Sequenced Flashing Light Unit

A Sequenced Flashing Light Unit is an individual high‑intensity light fixture that operates as part of a Sequenced Flashing Lights (SFL) system — called Runway Alignment Indicator Lights (RAIL) where the flashers extend outboard beyond the end of the steady‑burning centreline, as in MALSR and SSALR, and simply called sequenced flashers where they sit at the same stations as the centreline barrettes, as in ALSF‑1 and ALSF‑2. The equipment is the same; the name follows the position in the system. These units are installed along the approach lighting system and are triggered in rapid succession from the outer end toward the runway threshold, creating a dynamic "rolling" or "chasing" effect that provides pilots with directional cues during final approach. Each unit is controlled by an SFL controller that precisely times the flash sequence, ensuring a consistent and seamless visual pattern. The units are high‑intensity unfiltered‑white condenser‑discharge (xenon) or, increasingly, LED flash heads, elevated on frangible mountings, and are a component of precision (ALSF‑1/ALSF‑2), medium‑intensity (MALSF/MALSR, SSALF/SSALR) and omnidirectional (ODALS) approach lighting systems.

The flash itself is a very short, very bright pulse — a xenon discharge lasts well under a millisecond — so the light is rated in effective intensity (the steady intensity that would give the same visual range), not in raw peak candela. The sweep from the outermost unit to the innermost repeats twice per second in ALSF and MALSR/RAIL installations, and once per second in ODALS, producing the ball of light that appears to travel toward the threshold. That apparent motion is what makes the pattern conspicuous in fog, rain and snow, where a field of steady lights alone can be slow to interpret.

The purpose of the sequenced flashing light unit is to give a pilot's eye something a steady light cannot: a sense of motion and direction. In marginal visibility, a field of steady lights can be difficult to parse quickly, especially when the pilot is still transitioning from instruments to outside visual reference on short final. The rolling flash draws the eye and unmistakably points toward the runway, effectively saying "this way" rather than simply "here is a light," which shortens the time it takes a pilot to acquire and interpret the approach lighting pattern at the moment visual contact is first made. That is why the unit is concentrated in precision and medium-intensity approach systems — it earns its place by adding a directional cue exactly where and when visual guidance is most marginal, complementing rather than duplicating the steady centreline and crossbar lights around it.

Sequenced Flashing Light Units are deployed as part of approach lighting systems that require dynamic visual guidance. They are used in:

• ALSF‑1 – high‑intensity approach lighting with sequenced flashers, serving CAT I precision approaches.
• ALSF‑2 – the CAT II/III configuration; typically 15 flashers at 100 ft (30 m) stations from 2,400 ft out to the 1,000 ft decision bar, where the sequence terminates.
• MALSR and SSALR – five sequenced flashers form the RAIL section outboard of the steady‑burning centreline, supporting non‑precision and CAT I approaches.
• MALSF and SSALF – three flashers added at the outer end where approach‑area identification is difficult (bright surrounding lighting, confusing terrain).
• ODALS – seven omnidirectional flashers (five on the extended centreline plus a REIL pair), an economy system for runways without a full approach lighting installation.
• ICAO precision approach category I/II/III lighting – Annex 14 permits sequenced flashing lights to be added to the approach lighting system where an operational need is identified; they are not a mandatory element of the basic Calvert‑type configuration.
• Low‑visibility operations and military airfields – where fast, unambiguous acquisition of the approach pattern at the transition to visual reference is the deciding requirement.

Individual SFL units are set out at the same longitudinal stations as the approach lighting centreline barrettes — 100 ft (30 m) spacing in ALSF systems, 200 ft (60 m) for the RAIL section of a MALSR — each flash head on its own frangible mounting adjacent to the steady‑burning fixture at that station, rather than sharing the fixture’s pole. Each flasher head sits atop its pole alongside the steady-burning fixture, fed from a dedicated sequencing controller rather than the CCR series circuit, so a separate control cable run is needed back to the approach lighting control cabinet in addition to the series-circuit cabling. On approaches over water or uneven terrain, SFL poles are carried on the same raised platform or bridge structure as the adjacent centreline piers, with pier setout tied to the same extended-centreline survey as the rest of the approach lighting system.

A Sequenced Flashing Light Unit operates in a precisely timed sequence with other units in the SFL system:

• Sequence Control: Each unit receives a trigger pulse from the system controller at its allotted point in the cycle, firing in order from the outermost unit inward. The interval between adjacent units is a few tens of milliseconds — ODALS, for example, fixes it at 1/15 second (≈67 ms) between centreline units — so that the entire sweep completes well inside one cycle.
• Flash Rate: The complete sequence repeats twice per second (120 cycles per minute) in ALSF and MALSR/RAIL installations. ODALS repeats at 60 cycles per minute, with the two REIL‑configuration lights firing together 4/15 second after the last centreline flash. Tolerance on these intervals is typically ±10%.
• Flash Duration: The light pulse from a condenser‑discharge (xenon) head lasts well under a millisecond; LED flash heads use a slightly longer pulse or a burst of pulses that the eye reads as one flash. Because the pulse is far shorter than the eye’s integration time, performance is specified as effective intensity using the Blondel–Rey relationship (0.2 s visual constant), not as instantaneous peak candela.
• Rolling Effect: The sequential firing creates the "rolling" or "chasing" ball of light travelling toward the runway, giving a dynamic alignment cue that a steady light cannot.
• Intensity Control: Discharge flashing light equipment provides three brightness steps — high, medium and low — not the five steps used for steady‑burning runway lighting. Where the flasher is fed from a series circuit, a series‑circuit adapter maps the regulator’s current steps onto those three flash levels (on a 5‑step 6.6 A regulator: 6.6 A and 5.2 A → high, 4.1 A → medium, 3.4 A and 2.8 A → low).
• Electrical Interface: The flash energy comes from a local power supply/control unit, typically fed at 120/240 V AC, which charges the discharge capacitor and delivers the trigger pulse to the optical head — the head is not energised directly from the series loop. Where the installation is series‑fed, an optional series‑circuit adapter takes 6.6 A through a series‑to‑series isolation transformer so the flashers switch with the approach/runway circuit.
• Mounting: Flash heads are elevated on frangible mountings (frangible couplings to FAA drawing C‑6046 or equivalent), on low‑impact‑resistant approach light towers where the terrain falls away. In‑pavement flash heads are offered by some manufacturers for specific applications but are not the normal arrangement.

SFL units are not commanded individually — timing authority sits with a dedicated SFL sequencer/flasher controller that fires each unit's capacitor-discharge trigger in turn along the row, independent of the steady-burn approach lighting circuit the row is embedded within. The ALCMS enables or disables the sequenced flasher function as a whole (based on RVR category or ATC instruction) and selects one of the flasher system’s three brightness steps — high, medium or low — which the controller applies uniformly to every unit in the row. The rolling effect is a timing offset layered on top of a single system-level brightness, not independent per-unit dimming. Note that the flasher’s three steps are coarser than the five steps of the steady-burning approach lighting they sit within: where the flashers track a 5-step constant current regulator through a series-circuit adapter, two regulator steps map onto each of the outer flash levels, so an ATC brightness change does not always produce a visible change in flash intensity. Fault detection is correspondingly split: the CCR/SFL controller watches overall loop current for a suppressed-circuit fault, while the sequencer separately monitors trigger continuity to each position, so a single lamp or capacitor failure can often be isolated to its place in the row even though intensity control itself remains loop-wide.

Each SFL unit is built around a high-intensity xenon strobe tube or high-output LED cluster housed in a rugged die-cast aluminium enclosure engineered to withstand the heat cycling and mechanical stress of repeated high-energy flashes. The lens is a hardened, UV-stabilised polycarbonate or tempered glass dome, gasket-sealed to the housing to keep the unit watertight, with a narrow, forward-directed optical system concentrating the flash toward the approach path. Elevated units mount on a frangible coupling (FAA drawing C‑6046 Type 1/1A or equivalent) on a concrete foundation or on the low-impact-resistant approach light structure serving that station, while the discharge capacitor bank and trigger electronics sit in a separate weatherproof power supply enclosure — NEMA 4 or equivalent, with a hinged lockable door — at the base or in an adjacent equipment case, linked to the lamp head by heavy-gauge cable rated for the high peak currents of a xenon discharge. This separation keeps the failure-prone electronic components accessible for servicing without disturbing the elevated optical head. Door interlocks and bleeder resistors are built into the power supply and control enclosures so that opening a door disconnects input power and bleeds voltages above 150 V down to 50 V within 30 seconds, with the bleeder resistors alone doing so within five minutes as a back-up. Internal wiring terminates at sealed L-823 connectors where the unit is series-fed, and all fasteners and brackets are stainless or otherwise corrosion-resistant given the fixture's exposed siting along the approach lighting corridor.