Lead‑in Lighting

Lead‑in Lighting (also known as approach path guidance lighting) is a series of lights that provide a visual track for pilots to follow when the runway is not aligned with the standard straight‑in approach. These lights are typically installed on a curved or angled path that guides the aircraft from a point in the en‑route airspace to the runway threshold. Lead‑in lighting is essential at airports where terrain, obstacles, noise abatement, or airspace constraints prevent a conventional straight‑in approach.

The system consists of a row of lights (often omni‑directional or uni‑directional) that are spaced at regular intervals along the desired flight path. They may be steady‑burning or flashing, and are often supplemented with sequenced flashing lights to indicate direction. Lead‑in lighting is a cost‑effective alternative to full precision approach systems for airports with non‑standard approach geometries.

Lead-in lighting exists to make an otherwise unattainable approach flyable at night or in marginal visibility, at airports where terrain, obstacles, noise restrictions, or airspace conflicts rule out a standard straight-in approach altogether. Without a visual path to follow, a pilot flying a curved or offset approach in the dark has no reliable way to judge whether they're tracking the safe corridor toward the runway, so the lighting substitutes for the straight-line visual cues a conventional approach would otherwise provide. Its purpose is therefore not to refine an already-standard approach but to enable a non-standard one to be flown safely at all, offering airports with awkward geometry a workable alternative to the far more expensive option of re-routing traffic or restricting operations to daylight visual conditions only.

Lead‑in lighting is deployed at airports and heliports where approach paths are not straight‑in:

• Airports with offset approaches – where the runway is not aligned with the prevailing approach direction due to terrain or obstacles.
• Heliports – where helicopters approach from varying directions and require visual guidance.
• Water aerodromes – where floatplanes require a marked approach path over water.
• Noise‑sensitive airports – where approaches are designed to avoid residential areas.
• Military airfields – where tactical approaches require non‑standard flight paths.
• Temporary or remote airstrips – where a simple visual guidance system is needed.

Lead-in fixtures are set out along the curved offset path itself rather than on a straight pier line, each fixture mounted on its own frangible pole and foundation positioned to trace the surveyed curve from the point where the approach deviates from straight-in through to where it merges into the standard approach lighting centreline. Pole spacing follows the curvature rather than a fixed interval, tightening through the sharpest part of the bend so the row reads as a continuous curved line to the pilot. Cabling follows the same curved alignment in duct or direct-buried cable back to the approach lighting control cabinet, with each fixture's isolating transformer at its pole base; the whole layout is set out and re-surveyed against the offset-approach procedure design rather than the runway centreline extension.

Lead‑in lighting provides a visual path that the pilot follows from the approach start to the runway threshold:

• Configuration: A row of lights is installed along the desired approach path, typically starting several kilometres from the runway and ending at the threshold. The lights are spaced at intervals (e.g., 100–300 m) and may be elevated or flush‑mounted.
• Lighting Characteristics: The lights are typically white, steady‑burning or flashing, with intensity controlled by a CCR. In some systems, they flash in sequence to indicate the direction of travel (like a “chase” effect).
• Guidance: The pilot visually aligns the aircraft with the row of lights, maintaining the path. The spacing and colour provide distance and alignment cues. When the lead‑in lights merge with the approach lighting system, the pilot transitions to the standard approach lights for final alignment.
• Integration: Lead‑in lighting is often integrated with the runway approach lighting system, with the final lights merging into the approach lighting structure (e.g., ALSF‑1/2, SALS, etc.). They are controlled by the same CCR and ALCMS.

Because a lead-in row can run for several kilometres before it ever reaches the approach lighting system, its control architecture has to solve a distance problem that a compact runway or taxiway loop does not. Brightness is still set by the ALCMS selecting a CCR step, but a single series circuit run over that distance would be impractical to regulate and fault-find, so the path is typically fed from one or more intermediate control points along its length rather than a single CCR at the runway end — keeping voltage drop and fault-loop length within workable limits. Where the lights flash in a "chase" sequence rather than burning steady, a dedicated sequence controller drives that timing layer independently of the CCR brightness layer, the same separation used on sequenced flashing lights. At the point where the lead-in row physically merges into the approach lighting structure, control also merges: the final lead-in fixtures are typically absorbed into the approach lighting circuit and ALCMS command set, so from that point onward the lead-in system is switched and monitored as part of the approach lighting system rather than as a separate circuit.

Lead-in lighting fixtures follow the same elevated or flush-mounted construction as standard approach lights, adapted for installation over varied and often uneven terrain along the curved or angled path. Elevated units use a die-cast aluminium housing on a frangible coupling, seated on an L-867 base can whose mounting stake or footing is engineered for the specific ground conditions encountered along the lead-in route, which may include soft ground, embankments, or shallow water crossings; flush units instead sit in an L-868 base can where the terrain permits pavement or hard-standing installation. The optical assembly is gasket-sealed behind a polycarbonate or glass lens to achieve an IP67/IP68 rating, since fixtures along a multi-kilometre lead-in route are frequently far from routine access and must tolerate prolonged exposure to moisture and weather. Because the lead-in path can run for a kilometre or more, cable runs between fixtures are longer than in compact approach arrays, so L-823 connectors and the associated isolating transformer per light are sized and routed with attention to voltage drop and cable protection along the full length. Stainless-steel fasteners and corrosion-resistant coatings are standard, given the extended, often exposed, siting of these fixtures away from the immediate airfield.