Flashing Approach Lights

Flashing Approach Lights are a set of high‑intensity white lights installed along the approach lighting system that flash at a regular interval (typically 60‑120 flashes per minute) to provide a dynamic visual cue for pilots during final approach. Unlike Sequenced Flashing Lights (SFL), which flash in a progressive sequence to create a "rolling" effect, these lights typically flash simultaneously or in a simple alternating pattern, offering a pulsing visual reference that enhances the pilot's awareness of the approach path and helps maintain proper descent angle.

They are often used in systems such as Runway End Identifier Lights (REIL), as part of ALSF configurations, or as standalone flashing beacons at airports where basic flashing guidance is sufficient. Their primary advantage is the increased conspicuity compared to steady‑burning lights, making them particularly valuable in fog, rain, or snow.

These lights exist because a flashing signal is inherently more conspicuous to the human eye than a steady one at equivalent intensity, particularly when visibility is degraded by fog, rain, or snow and the pilot's attention is divided across multiple approach references. Their purpose is to draw and hold the pilot's attention to the approach path itself during the highest-workload phase of flight, functioning as a supplementary conspicuity layer rather than a replacement for the steady-burning centreline, barrette, and crossbar lights that carry the primary lateral and distance guidance. By reinforcing the approach lighting system's overall visibility rather than adding new geometric information, they serve a purpose distinct from sequenced flashing lights, which are used specifically for directional "rolling" guidance rather than general conspicuity.

Flashing approach lights are installed as part of approach lighting systems at airports that require enhanced visual cues. They are typically used in:

• Runway End Identifier Lights (REIL) – pairs of synchronized flashing lights at the runway threshold
• ALSF‑1 and ALSF‑2 systems – as part of the approach lighting configuration
• MALSF and MALSR systems – providing flashing cues in medium‑intensity systems
• Simple approach lighting systems where basic flashing guidance is needed
• Military airfields requiring conspicuous approach markers
• Heliports and remote airstrips where low‑cost flashing beacons are used

Flashing units are integrated into the same pier or pole line as the steady-burning approach lights they augment, typically at the outer end of the approach lighting system and/or paired at the runway threshold as REIL. Approach-lighting-pier units mount the flasher head atop the same frangible pole as the associated centreline or crossbar light, sharing its foundation and cable route back to the approach lighting control cabinet; threshold REIL units mount on their own low frangible masts just clear of the runway strip, one on each side of the runway. Each flasher head is fed from a dedicated flasher control unit rather than the CCR series circuit, so a separate control cable run is required alongside the series-circuit cabling.

Flashing approach lights operate on a simple but effective principle:

• Flashing Pattern: The lights flash at a consistent rate (typically 60‑120 flashes per minute) – either simultaneously (all on/off together) or in a simple alternating pattern between groups.
• Visual Cue: The pulsing effect draws the pilot's attention and provides a clear contrast against steady‑burning lights, helping to distinguish the approach path from background lighting.
• Depth Perception: The flashing action helps pilots judge distance and closure rate, as the apparent brightness and flash timing change with distance.
• Directional Guidance: When placed along the approach centreline, the lights provide a directional reference, especially when combined with steady‑burning barrettes or crossbars.
• Intensity Control: Like most AGL lighting, the intensity is typically controlled by a CCR with multiple steps, allowing ATC to adjust brightness for visibility conditions – higher intensity in fog, lower in clear conditions.
• Integration: They are often integrated with the ALCMS for remote control and monitoring.

Flashing approach lights are governed by two independent control layers rather than the single CCR loop that runs a plain edge or centreline light. A dedicated flasher control unit — separate from the constant current series circuit — generates the flash timing (simultaneous or alternating, 60‑120 flashes per minute) and requires its own control cable run back to the approach lighting control cabinet. Brightness, by contrast, is still set the conventional way: the flasher head is fed through an isolating transformer from the CCR series circuit, so the ALCMS-selected intensity step applies to the flash just as it would to a steady-burning fixture. In practice this means the ALCMS issues two separate commands to the same fixture — a brightness step to the CCR loop, and a flash mode/rate to the flasher control unit — and a fault in one layer (e.g., a stuck flasher relay) does not necessarily affect the other.

The flasher head houses a xenon flash tube (or, in newer installations, a high-output LED array driven in pulse mode) behind a toughened-glass or polycarbonate dome, gasket-sealed for an IP65/IP66 rating against the weather exposure typical of an approach lighting site. The flash tube's discharge capacitor and trigger electronics are housed either within the head itself or in a separate weatherproof enclosure at the pole base, since this circuitry is driven by the dedicated flasher control unit rather than the CCR series loop. On approach-lighting piers the head mounts atop the same frangible pole used by the associated centreline or crossbar light, sharing its foundation; threshold REIL units instead mount on their own low frangible masts set just clear of the runway strip. Because the fixture carries two separate feeds — series-circuit power for brightness and a control cable for flash timing — its base enclosure has separate sealed glands for each, along with the isolating transformer for the series-circuit connection.