Decision Bar

The Decision Bar (also known as the Decision Height Bar) is a row of lights installed perpendicular to the approach centreline at a specific distance from the runway threshold. It marks the point on the approach path where the pilot must make the go/no-go decision — if the runway environment is not visible at this point, a missed approach must be initiated. The Decision Bar is a key component of precision approach lighting systems (ALSF‑1, ALSF‑2, and other CAT II/III configurations), providing a clear visual reference that enhances safety during low visibility operations.

The Decision Bar is fixed at 1,000 ft (≈300 m) from the threshold on virtually all US approach lighting system types, regardless of the runway's certified decision height. It does not mark the physical point where the aircraft reaches DH — on a standard 3° glide slope, a CAT I 200 ft DH is actually reached several thousand feet farther out, well beyond the approach lighting system itself. Instead, the bar is positioned so that, on a normal glide path, it passes beneath the aircraft at roughly 100 ft AGL, giving the pilot a large, fixed visual horizon to reference during the transition from instrument to visual flight around the decision-height area — not a literal marker of the decision altitude itself. The bar consists of high‑intensity white lights, steady‑burning or flashing depending on system type.

The Decision Bar exists to give the pilot a large, fixed visual horizon during the most consequential moment of an instrument approach: the transition from instrument reference to visual flight around decision height, when the runway environment must be in sight or the pilot must commit to a missed approach. Rather than being surveyed to the exact distance that corresponds to each runway's certified decision height (which varies by category and glide slope, and in most cases falls well beyond the approach lighting system itself), the bar is sited at a single standard distance — 1,000 ft (300 m) from the threshold — used consistently across US approach lighting system configurations. Its value comes from being a large, conspicuous, and reliably positioned reference the pilot can use to judge attitude and closure during that critical phase, not from marking the decision altitude itself.

The Decision Bar is an essential component of precision approach lighting systems and is used on:

• CAT I precision approach runways (ALSF‑1 configuration) – provides a clear decision point for approaches with 200 ft decision height.
• CAT II and III precision approach runways (ALSF‑2 configuration) – critical for low visibility operations with decision heights as low as 100 ft or less.
• Runways with low visibility operations (RVR below 800 m) – ensures pilots have a visual reference at the decision height.
• Major international airports – where high traffic volumes and adverse weather demand reliable approach guidance.
• Military airfields – requiring precision approach capability for all‑weather operations.

The Decision Bar is erected on its own row of piers set perpendicular to the extended runway centreline at the 300 m (≈1,000 ft) point from the threshold — a fixed siting used across US approach lighting system layouts, including ALSF‑1, rather than a position calculated from any particular runway's certified decision height. Poles either side of the centreline are tied to a common foundation line so the bar presents as a single unbroken span at the same elevation as the centreline barrettes it crosses. Where the approach passes over water or uneven ground, the bar's piers are raised on a platform or bridge structure matching the adjacent approach lighting piers. Cabling loops along the bar from pole to pole back to the approach lighting control cabinet, with an isolating transformer at each pole base, and the whole assembly is set out from the ILS/runway centreline survey to keep the bar square to the approach.

The Decision Bar operates as a key visual cue within the approach lighting system:

• Positioning: The bar is installed at a fixed distance of 1,000 ft (300 m) from the threshold, a standard siting used across US approach lighting configurations rather than one recalculated per runway's glide slope or decision height. It is placed perpendicular to the centreline, spanning the approach corridor.
• Lighting: The bar comprises multiple high‑intensity white lights (usually 5–7 per side, or a full row across the approach path). They are steady‑burning or flashing, with intensity controlled by the CCR (5‑step). In some systems, the bar may flash in sequence to draw attention.
• Guidance: As the aircraft reaches the decision height, the pilot looks for the Decision Bar. If the bar is clearly visible and the runway environment is acquired, the pilot may continue the approach. If not, a missed approach is executed. The bar provides a positive visual reference that confirms the aircraft is on the correct glide path and at the correct position.
• Integration: The Decision Bar is integrated with other approach lighting components – centreline barrettes, crossbars, side row bars, and sequenced flashing lights – to form a comprehensive visual guidance system. It is typically the last bar before the threshold, marking the critical decision point.

The Decision Bar is not individually addressable — control is exercised at the loop level through the approach lighting series circuit, the same as the crossbars and barrettes it crosses. An Airfield Lighting Control and Monitoring System (ALCMS), operated by the tower or an automated visibility-linked routine, selects the required brightness step based on prevailing RVR or ATC instruction. This command is passed to the approach lighting CCR, which regulates the entire bar's loop to the corresponding fixed current (5-step). Every lamp across the bar, via its own isolating transformer at the base of its pole, draws from the same current and therefore burns at the same intensity simultaneously — there is no way to brighten one section of the bar to draw more attention than another; visual prominence at decision height comes from the bar's position and lamp count, not from any per-fixture control. Fault detection is loop-based: the CCR continuously monitors loop voltage against the commanded current, and a voltage rise at constant current signals increased circuit impedance, typically an open-circuit lamp or a failed isolating transformer, which is reported to the ALCMS as a loop fault rather than pinpointed to an exact lamp without further sectionalised testing. Given the bar's role as the last positive visual reference before the go/no-go decision, any reported loop fault on this circuit is treated as high priority for field verification ahead of CAT I operations.

The Decision Bar is built like a crossbar: a row of high-intensity lamp heads, each a die-cast aluminium housing with a toughened-glass or polycarbonate lens gasket-sealed for an IP65/IP66 rating, mounted across several frangible poles tied to a common foundation line and set to the same elevation as the centreline barrettes it crosses. Because the bar marks a safety-critical decision point, lamp heads are typically higher-output units (steady-burning or flashing) than the plain centreline barrettes, but share the same housing and lens construction. Each pole incorporates a frangible coupling near its base to shear cleanly on impact, and where the approach crosses water or uneven ground the bar is carried on a raised platform or bridge structure matching the adjacent piers. Cabling loops pole to pole back to the approach lighting control cabinet, with an isolating transformer at each pole base, and stainless-steel fasteners and sealed junction boxes used throughout.