Elevated versus inset fixtures
One stands above the ground and breaks away if struck. The other sits in the pavement and takes the wheels. Where the light goes decides which one it is.
An elevated fixture is a lamp head on a short riser, mounted on a frangible coupling above a light base set in the ground. It stays low for propeller and engine-pod clearance and is designed to break at the coupling rather than damage an aircraft. An inset fixture is set flush in the pavement on a shallow base, so aircraft and vehicles can roll over it and it needs no frangible riser.
The choice follows position. Where wheels run, as with runway centreline and touchdown zone lights, the light has to be inset. Beside the pavement, such as runway edge, threshold and end lights, an elevated fixture is usual, with inset used where the edge is paved or elevated fixtures are unsuitable. Elevated is quicker to install and replace; inset trades a more involved installation for a clear surface.
Above grade, on a frangible coupling. Quick to install and change, but it is an object that can be struck or buried.
Flush in the pavement, carrying wheel loads. No obstruction, but harder to install, clean and re-set.
| Aspect | Elevated | Inset |
|---|---|---|
| Position | Beside the pavement or at its edge | Within the pavement, where aircraft run |
| Typical uses | Runway edge, threshold and end lights, approach lights, taxiway edge | Runway centreline, touchdown zone, taxiway centreline, in-pavement edge and stop bars |
| Height | Low profile, about 0.36 m or less (FAA edge-light practice) | Minimal, about 13 mm or less above surface (FAA practice) |
| Frangibility | Required: breaks at the coupling | Not required: not a raised object |
| Wheel loads | Not designed to be driven over | Built to carry aircraft wheel loads |
| Snow and ice | Can be buried or struck by clearing equipment | Below the plough, but window can be covered by compacted snow or ice |
| Dirt and rubber | No tyre rubber, but dust, mud splash and snow still soil the lens | Rubber, water and de-icer collect on the window |
| Installation | Simpler: base set in ground beside the pavement | More involved: base set into pavement structure |
| Maintenance | Head replaced from above; coupling replaced after a strike | Window cleaning, height re-set after resurfacing, often under a closure |
| Power | Series circuit via isolation transformer (both). Elevated: transformer sits in the base can below the head. Inset: in a deep base under the light, or in a separate can beside the pavement with secondary leads to the fixture. | |
What decides elevated or inset
Where traffic runs
If aircraft or vehicles drive over the position, it has to be inset. If not, elevated is usually simpler.
Obstruction and frangibility
A raised light must be low and frangible. Check height and the breakaway point against the standard.
Snow and climate
Consider drifts and ploughs for elevated lights, and compaction and ice on inset windows. Check heating options.
Pavement works
Inset bases must be re-set after resurfacing. Plan for this in the pavement maintenance programme.
Access for maintenance
Inset work often needs a closure. Elevated work is usually quicker, but check strike damage after incidents.
Standard and design
Follow ICAO Annex 14 and the aerodrome design for the required type at each position.
Elevated versus Inset: Key Points
An elevated light is a low lamp head on a frangible coupling beside the pavement: quick to install and replace, but it can be struck or buried by snow. An inset runway light is set flush in the pavement on a shallow base, so aircraft and vehicles can roll over it and it needs no frangible riser. Where wheels run, as with runway centreline and touchdown zone lights, the light has to be inset. Installation is more involved than for an elevated light because the base is set into the pavement structure. In service the window needs cleaning, and the height must be re-set after resurfacing, often under a closure. Confirm the requirement against the applicable standard (ICAO / FAA) and the manufacturer's data.
Frequently Asked Questions
Q1What is the difference between an elevated and an inset light?
An elevated light stands above ground on a short frangible riser, beside the pavement or at its edge. An inset (flush) light is set into the pavement surface so aircraft and vehicles can roll over it. Both are fed from the same kind of series circuit through an isolation transformer.
Q2Why are runway centreline and touchdown zone lights inset?
Because they sit where aircraft wheels run. An elevated light there would be an obstruction and a hazard, so these lights are built into the pavement and carry wheel loads. The application and standard decide the exact requirement.
Q3Why must elevated lights be frangible?
A raised fixture is an object an aircraft could strike. It is kept low for propeller and engine-pod clearance and mounted on a frangible coupling designed to break at a set point, so the aircraft is not damaged. The base, transformer and cable below ground normally stay in place.
Q4Can edge lights be inset?
Yes. Runway and taxiway edge lights are normally elevated, but may be inset where the edge is paved or an elevated fixture would be an obstruction or be unsuitable. Check the standard and the aerodrome's design.
Q5Which is easier to maintain?
Elevated lights are usually quicker to reach and replace, because the head is above grade and the base stays in the ground. Inset lights need more involved work: cleaning rubber and dirt from the optical window, re-setting height after resurfacing, and often working under a runway or taxiway closure.
Q6How do snow and ice affect each type?
Elevated lights can be buried by drifts or struck by snow-clearing equipment. Inset lights sit below the plough, but compacted snow, ice and slush can cover the window, and heat from the light source helps clear it. LED fixtures give off less heat, so check heating options. Behaviour depends on the product and climate.
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