Threshold Light

Threshold Light – Light Marking Runway Threshold – Airport Lighting

Overview

Threshold lights are a foundational element of airport runway lighting systems, acting as the definitive visual indicator for the beginning of the runway available for landing. Standardized worldwide, these green lights are critical for pilot situational awareness, especially during night operations, low-visibility conditions, and at airports with complex layouts or displaced thresholds.

How Threshold Lights Work

Threshold lights are installed at the runway threshold—the precise point where landing is permitted. Their main features include:

• Color: Always green from the approach direction, signaling “safe to land” per ICAO Annex 14 and FAA AC 150/5340-30.
• Directionality: Typically unidirectional, visible only to landing aircraft. On bidirectional fixtures, the reverse side emits red, serving as runway end lights.
• Placement: Spaced no more than 3 meters (10 feet) apart, forming a continuous line across the full runway width. Lights are mounted either on the surface or slightly off the edge, depending on airport design.
• Intensity: Adjustable, coordinated with other lighting systems for optimal visibility and to prevent glare.
• Visibility: Must be seen from at least 2 nautical miles on approach under clear weather.

Their unmistakable green glow, distinct from white/yellow edge lights and red runway end lights, creates a universal, high-contrast cue for pilots, supporting safe approaches even when painted pavement markings are not visible.

Displaced Thresholds and Their Lighting

A displaced threshold is used when the intended landing area must be shifted farther down the runway, often for obstacle clearance, noise abatement, or terrain considerations. In such cases:

• The threshold lights are installed at the declared landing start point, not at the pavement’s end.
• The pavement prior to the lights is marked with white arrows or chevrons, indicating it cannot be used for landing but may be used for taxiing, takeoff, or rollout.
• Pilots must never land before the threshold lights, as this area may lack adequate obstacle clearance.

Displaced thresholds are common at airports with challenging approach paths or nearby obstacles, and clear lighting is crucial to prevent regulatory breaches or accidents.

Runway End Lights

Runway end lights are red, unidirectional lights marking the physical end of the runway. Key points:

• Installed in line with threshold lights, across the full width of the runway.
• Typically bidirectional fixtures: green for threshold, red for runway end.
• Serve as a last visual warning for both landings and takeoffs.
• Spaced at intervals not exceeding 3 meters (10 feet).
• Must not be confused with stop bars or taxiway lights.

Their unequivocal red color universally communicates “do not proceed,” enhancing runway safety for all operations.

Runway Edge Lights

Runway edge lights outline the lateral boundaries of the usable runway:

• Color: Mainly white, turning yellow in the caution zone (last 600 meters of instrument runways).
• Spacing: Usually not exceeding 60 meters (200 feet), or less for precision approach runways.
• Installation: On frangible mounts, able to withstand jet blast and snowplows.
• Intensity: Adjustable to adapt to varying visibility conditions.

Edge lights help pilots maintain alignment, prevent excursions, and provide essential distance cues, especially at night or in fog.

Runway End Identifier Lights (REIL)

REILs are high-intensity, synchronized flashing white lights on each side of the runway threshold. Their function:

• Provide rapid, positive identification of the threshold, especially where it is hard to distinguish from background lights or terrain.
• Flash at 60–120 per minute, visible from at least 3 nautical miles.
• Used at non-precision approach runways or where threshold confusion is likely.
• Supplement threshold lights, not replace them.

REILs are essential for quick, unambiguous threshold identification during critical approach phases.

Approach Lighting System (ALS)

The Approach Lighting System extends from the runway threshold into the approach area:

• Aligns with the extended centerline.
• Configuration varies by airport category: complex systems (e.g., ALSF-2) include steady and flashing lights, simpler ones use bars or barrettes.
• Provides alignment, orientation, and glide path cues.
• Integrated with threshold lights, REILs, and runway centerline lights for a seamless visual approach path.

ALS is vital for safe instrument approaches and smooth transition from approach to landing.

Touchdown Zone Lights (TDZL)

TDZLs indicate the optimal touchdown area on precision runways:

• Two rows of white light bars, extending from threshold up to 900 meters or runway midpoint.
• Recessed, durable, and intensity-adjustable.
• Activated when visibility is low or during Category II/III approaches.
• Help prevent excessively long or short landings by providing clear touchdown cues.

TDZLs work with threshold and centerline lights to support precise landings.

Runway Centerline Lights

Runway centerline lights provide continuous guidance along the runway’s length:

• White, in-pavement, spaced at 15 meters (50 feet).
• Last 900 meters alternate red/white; last 300 meters all red, warning of runway end.
• Required for high-category instrument runways and high-traffic airports.
• Intensity-adjustable and rigorously maintained.

They are crucial for alignment and distance judgment during takeoff, landing, and taxiing in low visibility.

Taxiway Lights

Taxiway lighting ensures safe ground movement:

• Blue edge lights outline taxiway boundaries.
• Green centerline lights mark the precise taxi route.
• Lower intensity than runway lights, frangible, and robust.
• Controlled by ATC, intensity can be adjusted or turned off as needed.
• Prevent confusion and runway incursions by using distinct color codes.

Properly lit taxiways are essential for safe and efficient airport operations.

Visual Approach Aids: PAPI & VASI

PAPI (Precision Approach Path Indicator) and VASI (Visual Approach Slope Indicator) provide glide path cues:

• PAPIs: Four lights, red or white depending on approach angle.
• VASIs: Two-bar system, simplified color pattern.
• Visible from 5 NM (day) or 20 NM (night).
• Installed beside the runway, outside the touchdown zone.
• Regularly calibrated for accuracy.

These aids support correct approach angles and work together with threshold lights for optimal landings.

Stop Bar Lights

Stop bars are safety-critical at runway/taxiway intersections:

• Row of red, in-pavement lights at holding positions.
• Supplemented by elevated red lights at the sides.
• Controlled by ATC: illuminated when unsafe to enter runway; extinguished when cleared.
• Mandated at complex or low-visibility airports.
• Strict compliance is required by pilots and ground vehicles.

Stop bars are a frontline defense against runway incursions.

Maintenance and Inspection

Regular maintenance and inspection are vital for all lighting systems:

• Follow ICAO Annex 14 and FAA AC 150/5340-26C guidelines.
• Scheduled checks for alignment, intensity, color, and function.
• Immediate repair or replacement of any faulty or misaligned fixture.
• Modern LEDs reduce maintenance but routine cleaning and inspection remain critical.
• Accurate records are mandatory for compliance and safety audits.

A well-maintained lighting system is foundational to runway and airport safety.

Conclusion

Threshold lights, and the broader family of runway and taxiway lighting systems, are indispensable in ensuring safe, efficient, and globally standardized airport operations. Their green illumination at the runway threshold is a universal signal enabling pilots to identify the precise start of the landing area, regardless of weather or visibility. Together with REILs, ALS, edge, centerline, and touchdown zone lights, they form a cohesive visual environment that supports every stage of aircraft ground movement and approach.

Continuous adherence to international standards, rigorous maintenance, and ongoing technological upgrades are essential to maximize the safety and operational effectiveness of these lighting systems. For airport operators, integrating reliable, energy-efficient lighting is both a regulatory requirement and a vital contribution to global aviation safety.