Sequenced Flashing Light System for Airfield Ground Lighting

What It Is

Instead of a continuous steady-burning pattern, the system flashes individual units in sequence. This helps pilots identify the approach-lighting system at greater range and supports approach-path identification, runway alignment, the transition from instrument to visual flight, early recognition of the approach axis and improved conspicuity against high background lighting.

FAA describes the effect as a ball of light travelling toward the runway.

Selected According To

• Approach lighting system
• Runway classification and approach category
• System length
• Number of flasher stations and spacing
• Flash sequence, rate and synchronization
• Light intensity, optical direction and beam distribution
• Control architecture, power supply and monitoring
• Mounting arrangement and environmental conditions
• Project technical specification

Why the Motion Matters

The principal purpose is an early and conspicuous dynamic reference toward the runway. A steady-burning light creates a static pattern; the sequence adds a moving cue that helps the pilot identify the correct system and runway alignment.

FAA states that MALSR sequenced flashers appear as a ball of light travelling toward the runway.

Sequence Progression

Steady-Burning Approach Lights

• Continuous longitudinal reference
• Horizontal reference
• Approach geometry
• Runway alignment

Sequenced Flashing Lights

• Dynamic identification
• Long-range conspicuity
• Directional movement cue
• Additional lead-in information

Simplified Concept

════ RUNWAY ════
      THRESHOLD
          │
    ●  FLASH 5   (innermost)
    ●  FLASH 4
    ●  FLASH 3
    ●  FLASH 2
    ●  FLASH 1   (outermost)
        APPROACH

Notes

Flashes occur sequentially from the outer approach area toward the runway. The exact number of flashers and their spacing depend on the selected approach-lighting system.

One of the Most Important Characteristics

For the principal runway approach systems the sequence progresses outermost → innermost → threshold, creating apparent motion toward the runway.

Standards Basis

ICAO precision approach provisions specify sequential flashing beginning with the outermost light and progressing toward the threshold. FAA approach systems similarly operate the flashers from the outer approach area toward the threshold.

Controlled Accurately

FAA guidance for systems such as MALSF identifies sequenced flashers operating at approximately twice per second, meaning the whole sequence repeats about 120 times per minute, with each flasher firing once per cycle. FAA AC 150/5345-51B sets different rates for unidirectional and omnidirectional flashers (for example, runway end identifier lights), so take the rate for your flasher type from the current edition of that document and the approved system drawing.

Take the Exact Characteristic From

• Applicable system standard
• Current FAA / ICAO requirement
• Approved project drawing
• Manufacturer specification

System-Specific Stationing

For example, FAA MALSR flashers (typically five, up to eight) are spaced at 200 ft intervals, beginning 1,600 ft from the threshold. ALSF configurations use different arrangements and stationing according to the applicable system.

Final Spacing

The final spacing must be taken from the approved system drawing.

Normally White

The flashing element is normally associated with the white approach-light field. FAA identifies the flashers in systems such as MALSR and ALSF-2 as white flashing lights.

Colour Verification

• Chromaticity
• White output
• Consistency between flashers
• Intensity
• Optical distribution

The flasher colour must remain visually consistent with the approach-lighting system.

The Optical System Should Provide

• Appropriate horizontal coverage
• Appropriate vertical coverage
• Correct main beam direction
• Consistent output between stations
• Visibility from the intended approach sector
• Controlled glare / spill

FAA MALSF guidance aims the steady and flashing lights with beam axes parallel to the runway centreline and intersecting the assumed approach glide-slope geometry.

Photometric Review Considers

• Horizontal and vertical beam spread
• Main beam axis
• Approach angle
• Aircraft height and viewing distance
• Background lighting
• Threshold relationship

Each flasher must be clearly visible from the intended approach area and the sequential effect must remain continuous.

Intensity Selection Depends On

• Approach-lighting system and approach category
• Ambient background lighting and visibility
• Optical design and flasher type
• Control configuration

FAA material notes effective intensity can require adjustment in high-background-light environments. Coordinate settings with the steady-burning elements.

Control Characteristics

The flashing subsystem may differ from the steady lights. FAA inspection guidance states the three MALSF flashers do not have their own intensity control and follow the steady-burning lights' settings.

Establish flasher intensity, available steps, relationship to steady-burning lighting, CCR / control architecture and independent or shared intensity control.

The Controller Must Maintain

• Correct order and timing
• Correct synchronization
• Correct intensity
• Reliable triggering
• Fault monitoring

A timing fault can make the system appear static, irregular or out of sequence.

Synchronization

• Correct sequence order
• Consistent interval and repeatable cycle
• Stable timing
• No unintended simultaneous operation
• Correct restart after power interruption

Verify synchronization during commissioning where several independent flasher modules are used.

May Include

• Dedicated flasher circuit
• CCR supply or dedicated power supply
• Sequencer
• Individual flasher channels
• Control cabinet
• Monitoring circuit
• ALCMS interface

Simplified Concept

ICAO Provision

For certain precision approach systems, ICAO requires the electrical circuit to be arranged so the flashing lights can be operated independently of other approach-lighting lights.

The Technical Submission Should State

• Flasher and steady-light circuit arrangement
• Control relationship and CCR allocation
• Fault isolation
• Maintenance isolation

This can simplify testing and fault diagnosis.

May Interface With

• AGL control panel
• Approach-lighting control cabinet
• ALCMS and CCR
• Sequencer / controller
• Local maintenance panel
• Remote monitoring
• Airport operations system

Potential Functions

• ON / OFF and intensity selection
• Sequence activation
• Status monitoring and fault indication
• Remote control
• Maintenance mode
• Alarm reporting

Potential Advantages

• Fast switching
• Long service life
• Stable optical output
• Reduced maintenance
• Consistent colour
• Lower energy consumption
• Precise electronic timing
• Diagnostic capability

The complete flasher must satisfy optical, timing, intensity, electrical, mechanical and environmental performance.

Switching Parameters

• Rise / fall time
• Pulse duration
• Flash intensity
• Timing accuracy
• Repetition rate
• Thermal management
• Driver performance
• EMC performance
• Surge protection

Obtain values from the manufacturer's technical documentation.

Installation Forms

• Elevated fixtures
• Light-bar assemblies
• Low-level approach fixtures
• Dedicated flasher heads
• Frangible-mounted equipment
• Modular LED units

Coordinate supports with approach geometry, frangibility, height, beam axis, wind loading, ground conditions, cable routing and maintenance access.

Approach Area Geometry

• Runway extended centreline
• Approach-lighting stationing
• Glide-slope reference
• Light-support elevation
• Obstruction environment
• Cable routes and drainage

FAA MALSF layout guidance specifies the optimum approach-light plane, slope tolerances and beam-aiming requirements.

May Include

• Frangible supports
• Low-impact structures
• Breakaway mounting
• Controlled fixture elevation

FAA Projects

Select support structures according to the applicable FAA low-impact / frangibility requirements for approach lighting.

Potential Sources

• City lighting
• Roads and buildings
• Industrial facilities
• Airport apron lighting
• Stadiums
• Commercial development

Evaluation

Evaluate the system for sufficient conspicuity under the actual operational environment.

The Control System Should Consider

• Flasher and controller failure
• Power failure
• Timing and communication failure
• Channel failure
• Loss of synchronization

Operational Effect

The effect of individual failures depends on the governing standard and approach category. FAA material notes that the effect of inoperative SFL / RAIL equipment on approach serviceability depends on approach category and system configuration.

Flash-Rate Testing

• Flash rate
• Inter-flasher timing
• Pulse duration
• Repeatability

Photometric Testing

• White colour
• Intensity
• Beam distribution and optical direction
• Uniformity and visibility

Timing

• Sequence and flash rate
• Synchronization
• Missing flashes
• Delayed channels

Control

• ON / OFF and intensity
• Remote status
• Fault indication
• Alarm reporting

Do Not Treat as Identical

RAIL and sequenced flashing lights are closely related in some FAA approach systems but should not be treated as universally identical products.

The FAA System Designation Determines

• Number of units
• Location and sequence
• Operational effect
• Approach-category implications

FAA inspection guidance distinguishes SFL and RAIL and describes different consequences for inoperative units by approach category.