Approach lighting system (ALS). Provides lights that will penetrate the atmosphere far enough from touchdown to give directional, distance, and glidepath information for safe transition from instrument to visual flight.
Source: US FAA Pilot's Handbook of Aeronautical Knowledge
An approach lighting system (ALS) is a configuration of signal lights placed symmetrically on both sides of the runway extended centreline, starting at the landing threshold and extending outward into the approach zone. This system provides visual information about runway alignment, height perception, roll guidance, horizon references and limited distance-to-go information.
The fillowing types of ALS are defined by ICAO:
- Simple ALS. These are used for non-instrument runways and non-precision approach runways.
- Precision approach category I lighting system.
- Precision approach category II lighting system.
Generally, ALSs consist of a row of lights that extends from the runway threshold up to several hundred metres and one or more crossbars. The length of the row, the number of crossbars and the spacing between the lights depend on the type of the ALS, the lights used (separate light sources or barettes) and whether or not the system meets the required serviceability levels specified in ICAO Annex 14.
A simple approach lighting system normally consists of a row of lights on the extended centre line of the runway extending, whenever possible, over a distance of not less than 420 m from the threshold with a row of lights forming a crossbar 18 m or 30 m in length at a distance of 300 m from the threshold (see picture below). The lights forming the central line are usually spaced at 60 m but this can be reduced to 30 m to improve guidance.
A CAT I lighting system normally consists of a row of lights on the extended centre line of the runway extending over a distance of 900 m from the runway threshold, spaced at 30 m, and crossbar(s). For the first 300 m, a single light source is used. For the second 300 m (i.e. between 300 and 600 m from the threshold), two light sources are used and for the last 300 m, three light sources are used. Crossbars are added at 150, 300, 450, 600 and 750 m. The one at 300 m is 30 m in length and the others are either of the same length or adjusted so that the lines connecting the outer edges converge 300 m after the runway threshold (see picture below). The lights are showing white colour.
A CAT II/III lighting system normally consists of a row of lights on the extended centre line of the runway extending over a distance of 900 m from the runway threshold, spaced at 30 m, two side rows, extending to 270 m from the threshold, and crossbars. The spacing between the lights of the side rows is usually 30 m but can be increased to 60 m if the required serviceability levels are demonstrated (in this case, these rows only extend to 240 m from the threshold). The centre line is showing white colour and the side rows are showing red. Crossbars showing white colour are added at 150, 300, 450, 600 and 750 m. The picture below shows the first 300 m of a CAT II/III system.
If barettes are used instead of light sources, they must be at least 3 m long in the simple ALS case, or at least 4 m in the other cases. In this case, only one crossbar (at 300 m) is installed in the CAT I case and only two (at 150 and 300 m) are installed in the CAT II/III case.