What is the decision bar?

It is the white bar of lights that crosses the centreline 1,000 ft from the threshold in the FAA ALSF-1 and ALSF-2 approach lighting systems. It is about 100 ft wide and is the widest bar in the pattern. It is called the decision bar because, at low decision heights, it is the part of the pattern the aircraft is passing over or approaching as the pilot decides whether to continue.

Not as a regulatory name. FAA documents call it the 1,000-ft bar, and the name decision bar is a teaching label for what it does. The decision itself is set by the decision altitude or height on the instrument procedure, not by the bar. The bar is a visual reference that helps the pilot judge position and distance at that point.

It puts a distinct reference near where a low-minimum approach reaches its decision point. As an illustration only, take a 3-degree glidepath that meets the runway about 1,000 ft in from the threshold: an aircraft 100 ft above the touchdown zone is then roughly 900 ft from the threshold, close to the bar, while at 200 ft it is roughly 2,800 ft out, near the outer end of a 2,400 to 3,000 ft system. Real figures depend on the glidepath angle and the glidepath reference point of the runway.

On ALSF-1 and ALSF-2 the sequenced flashers run from 1,000 ft outward and stop at the 1,000-ft bar. They lead the eye in towards the bar, and ending there means they do not distract the pilot during the final transition to visual references. The bar then gives a steady white line across the centreline.

ALSF-1 and ALSF-2 have the 1,000-ft bar, about 100 ft wide; ALSF-2 has 21 lights in it. MALS, MALSF and MALSR have a 1,000-ft crossbar about 66 ft long, SSALS has one about 70 ft wide, and SALS keeps the ALSF-1 bar. ICAO patterns have a 30 m crossbar at 300 m from the threshold, which is a little under 1,000 ft, and Category II and III add a second crossbar at 150 m.

A distance reference, because it is the one wide bar the pilot can pick out along the centreline, and a roll reference, because its width shows how level the aircraft is relative to the runway. It is also the point from which the red side rows and the 500-ft bar of ALSF-2 are read towards the threshold.

Lamp outages and misalignment in the bar, because a gap or a skewed line weakens the distance and roll cue. Check the bar is centred on the extended centreline and square to it. Treat the circuit as live until it is isolated, locked out and proved dead under the permit-to-work, and discharge capacitor-discharge flashers as the manufacturer instructs before opening a head or cabinet.