AGL Visual Encyclopedia ENTRY 29 · APPROACH LIGHTING SYSTEMS

What is approach-light intensity?

The adjustable brightness of the approach lights, stepped up or down so the pattern is clear in any light without dazzling the pilot.

approach centreline light

relative intensity (log scale)

step being selected (time-lapse)

The Fact ICAO Annex 14

Approach lights must be seen at a distance in all the conditions the system is meant for, from a clear night to daylight and fog. A single fixed brightness cannot do that, so the intensity is adjustable in steps.

The constant current regulator changes the circuit current to move between steps. Lower steps suit a clear night, where bright lights would dazzle. Higher steps suit daylight or poor visibility, where dim lights would be lost. The setting is chosen from the control system and coordinated with the threshold and runway edge lights.

ColourWhite (centreline and crossbars; red side rows and green threshold lights on precision systems)

ControlStepped (commonly 3 or 5 steps)

Driven byConstant current regulator

Selected byATC or control system (visibility and daylight)

ReferenceICAO Annex 14 § 5.3.4

Diagram not to scale. Step count, current values and the brightness at each step vary by system type and regulator. In FAA practice the ALSF-2 has five steps (100, 20, 4, 0.8 and 0.16 % relative intensity) and MALS, MALSF and MALSR have three (100, 20 and 4 %), per JO 6850.2C Table 2-3. The bars are drawn on a logarithmic scale because each step is a large ratio, not a small increment. Capacitor-discharge sequenced flashers set their intensity steps through the flasher control, not by changing series-circuit current.