What is inside a runway light fixture?
A sealed, rigid body that holds the optics and the light source, with a lead that runs down into the base can. Here is how an inset fixture is put together, from the window to the plug.
An inset runway light sits in the pavement, where aircraft wheels, rain, fuel, heat and vibration all reach it. Its construction has to protect a small light source and its optics from all of that, and still let the light out through one window.
1Body, window and seal
A rugged body, usually metal, holds everything and carries the load of aircraft passing over it. A prism or window closes the top, seated on a gasket or O-ring and held by a retaining ring and bolts. That seal keeps out water, dirt and fuel, so a damaged or badly seated seal is a common cause of faults.
2Optics and source inside
Below the window sit the optical assembly and a mount that holds the lamp or LED board in the right position for the optics. In a lamp-based fixture the lamp’s heat helps keep the window clear of snow. In an LED fixture a heat path carries the heat away from the board into the body. Short internal wiring joins the source to the lead.
3How it connects
The secondary lead leaves through a cable entry in the base of the fixture, passes down into the base can and ends in a plug that connects to the secondary of the isolating transformer. The fixture bolts to the base can, and the bolts must be tightened to the manufacturer’s torque so the seal and the mounting both hold.
| Part | Job |
|---|---|
| Body | Holds the parts and carries wheel loads |
| Prism or window | Lets the beam out and closes the top |
| Gasket or O-ring | Seals the window against water, dirt and fuel |
| Retaining ring and bolts | Hold the window and seal in place |
| Optical assembly | Shapes the beam (see Entry 30 · Fixture optical system) |
| Lamp holder or LED board | Holds the source in position and, for LEDs, carries heat away |
| Cable entry | Lets the lead leave the body and stays sealed |
| Secondary lead and plug | Connect the fixture to the isolating transformer secondary |
Frequently Asked Questions
Q1What is inside a runway light fixture?
A rugged body holds a sealed window, an optical assembly, a mount for the lamp or LED board, and the internal wiring. A secondary lead leaves through the base and ends in a plug for the isolating transformer.
Q2Why are the seals so important?
Water, dirt and fuel inside the fixture can damage the source, the wiring and the optics. A gasket or O-ring under the window, held by the retaining ring and bolts, keeps them out, so a damaged or badly seated seal is a common cause of faults.
Q3How does the fixture connect to the circuit?
The secondary lead passes down into the base can and ends in a plug that connects to the secondary of the isolating transformer. The transformer, not the fixture, is what connects to the series circuit.
Q4Is the construction the same for inset and elevated fixtures?
The main parts are similar, but the housing differs. An inset fixture has a low body that sits flush in the pavement and bolts to a base can. An elevated fixture sits on a frangible mounting so that it breaks away if an aircraft hits it.
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