What is an approach lighting system?
The lights that lead a pilot from instrument guidance to the runway — a pattern on the extended centreline you can see from far out, and the structures, cables, transformers and controls that keep it lit.
An approach lighting system is a pattern of lights on the extended runway centreline, running out from the threshold. Its job is to help the pilot move from instrument guidance to visual flight, find the runway, line up with it and judge the approach. Because the pattern is set out from the threshold, it reads the same way at every runway end.
1The pattern
White centreline lights and crossbars carry alignment, bank and distance. In CAT II/III, red side rows fill the inner approach. Sequenced flashing lights supplement the steady lights in the outer approach, and the green threshold marks where the runway starts.
2Three kinds of system
A simple system runs not less than 420 m for a non-precision approach. A precision CAT I system runs 900 m wherever possible. A CAT II/III system is also 900 m, with barrettes and red side rows in the inner part and two crossbars. These are the ICAO minimum crossbars; the full Calvert system uses five.
3What holds and feeds the lights
Each light stands on a frangible mounting on a foundation, so it gives way if an aircraft hits it. An isolating transformer in a can below ground feeds each lamp from a series circuit run by a constant current regulator in the substation.
4How it is run
The control and monitoring system selects the brightness step from the tower, coordinates the approach lights with the threshold and runway lights, and returns status and faults. The system is then aligned, inspected and tested through its life.
| Group | Entry | What it covers |
|---|---|---|
| Layout | Approach Lighting Dimensions | Length, spacing and crossbar widths, along and across the centreline |
| Layout | Approach Light Layout | Simple, CAT I and CAT II/III patterns, measured from the threshold |
| Pattern | Centreline Approach Lights | The white backbone on the extended centreline |
| Pattern | Crossbar | Roll and distance reference across the centreline |
| Pattern | Wing Bars | Green threshold line carried beyond the runway edges |
| Pattern | Sequenced Flashing Lights | Moving flash that leads the eye in |
| Structure | Approach-Light Structures | Stems, towers and masts that carry the lights |
| Structure | Approach-Light Foundations | What holds each structure in the ground |
| Structure | Frangible Mounting | Breaks or bends on impact |
| Electrical | Approach-Light Cable Routing | How the series cable runs out to the lights |
| Electrical | Transformer Arrangement | Isolating transformer in a can at each light |
| Electrical | Approach-Light Circuit | The series loop from the regulator |
| Control | Approach-Light Control | Switching and brightness step from ATC |
| Control | Approach-Light Intensity | Stepped brightness for night, day and fog |
| Work | Approach-Light Alignment | Aiming and levelling the fixtures |
| Work | Installation Sequence | Order of work on site |
| Work | Approach-Light Inspection | Visual checks |
| Work | Approach-Light Testing | Instrument measurements |
| Work | Approach-Light Installation | Setting out, mounting, connecting and aiming each light |
| Work | Approach-Light Maintenance | Keeping the pattern complete and serviceable |
| Work | Approach-Light Work Safety | Clearance, isolation and work at height |
| Selection | Approach Light Selection | Four decisions that narrow the system choice |
| Whole system | Complete Configuration | All the parts read together |
Frequently Asked Questions
Q1What is an approach lighting system?
It is a pattern of lights on the extended runway centreline, running out from the threshold, that helps a pilot make the transition from instrument guidance to visual flight and line up with the runway. It is made of white centreline lights and crossbars, red side rows in CAT II/III, sequenced flashing lights and a green threshold row, supported by the structures, cables and circuits that power them.
Q2What does an approach lighting system do for the pilot?
It shows where the runway is before the runway itself can be seen, whether the aircraft is lined up and level, and roughly how far there is still to go. Because the pattern is fixed relative to the threshold, it reads the same way at every runway end.
Q3What are the types of approach lighting system?
A simple system runs not less than 420 m and serves a non-precision approach runway. Precision systems for CAT I, and CAT II/III, run 900 m wherever possible, with CAT II/III adding red side rows, barrettes in the inner part and a second crossbar. Which one a runway gets depends on the type of approach it supports.
Q4What is behind the lights themselves?
Each light sits on a frangible stem on a foundation, fed by an isolating transformer in a can below ground. Series cables loop out from a constant current regulator in the substation, and the airfield lighting control and monitoring system sets the brightness step and reports status.
Q5Why are approach light structures frangible?
Approach lights stand in the approach area, where an aircraft that touches down short could hit them. Frangible mountings are designed to break or bend on impact so the structure does less damage to the aircraft.
Q6Where do I read about each part of the system?
Each part has its own entry in the Approach Lighting Systems section of the AGL Visual Encyclopedia: layout, centreline, crossbar, wing bars, sequenced flashers, structures, foundations, cable routing, transformers, circuit, control, intensity, alignment, installation, inspection and testing. The table on this page links to each.
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