Blue Edge Light VS Green Centreline Light
Boundary vs Path — the blue taxiway edge light defines the lateral limits of the taxiway, while the green taxiway centreline light provides directional guidance along the intended taxi route. Colour is the fastest way to tell them apart, day or night: blue means "stay within these limits," green means "follow this path."
At a Glance
| Feature | Blue Edge Light | Green Centreline Light |
|---|---|---|
| Colour | Blue | Green |
| Location | Along taxiway edges | Along taxiway centreline |
| Main purpose | Defines taxiway boundary | Provides directional guidance |
| Pilot message | "Stay within these limits" | "Follow this path" |
| Direction | Lateral reference | Longitudinal reference |
| Typical fixture | Elevated or inset | Mainly inset |
| Use on curves | Defines outer boundary | Guides aircraft around curve |
| Low visibility | Boundary identification | Primary route guidance |
| Easy meaning | EDGE | PATH |
What Is a Blue Edge & Green Centreline Light?
Blue Edge Light
Identifies the lateral limits of the taxiway. The pilot uses them to answer: "Where are the sides of the taxiway?"
Provides a reference for
- Taxiway boundary identification
- Lateral limits, not a route to follow
- Where the taxiway physically is
- Boundary awareness in low visibility
Green Centreline Light
Provides a visual indication of the taxi route centreline. The pilot uses them to answer: "Where should I steer?"
Provides a reference for
- The intended taxi path
- Steering guidance along the route
- Guidance through curves and intersections
- Primary route guidance in low visibility
Blue = Edge · Green = Centreline
This is the fastest way to distinguish the taxiway boundary from the taxi route, and the easiest thing to memorise: blue → edge → boundary, green → centreline → guidance.
On a straight section, blue lights run down both sides of the taxiway while a single line of green lights runs down the middle — width on the outside, path in the middle.
How They Sit on the Taxiway
The blue lights establish the width and limits of the taxiway. The green lights establish the route to follow within it — both pieces of information are given to the pilot at once.
Path vs Boundary
The simplest way to remember it: green centreline lights answer "where should I steer?" while blue edge lights answer "where should I stay within?" Do not read blue as "stop" or green as "permission to taxi" — they are lighting identification and guidance colours, not clearance. Operational clearance still comes from the applicable ATC procedures, markings, signs and other aerodrome controls.
Elevated & Inset Fixtures
Blue edge lights may use an elevated fixture positioned beside the taxiway, since they sit outside the wheel path. Green centreline lights are commonly inset, because aircraft may pass directly over the centreline itself.
| Installation | Blue Edge | Green Centreline |
|---|---|---|
| Typical fixture | Elevated or inset | Mainly inset |
| Why | Sits clear of aircraft wheel paths | Aircraft may pass directly over it |
| Curves | Defines outer boundary | Guides aircraft around curve |
How Each System Behaves Along the Route
At a Curve
Blue shows where the taxiway boundary curves. It continues to define the lateral limits, but does not itself describe the path an aircraft should follow through the turn.
At a Curve
Green shows the aircraft's intended centreline path around the curve — a continuous line the pilot can track through the turn.
At intersections, centreline lighting can indicate the intended taxi route while edge lights continue to identify taxiway boundaries. Other systems, such as stop bars and runway guard lights, may provide additional runway-incursion protection.
Important exception near runway holding positions: where a taxiway centreline continues toward a runway, the taxiway centreline lights are enhanced by alternating green and yellow lights for a defined distance before the runway holding position (enhanced taxiway centreline lighting). This gives the pilot an additional visual cue that the runway environment is close, and is separate from the plain green centreline lighting used elsewhere on the taxiway. Blue edge lighting does not carry an equivalent colour change.
Which Matters More When Visibility Drops?
Both systems become especially valuable when visual references are reduced, but they serve different roles. Blue edge lights provide awareness of the taxiway limits. Green centreline lights provide a much clearer visual route for aircraft movement — for basic directional guidance, the green centreline is generally the more direct aid.
Easy memory: green = where to taxi, blue = where the taxiway is. In low visibility, the pilot leans on the green centreline to know where to go, and the blue edge to know the limits they must stay inside.
What Each Light Answers
| Question | Light |
|---|---|
| Where are the taxiway limits? | Blue Edge |
| Where is the taxi route? | Green Centreline |
| Where should I steer? | Green Centreline |
| Where should I stay within? | Blue Edge |
| What defines the taxiway sides? | Blue Edge |
| What provides longitudinal guidance? | Green Centreline |
Blue Edge Light vs Green Centreline Light
| Parameter | Blue Edge Light | Green Centreline Light |
|---|---|---|
| Primary function | Boundary | Guidance |
| Position | Taxiway edge | Taxiway centre |
| Visual reference | Lateral | Longitudinal |
| Pilot interpretation | Taxiway limits | Taxi route |
| Typical fixture | Elevated/inset | Inset |
| Curves | Defines boundary | Guides path |
| Intersections | Boundary reference | Route guidance |
| Low visibility | Important | Very important |
Common Colour Confusion
They are primarily lighting identification and guidance colours in this context, not clearance signals. Operational clearance still comes from the applicable ATC procedures, markings, signs and other aerodrome controls.
They serve different purposes. Blue edge lights define the lateral limits of the taxiway; green centreline lights provide guidance along the intended taxiway path — one is a boundary reference, the other is a route reference.
Green centreline lights are commonly inset because aircraft may pass directly over the centreline. Blue edge lights, sitting outside the wheel path, may be elevated or inset.
Blue Edge Light vs Green Centreline Light
Blue edge and green centreline lights are complementary, not interchangeable, systems. One defines where the taxiway ends; the other defines where to steer within it.
Together, edge lighting keeps the aircraft within the taxiway limits while centreline lighting keeps it on the designated taxi path — a pilot relies on both for safe ground movement, especially in low visibility.
Easy memory: Blue → edge → boundary. Green → centreline → guidance.
Engineering & Expert Notes
Colour Verification Is Part of Acceptance
ColourConfirm chromaticity with instruments, since similar-looking units can differ.
Check the Pattern, Not Just Individual Lights
LayoutSpacing and position define the guidance. A missing light changes what the pilot sees.
Inspect From the Cockpit Viewpoint
VisibilityWalk or drive the taxi route at night and check the view from the crew's eye position.
Avoid Swapping Parts Between Types
SparesColour filters and optics differ. Use approved parts for each type.
Frequently Asked Questions
Colour and function — blue edge lights define the lateral limits of the taxiway, while green centreline lights provide guidance along the intended taxi route.
No. These are primarily lighting identification and guidance colours, not stop/go signals. Operational clearance still comes from the applicable ATC procedures, markings, signs and other aerodrome controls.
Because aircraft may pass directly over the centreline. Inset fixtures avoid interfering with wheel contact, which is especially important at intersections and other crossing points.
Green centreline lighting is generally the more direct aid for basic route guidance, while blue edge lighting provides the complementary boundary reference.
Yes. Edge lights continue to define the lateral limits around curves and at intersections, while centreline lights provide the continuous path to follow through them.
Neither — stop bars and runway guard lights are separate systems that may provide additional runway-incursion protection alongside centreline lighting at intersections.
Not always. On a taxiway centreline that continues toward a runway, the lights are enhanced with alternating green and yellow for a defined distance before the runway holding position, warning the pilot that the runway environment is close. Plain green is used on centreline sections away from this transition. Blue edge lighting has no equivalent colour change.
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 — taxiway edge and centreline lighting colour, siting, intensity and operational requirements.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — taxiway edge and centreline lighting configuration, installation, photometric requirements and electrical systems.
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 taxiway 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.
Blue edge = where the taxiway is. Green centreline = where to taxi — two complementary systems, not one.
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