What are sequenced flashing lights in approach lighting?
A row of white flashers on the extended centreline that fire one after another toward the threshold — a ball of light that seems to race to the runway and leads the pilot in when steady lights are hard to see.
Sequenced flashing lights are bright white flashers on the extended runway centreline that flash one after another toward the threshold, starting with the light farthest from the runway. The sequence repeats about twice a second, so the pilot sees a ball of light running down the centreline to the runway.
They supplement the steady-burning approach lights; they do not replace them. A moving flash is far easier to pick out than a steady light in fog, haze or rain, so it helps the pilot find the centreline early and judge alignment and distance. A master timer keeps the flashers in order, and each one is normally a capacitor-discharge flash lamp in a head with its own control box.
Frequently Asked Questions
Q1What are sequenced flashing lights in approach lighting?
Sequenced flashing lights are a row of bright white flashing lights on the extended runway centreline, supplementing the steady barrettes: along the whole system for Category I, and in the outer part (from about 300 m) for Category II and III. They flash one after another toward the runway threshold. To the pilot they look like a ball of light racing along the centreline to the runway, which gives a strong cue to the runway's position and alignment.
Q2How do they flash?
Each light flashes once in turn, starting with the one farthest from the runway and ending with the one nearest the threshold, and the sequence then repeats. The usual rate is two cycles per second. A master timer controls the order, so the lights fire in step rather than at random.
Q3Why are they added to the steady lights?
Steady lights can blend into fog, haze, rain or the lights of the surrounding area. A moving flash is much easier to pick out, so it helps the pilot find the centreline early and judge direction and distance. ICAO Annex 14 Volume I, section 5.3.4, allows flashing lights to supplement CAT I, II and III steady-burning approach lighting systems.
Q4Do sequenced flashing lights replace the steady approach lights?
No. They supplement them. The steady centreline, crossbars and threshold lights still form the pattern; the flashers are an extra cue on the same centreline. Barrette centreline systems are typically supplemented with sequenced flashing lights that flash toward the threshold.
Q5What are ALSF-2, MALSF and RAIL?
They are system names. ALSF-2 is the FAA high intensity approach lighting system with sequenced flashing lights. MALSF is a medium intensity system with three sequenced flashers at three light-bar stations near its outer end, and MALSR is a medium intensity system with runway alignment indicator lights (RAIL), which are the same kind of sequenced flasher. ICAO describes systems by type and category rather than by these names.
Q6How is a failed flasher recognised, and is it hazardous to work on?
A flasher is treated as out of service if it stops flashing, or if its flash frequency or duration falls outside the specified range. A gap in the running light is often what the pilot, or the airfield checker, notices first. Flashers are normally capacitor-discharge units, so the stored charge is treated as hazardous: isolate, lock out and discharge as the manufacturer instructs before opening a head or control box, under the permit-to-work.
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