Runway Threshold Light VS Runway End Light
Same Fixture Line, Two Different Messages — runway threshold lights identify the beginning of the landing portion of the runway, while runway end lights identify the physical termination of the runway. Both are directional, and the same physical runway extremity can be a green threshold from one direction and a red end from the other, which is exactly why the two terms should never be used interchangeably.
At a Glance
| Feature | Runway Threshold Light | Runway End Light |
|---|---|---|
| Main purpose | Identifies the threshold | Identifies the runway end |
| Location | At/near threshold | At/near runway end |
| Pilot approaching for landing | Very important | Also provides end-of-runway reference |
| Typical colour facing approach | Green | Red |
| Colour viewed from opposite direction | Red | Green |
| Directional nature | Yes | Yes |
| Installed across runway width | Generally yes | Generally yes |
| Indicates landing beginning point | Yes | No |
| Indicates runway termination | No | Yes |
Two Names, One Physical Line
The key difference is direction and function. Threshold lights are associated with the runway threshold — the beginning of the portion of the runway available for landing — while runway end lights identify the physical end of the runway. The same transverse line of fixtures can serve both roles depending on which direction it is viewed from.
(Approach A)
(seen by Approach B)
(seen by Approach A)
(Approach B)
What Is a Threshold Light?
Runway Threshold Light
Marks the beginning of the portion of the runway available for landing. When viewed by an aircraft approaching to land, threshold lights are green, forming a transverse line across the runway width.
Tells the pilot
- This is the landing threshold
- Landing may begin from this point
- Runway alignment on approach
Runway End Light
Marks the physical termination of the runway. When viewed by an aircraft approaching the runway end from the runway direction, the lights are red.
Tells the pilot
- This is the end of the runway
- No further runway pavement beyond this point
- A clear stop/termination warning
Threshold = Green · End = Red
This is the fastest way to tell the two functions apart, and it is the first thing every AGL engineer memorises — with one important caveat: the colour depends entirely on which direction the light is viewed from.
Green = threshold when viewed from the approach side. Red = runway end when viewed from the runway side. This is why threshold/end lighting is directional in nature.
The Important Colour Relationship
Same physical fixtures, opposite meaning: the same runway extremity can be a green threshold on approach from one direction, and a red end when viewed from the runway side on approach from the opposite direction. Landing toward the runway, the pilot sees green (threshold); moving toward the runway end, the pilot sees red (end).
What Each Light Answers
Runway Threshold Light
Answers: "Where does the landing portion of the runway begin?" Very important to a pilot approaching to land, since it fixes the start point of the usable landing distance.
Runway End Light
Answers: "Where does the runway terminate?" Provides a clear end-of-runway reference and warning, distinct from the threshold indication.
Threshold/End Lights vs Runway Edge Lights
This is an important distinction. Runway edge lights are installed along the sides of the runway and define runway width. Threshold lights are installed across the threshold and identify the landing threshold. Runway end lights identify runway termination. All three are different functional roles within the same overall runway lighting system.
When Threshold ≠ Physical Runway Start
A particularly important case is a displaced threshold, where the physical runway pavement continues before the threshold but that portion is not available for landing. In this case, the threshold lights identify the point from which the landing portion begins — not necessarily the physical beginning of the paved runway.
Key takeaway: threshold and runway end should not be treated as interchangeable terms. A displaced threshold makes this distinction operationally significant, not just semantic.
Runway Threshold vs Runway End
| Parameter | Threshold Light | Runway End Light |
|---|---|---|
| Function | Identifies threshold | Identifies runway end |
| Pilot landing view | Green | Red |
| Pilot looking toward runway end | Not applicable as threshold indication | Red |
| Position | Threshold | Runway end |
| Directionality | Directional | Directional |
| Transverse arrangement | Yes, per applicable standard | Yes, per applicable standard |
| Relation to runway | Beginning of landing portion | Termination |
| Displaced threshold relevance | Very important | Less affected |
| Main message | Landing starts here | Runway ends here |
Fixtures and Circuit Arrangement
Threshold/end lights can use elevated fixtures, inset fixtures, or unidirectional/bidirectional optical configurations depending on the applicable runway design. The important point is that the optical output must be directed correctly for the required viewing direction.
Simplified path: CCR → series circuit → isolation transformer → threshold/end light. The actual circuit arrangement depends on the runway lighting design and applicable standard.
Common AGL Terminology Confusion
They are very often the same physical line of fixtures at the same runway extremity — the colour and role simply depend on which direction it is being viewed from.
Runway edge lights run along the sides of the runway and define its width. Threshold lights are installed across the threshold and identify the landing threshold — a different function entirely.
Not with a displaced threshold. There, the threshold marks where the landing portion begins, which can be some distance beyond the physical start of the pavement.
Runway Threshold Light vs Runway End Light
Threshold and runway end lights are directional roles, not fixed physical categories. The fundamental difference is which point on the runway is being identified and from which direction it is viewed.
Threshold lighting tells a landing pilot where the usable landing distance begins; runway end lighting tells a pilot moving toward that extremity where the runway physically terminates.
Easy memory: Green → threshold → landing starts here. Red → runway end → runway ends here.
Engineering & Expert Notes
Check Orientation and Colour Seen From Each Direction
OrientationVerify what each approach sees.
Verify Position Against Survey
PositionDisplaced thresholds need accurate records.
Measure Colour
ColourVerify chromaticity.
Walk the Pattern at Night
InspectionCheck for gaps.
Frequently Asked Questions
Function and viewing direction — threshold lights identify the start of the landing portion of the runway and appear green to a landing pilot, while runway end lights identify the physical termination of the runway and appear red when viewed from the runway side.
Yes. The same physical line of lights at a runway extremity is a green threshold when viewed by an aircraft approaching from that direction, and a red end when viewed from the opposite, runway-side direction.
No. Runway edge lights run along the sides of the runway and define runway width. Threshold lights are installed across the runway, transverse to its length, and identify the landing threshold.
A displaced threshold is when the physical runway pavement continues before the threshold, but that portion is not available for landing. The threshold lights mark where the landing portion begins, not the physical start of the pavement — which is why threshold and runway end are not interchangeable terms.
Because the colour presented depends on the direction of view — green when viewed from the approach side, red when viewed from the runway side — so the same fixture line serves two different roles for aircraft operating in opposite directions.
Commonly through an AGL series circuit via a CCR and isolation transformer, though the actual circuit arrangement depends on the runway lighting design and applicable standard.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — runway threshold and runway end lighting colour, siting, intensity and operational requirements, including displaced threshold provisions.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — threshold and runway end lighting configuration, installation, photometric requirements and electrical systems.
FAA AC 150/5345-46
Specification for Runway, Taxiway, Heliport, and Vertiport Light Fixtures — the fixture-level spec (bidirectional green/red lens construction) that threshold and runway end fixtures must be qualified against.
DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Do not design a threshold or runway-end lighting arrangement from a generic web diagram. The final installation drawing should always be developed from the current governing standard, approved airport design and project-specific requirements.
Threshold = green, marks the start of the landing portion. Runway end = red, marks physical termination. The same fixture line can be both, depending on the direction of approach.
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