What is ALSF-2?

ALSF-2 is the FAA's high intensity approach lighting system with sequenced flashing lights, configuration 2. It is the system for Category II and III runways. It combines white centreline barrettes, a 500-ft bar, a 1,000-ft bar, red side-row bars in the inner 900 ft, green threshold lights and a line of white sequenced flashers in the outer part of the system.

The FAA length is 2,400 ft from the threshold, or 3,000 ft where the ILS glide slope is below 2.75 degrees. The centreline has five-light white barrettes every 100 ft. A 1,000-ft bar about 100 ft wide and a 500-ft bar cross the centreline. Red side-row bars of three lights sit on each side at the nine stations from 100 to 900 ft. At 2,400 ft the system carries 15 sequenced flashers from 1,000 ft outward; the 3,000-ft version carries 21.

The outer 1,000 ft are alike. In the inner 1,000 ft, ALSF-2 adds the 500-ft bar and red side-row bars on both sides of the centreline. It has no separate red pre-threshold or terminating bar; the red lights of those bars become the side rows. ALSF-1 keeps the red pre-threshold and terminating bars, has no side rows, and serves Category I.

They show the pilot the edges of the inner approach and the aircraft's roll attitude, and they mark the part of the pattern closest to the runway. Under US instrument approach rules (14 CFR 91.175), approach lights alone allow descent to 100 ft above touchdown zone elevation only; the red side-row and terminating bars do not count toward that and must be in sight to go lower. Confirm the rule that applies to your approach procedure.

ALSF-2/SSALR means an ALSF-2 installation that can also run as a shorter SSALR when Category I conditions exist on a Category II or III runway. The red side rows, the 500-ft bar and the alternate 100-ft barrettes are switched off, leaving a white-only system about 1,400 ft long (bars every 200 ft and a crossbar at 1,000 ft) with five flashers at 200-ft spacing out to 2,400 ft acting as runway alignment indicator lights. It saves energy; the full ALSF-2 is switched back on when Category II or III conditions apply.

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, and work under the permit-to-work. Approach structures are elevated and sit in the runway protection area, so access, height and aircraft-movement controls also apply.