Runway Threshold and Runway End Lighting System Explained: ICAO Annex 14 Requirements and Practical Engineering Guide

A runway threshold is the beginning of the portion of the runway available for landing. It may be:

• A threshold located at the physical beginning of the runway
• A displaced threshold, where the landing area starts further down the runway due to obstacles, pavement limitations, or operational requirements

Regardless of its location, the threshold marks the beginning of the runway available for landing. It is not the touchdown point: aircraft cross the threshold on approach and touch down further along, in the touchdown zone. Providing a clear visual indication of the threshold is critical for flight safety, especially during night operations and low-visibility conditions.

Where a displaced threshold exists, the threshold lighting must be located at the declared threshold — not the physical beginning of the pavement. Runway edge lights on the pavement before the displaced threshold show red to approaching aircraft.

Runway Threshold Lights are green lights installed across the landing threshold of a runway. Their primary purpose is to indicate:

• The beginning of the landing surface
• The point where the runway available for landing begins
• The usable runway available for landing
• A lateral reference across the full runway width during the final approach

To an approaching pilot, these lights form a continuous line of green lights extending across the runway width, confirming that the aircraft is correctly aligned with the landing surface. Per ICAO Annex 14, threshold lights shall be provided for a runway equipped with runway edge lights, except on a non-instrument or non-precision approach runway where the threshold is displaced and wing bar lights are provided. They are fixed, unidirectional green lights showing in the direction of approach, and the system should:

• Extend across the full width of the runway, either equally spaced between the edge-light rows or in two symmetrical groups
• Be placed in a row at right angles to the runway axis, as near to the threshold as possible and, if outside it, not more than 3 m outside
• Clearly define the landing threshold
• Use a number and spacing of lights that depends on the approach category (non-precision, CAT I, CAT II/III), with closer spacing for the precision categories — take the exact figures from Annex 14 section 5.3.10
• Provide adequate intensity for the intended operational category

Green threshold wing bars — short groups of lights either side of the threshold — add conspicuity. They are used, for example, on precision approach runways and where a threshold is displaced; confirm the exact requirement for your category in Annex 14.

This ensures pilots receive accurate visual guidance regarding the available landing distance, even where the threshold has been displaced.

Runway End Lights mark the extremity of the runway. Unlike threshold lights, runway end lights are fixed, unidirectional red lights showing in the direction of the runway — warning pilots that no further runway is available beyond this point. Annex 14 provides runway end lights for a runway equipped with edge lights and specifies that they shall:

• Be placed in a row at right angles to the runway axis, as near to the end as possible and not more than 3 m outside it
• Consist of at least six lights, either equally spaced between the edge-light rows or in two symmetrical groups with a limited central gap
• On CAT III runways, be closely spaced (not more than about 6 m between lights, apart from any central gap)
• Show red toward the runway, so they are visible to aircraft operating on it

Their arrangement ensures that pilots can easily identify runway termination during the landing roll, take-off roll and rejected take-off.

A row of red lights signifies one thing to a pilot: end of the runway — no usable pavement beyond, so stop or turn off at a taxiway before reaching it.

The colour of these lights carries a specific, internationally standardised operational meaning — one that eliminates ambiguity regardless of airport location.

Where the threshold is at the runway extremity, the same fittings can serve as threshold lights for one direction and end lights for the other: combined threshold/end fittings show green toward the approach and red toward the runway. This bi-directional design reduces the number of fittings while giving pilots the correct information from each direction. Combined units are normally used only where the threshold coincides with the runway end; at a displaced threshold the arrangement differs and bi-directional units are rare.

Proper installation orientation is essential — an incorrectly installed combined fitting may display the wrong colour, creating a significant operational hazard.

Runway threshold and runway end lights must satisfy stringent photometric requirements to ensure consistent visibility under varying weather and ambient lighting conditions:

Annex 14 Appendix 2 gives isocandela diagrams for these light types, and performance is judged against the diagram rather than a single candela value. Modern LED fittings use precision optics to meet these requirements while reducing energy consumption and maintenance.

For AGL engineers, photometric compliance is not simply a testing activity at the end of installation — it is a consideration throughout design, fixture selection, installation, commissioning, and maintenance.

Both threshold and runway end lights are typically connected in an airport series lighting circuit. A standard installation includes:

This configuration ensures every light receives correct operating current while maintaining electrical isolation between fixtures — the electrical system and the lighting fixture must be considered as one integrated AGL system, not independent components.

Successful installation requires more than simply mounting the fittings. Engineers should verify:

• Correct orientation, accurate spacing, and proper elevation
• Secure frangible mounting and correct beam direction
• Waterproof cable connections and tight connector assembly
• Proper earthing and circuit continuity
• Photometric performance verified after installation

A final night inspection should always be performed to verify the visual appearance from both approach and runway directions — particularly important for combined fittings, where the correct colour must be visible from the intended direction, and for displaced thresholds, where the red edge-light zone must face approaching aircraft.

During inspections, engineers frequently encounter:

Most of these issues can be prevented through routine preventive maintenance and periodic inspections. A structured maintenance program should include:

• Routine visual inspections and cleaning of optical surfaces
• Tightening of mounting hardware
• Insulation resistance testing and electrical continuity checks
• Photometric verification and functional testing at all CCR brightness steps
• Replacement of damaged frangible components

Preventive maintenance not only improves system reliability but also reduces unexpected outages during airport operations.

Where the threshold is at the runway end they can share combined fittings, yet threshold and end lights communicate entirely different operational messages depending on the viewing direction.

• Threshold lights are green and mark where the runway available for landing begins; end lights are red and mark where the runway terminates
• A displaced threshold requires the threshold lighting to be at the declared threshold, not the physical pavement edge
• Where the threshold is at the runway end, combined fittings show green toward approach and red toward the runway — orientation errors are a genuine operational hazard
• Photometric compliance (beam spread, intensity, chromaticity, distribution, glare control) must be verified, not assumed
• The CCR, primary cable, isolation transformer, secondary cable, connector, and fitting must be treated as one integrated electrical system
• A structured preventive maintenance program materially reduces unexpected outages during airport operations

ICAO Annex 14, Volume I — Aerodrome Design and Operations: section 5.3.10 (runway threshold and wing bar lights), section 5.3.11 (runway end lights), Appendix 1 (colours) and Appendix 2 (photometric characteristics). Clause numbers change between editions and amendments; quote them from your own current copy and check your State’s regulations and AIP for differences.