What is ALSF-1?

ALSF-1 is the legacy FAA and US military high intensity approach lighting system with sequenced flashing lights, configuration 1. It extends from the runway threshold along the extended centreline for 2,400 to 3,000 ft and is used for Category I precision approaches. It combines white centreline barrettes, a 1,000-ft white bar, red bars close to the threshold, green threshold lights and a line of white flashers in the outer part.

The pattern has five parts: white centreline barrettes every 100 ft (five lights each), a white bar about 100 ft wide at 1,000 ft, a red pre-threshold bar at 100 ft, a red terminating bar at 200 ft, and green threshold lights. The full system is 3,000 ft long and carries 21 sequenced flashers. FAA practice sizes a precision approach lighting system at 2,400 ft where the ILS glide slope is 2.75 degrees or steeper and at 3,000 ft where it is shallower; the 2,400-ft length carries 15 flashers.

The outer 1,000 ft of the two systems are alike. In the inner 1,000 ft, ALSF-2 has red side-row bars on each side of the centreline, while ALSF-1 has no side rows and instead has red pre-threshold and terminating bars near the threshold. ALSF-2 is the system for Category II and III operations. ALSF-1 is used for Category I and is now a legacy pattern in FAA practice.

The flashers sit on the extended centreline at 100-ft intervals from 1,000 ft out to the end of the system. They flash one after another toward the runway, outermost first, and the sequence repeats about twice a second, so the pilot sees a ball of light running to the runway. They stop at the 1,000-ft bar so they do not distract the pilot in the final transition to visual references.

ALSF-1 is the high intensity pattern for Category I conditions. Category II and III runways are normally served by ALSF-2. The lowest visibility minimums that apply depend on the approach procedure and the approved lighting; follow the applicable standard and the authority's requirements rather than relying on the system name alone.

Yes. Sequenced flashers are normally capacitor-discharge units, so stored charge can remain after the circuit is off. Isolate, lock out and discharge as the manufacturer instructs before opening a flasher head or control cabinet, work under the permit-to-work, and remember that approach light structures are elevated and the system sits in the runway protection area.