for SFL integration in systems like ALSF-2, ALSF-1, MALSR, and their shortened variants.

The requirements include precise spacing (often 100-foot intervals), intensity, synchronization, and termination at a decision bar (usually 1,000 feet from the threshold).

How Do SFLs Work?

SFLs operate by creating a timed sequence of intense flashes, starting from the farthest approach light and progressing rapidly toward the runway threshold—typically at a rate of two cycles per second. This “running rabbit” or “chasing light” effect is highly conspicuous and draws the pilot’s attention along the correct approach path.

Key Features:

• Spacing: Units are positioned at fixed intervals (100–200 feet) along the centerline extension.
• Flash Sequence: Advanced control cabinets synchronize the flash pattern using digital protocols (CAN bus, RS485).
• Light Source: Modern SFLs use either xenon strobe lamps or high-intensity LEDs.
• Adjustable Intensity: Lighting intensity can be remotely controlled or adjusted automatically based on ambient conditions or pilot request.
• Integration: SFLs are coordinated with other approach lighting components (steady-burning lights, threshold bars, decision bars) for a seamless visual environment.

Types of Approach Lighting Systems Featuring SFL

ALSF-2 (Approach Lighting System with Sequenced Flashers, Type II)

• Runway Category: CAT II/III (precision approaches)
• Features: Up to 15 SFL units, 144 steady-burning centerline lights, side row bars, green threshold lights, and a 1,000-foot decision bar.
• Extension: 2,400–3,000 feet from the threshold.
• Purpose: Maximal guidance for the most demanding approaches.

MALSR (Medium Intensity Approach Lighting System with Runway Alignment Indicator Lights)

• Runway Category: Primarily CAT I, regional/secondary airports.
• Features: 5 RAIL (Runway Alignment Indicator Lights), steady-burning barrettes, simplified layout.
• Extension: Typically shorter than ALSF-2, using fewer SFL units.
• Purpose: Effective visual approach with reduced infrastructure.

SSALR, SSALF, SALSF (Shortened Approach Lighting Systems)

• Purpose: Provide essential approach guidance where space or operational constraints prevent installation of full-length ALSF-2 or MALSR systems.
• Features: 3–7 sequenced flashers, shorter overall system length (1,400–2,000 feet).

SFL Technology: Xenon vs. LED

Xenon SFLs

Traditional SFLs use xenon flash tubes. These produce extremely bright, short flashes by releasing stored electrical energy into a gas-filled lamp. While effective, they require more maintenance and have a shorter lifespan.

LED SFLs

Modern systems increasingly adopt high-powered LEDs. Benefits include:

• Energy Efficiency: Lower power consumption.
• Longevity: Lifespans exceeding 50,000 hours.
• Instant On/Off: Immediate response, no warm-up time.
• Robustness: Less sensitive to vibration and weather.
• Precision Control: Easy adjustment of flash sequence and intensity.

Airports are rapidly retrofitting xenon SFLs with LEDs following FAA and ICAO guidance.

Control and Monitoring

Sequenced Flashing Lights are integrated into the airport’s Airfield Lighting Control and Monitoring System (ALCMS). This allows:

• Real-Time Fault Detection: Instant notification of lamp failures.
• Remote Intensity Adjustment: Set light levels to suit weather and pilot needs.
• Remote Activation/Deactivation: Especially important for non-towered airports with pilot-controlled lighting.
• Compliance Logging: Automatic event and status recording for regulatory reporting.

Frangibility and Safety

All SFL fixtures and supports must be frangible up to 1.2 meters above ground, in compliance with ICAO and FAA standards. This ensures that if struck by an aircraft, the light structure will break away, minimizing the risk of damage or injury.

Frangibility is achieved using lightweight materials, breakaway couplings, and engineered mounts.

SFL in Practice: “Killing the Rabbit”

In clear weather, SFLs can be distracting. Pilots often request SFLs be turned off—referred to as “killing the rabbit.” This is done via ATC or pilot-controlled lighting systems, reducing unnecessary visual clutter while keeping the approach lighting system active.

Portable and Temporary SFL Solutions

Portable SFLs are available for expeditionary, temporary, or emergency airfields. These units feature:

• Battery or solar power
• Lightweight, quick-deploy mounts
• Full compliance with ICAO/FAA intensity and sequencing standards

Such systems are invaluable for military operations, disaster relief, or construction airfields.

Regulatory Framework

• ICAO Annex 14, Volume I: Global specification for SFL placement, performance, and frangibility.
• FAA AC 150/5340-30J & Engineering Brief 67: U.S. design, installation, and transition to LED technology.
• FAA Standard Drawings: Precise layouts and wiring for all ALSF/MALSR variants.

Related Terms

Approach Lighting System (ALS)

A structured array of lights extending from the runway threshold into the approach area, bridging instrument and visual flight. Includes SFL, steady-burning lights, threshold bars, and more.

RAIL (Runway Alignment Indicator Lights)

A short sequence (3–5 units) of SFL typically used in MALSR and SSALR systems for dynamic centerline guidance.

Decision Bar

A transverse bar of lights at 1,000 feet from the threshold, marking where SFL sequences terminate and pilots switch to steady-burning visual references.

Obstruction Lighting

Distinct from approach lighting, used to mark hazards (towers, buildings) with red or white lights—never to be confused with SFL or ALS components.

Pilot-Controlled Lighting (PCL)

At many airports, pilots can activate and adjust SFL and other approach lighting systems remotely via the radio (CTAF), conserving energy and allowing tailored visual cues for each approach.

The Future of SFL

Advances in LED technology, smart controls, and remote monitoring are making SFL systems more efficient, reliable, and adaptable. Airports worldwide are upgrading to meet evolving ICAO and FAA standards, ensuring that dynamic, visual approach guidance continues to enhance airfield safety for decades to come.

Summary

Sequenced Flashing Lights (SFL) are a vital visual aid for pilots, forming the backbone of approach lighting systems at airports worldwide. Their rapid, sequential flashes create unmistakable guidance toward the runway in challenging conditions, reducing the risk of misalignment, missed approaches, and runway incursions. With ongoing innovation in LED technology and digital control, SFLs continue to set the standard for safe, efficient, and reliable airport operations.

Further Reading

• ICAO Annex 14, Volume I – Aerodrome Design and Operations
• FAA AC 150/5340-30J: Design and Installation Details for Airport Visual Aids
• FAA Engineering Brief 67: LED Airfield Lighting
• Avlite AV-SSFL Portable Sequenced Flashers

For customized SFL solutions or to discuss upgrades, contact our experts or schedule a demo .