How are edge lights placed on wide fillets and apron entries?
Where a taxiway flares into an apron, the pavement edge swings away from the aircraft's path. The blue edge lights have to follow that edge round the fillet and along the apron boundary, closer together on the curve, so the outline stays true and the corner is not cut.
A fillet is the extra pavement that rounds the corner where a taxiway meets an apron or another taxiway. Wide fillets are built generously so long aircraft can turn without the main gear leaving the paved surface, so the pavement edge can sit well away from the path aircraft follow. The edge lights have one job there: show where the usable pavement ends.
1The fillet is a curve
Edge lights on a curve are spaced at less than 60 m so that a clear indication of the curve is provided. Annex 14 does not fix a single figure: the tighter the radius, the closer the lights, with guidance in Doc 9157 Part 4. A wide fillet has a larger radius, so it can sit at wider spacing than a tight one, but it is still closer than the straight-section limit. Design practice normally puts a light where each straight edge becomes the arc, so the change of direction is marked where it starts.
2The apron entry changes the edge line
The line runs along the taxiway, rounds the fillet, then continues along the apron boundary. On a holding bay, de-icing/anti-icing facility or apron, Annex 14 sets uniform spacing of not more than 60 m, the same limit as a straight taxiway. Lights sit as near as practicable to the edge, or outside it by not more than 3 m. Annex 14 also allows edge lights to be left out where surface illumination or other means give adequate guidance.
3What goes wrong
The usual fault is carrying the straight-section spacing across the fillet. The arc is then shown by a few widely spaced lights that read as straight segments, so the corner is poorly defined. Worse is running a straight line of lights across the fillet from one tangent point to the other (the red dashed line): the chord runs over unpaved ground outside the true edge, so the lit line shows pavement that is not there. The second is position: a light inside the edge sits in the aircraft's path, and one set too far outside stops being a reliable edge cue.
4How it fits with the rest
Edge lights mark the pavement; the green centreline lights mark the path across it, so the two are laid out together. Decide at design stage which circuit and switching section the fillet and apron-entry lights belong to, so a fillet is not left half lit when an adjacent section is switched off or a circuit is lost.
| Location | Edge-light arrangement | Purpose |
|---|---|---|
| Straight taxiway edge into the fillet | Uniform, ≤60 m | Continuous edge line |
| Wide fillet (curve) | <60 m, closer as radius shrinks; light at each end of the arc | Outline the true arc, not a chord |
| Apron entry and apron boundary | Uniform, ≤60 m | Outline the apron edge |
| Lateral position, all of the above | At the edge, or ≤3 m outside it | True outline, clear of the aircraft path |
| Adequate guidance by other means | May be omitted | Annex 14 allowance; confirm for your operation |
Frequently Asked Questions
Q1What is a wide fillet?
A fillet is the extra pavement that rounds the corner where a taxiway meets an apron, a junction or another taxiway. A wide fillet is built generously so long aircraft can turn without the main gear leaving the paved surface, which means the pavement edge can sit well away from the path the aircraft actually follows.
Q2How close should edge lights be on a fillet?
Closer than the straight-section maximum. ICAO Annex 14 calls for less than 60 m on a curve so that a clear indication of the curve is provided, but it does not fix one figure: the tighter the radius, the closer the lights. A wide fillet has a larger radius than a tight one, so it can use wider spacing, but it still has to be below 60 m. Guidance is in Doc 9157 Part 4 and, for FAA work, the radius table in AC 150/5340-30.
Q3Why not keep the 60 m spacing straight across the fillet?
Because the arc would be shown by a few widely spaced lights that read as straight segments, and a straight line of lights from tangent point to tangent point would cross unpaved ground outside the true edge. Either way the lit line no longer shows where the pavement ends. Closer spacing on the curve keeps the outline true.
Q4How are apron entries different from an ordinary taxiway edge?
The edge line has to change character. It runs along the taxiway, rounds the fillet and then continues along the apron boundary. Annex 14 spaces edge lights on a holding bay, de-icing/anti-icing facility or apron at uniform intervals of not more than 60 m, the same limit as a straight taxiway. Annex 14 also allows edge lights to be omitted where surface illumination or other means give adequate guidance, so check what the operation needs.
Q5How far from the pavement edge should the lights sit?
As near as practicable to the edge, or outside it by not more than 3 m. This is the same rule as on the straight sections. A light inside the edge sits in the aircraft's path, and one set too far outside stops being a reliable cue for where the pavement ends.
Q6Should fillet and apron-entry lights share the taxiway's circuit?
That is a design decision, not a default. Assign them deliberately so that a fillet is not left half lit when an adjacent section is switched off or a circuit is lost, and so the control system reports them against the section they belong to. See the taxiway circuit arrangement entry for how circuits are laid out.
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