Clearance Bar Light

A Clearance Bar Light — called an Intermediate Holding Position (IHP) Light under ICAO Annex 14 — is a row of three in-pavement steady-burning yellow lights that marks a holding position on a taxiway, increasing its conspicuity in low-visibility conditions. It may also be installed to mark the location of an intersecting taxiway during darkness. It is frequently co-located with a stop bar, but the two fixtures serve different roles: the stop bar's red lights are what pilots watch for the actual stop/proceed instruction — switched on to hold traffic, switched off (in conjunction with an explicit ATC clearance) to indicate traffic may proceed — while the clearance bar's yellow lights simply make that hold point or intersection visible, whether or not it is currently protected by an active stop bar.

The clearance bar light exists to make a hold point unmistakably findable, especially when visibility is poor enough that the pavement marking and any co-located stop bar are hard to pick out early. A pilot taxiing in fog or at night needs to identify the holding position with enough lead time to slow and stop correctly, not spot it only once already close; the row of yellow lights gives that early, unambiguous visual cue independent of whether a stop bar is currently on. Where it marks an intermediate holding position rather than a runway-holding position, it also protects against the more subtle risk of a pilot drifting past an intersection they didn't register was there at all. It is a conspicuity aid, not a substitute for the actual stop/proceed instruction, which pilots still take from ATC and, where fitted, the stop bar's own on/off state.

Clearance bar lights are installed at various locations on the airfield where a hold point or intersection needs added conspicuity, particularly in low visibility:

• Runway-holding positions, typically co-located with stop bar lights
• Intermediate holding positions on taxiways, marking the position itself
• Intersecting taxiways, to indicate their location during darkness
• ILS critical area boundaries, where the hold point needs to be conspicuous
• Low visibility operations, where conspicuity of the hold point is essential
• Military airfields requiring clearly marked hold points

Fixtures are set out in a row across the taxiway at the holding position, ILS critical area boundary, or apron entry/exit point they protect, using the standard load-rated inset base can recessed into the pavement (or an elevated frangible-mount fixture where the location falls on an unpaved shoulder). Because the bar is switched independently of the taxiway centreline lighting whenever it protects an ILS boundary or a holding point, each fixture is wired on its own control cable back to the local controller or Remote I/O in addition to the series-circuit power feed, with cabling and setout typically shared with the co-located stop bar or holding-position installation it accompanies.

Clearance bar lights operate as an integral part of the taxiway guidance and control system:

• Power: Powered from the constant current series circuit via an isolating transformer (typically 15W – 45W).
• Light Source: Utilizes yellow/amber LED arrays or yellow-filtered halogen lamps, per the standard aeronautical yellow used for taxiway and holding-position markings.
• Configuration: Installed as a row of three in-pavement lights across the taxiway at the holding position or intersection, typically adjacent to a co-located stop bar.
• Control: Illuminated continuously, or switched together with taxiway centreline lighting under low-RVR conditions, via the ALCMS.
• Indication: When illuminated, the clearance bar makes the hold point or intersection conspicuous; it does not itself indicate that a pilot may cross — that instruction comes from ATC and, where fitted, the co-located stop bar's on/off state.

Where the clearance bar protects an ILS critical area boundary or a runway-holding position alongside a stop bar, it is typically wired on its own control cable back to the local controller or Remote I/O, in addition to its series-circuit power feed, so the ALCMS can switch it independently of loop current selection elsewhere on the field. Its switching logic is separate from the stop bar's: the stop bar is what carries the actual stop/proceed instruction — on for hold, off (with an explicit ATC clearance) for proceed — while the clearance bar is commonly switched together with the taxiway centreline lighting it accompanies, illuminated to improve conspicuity of the hold point under low-RVR conditions rather than toggled in lockstep with the stop bar's state. Status and fault reporting flow back to the ALCMS per fixture rather than per loop, since — unlike the edge lighting series circuit — a fault at a protected hold point has direct operational significance and needs to be pinpointed quickly, not just flagged as a general loop-level anomaly.

A clearance bar light fixture is typically built as a set of three inset (or occasionally elevated) lamp units housed in a single die-cast aluminium or heavy-duty polymer base, set flush with the pavement at the holding position. Each of the three lamp positions uses an LED array or incandescent/halogen lamp behind an amber/yellow filtered lens or coloured LED optic, gasket-sealed with an O-ring to the housing to achieve an IP67/IP68 rating suitable for continuous wheel loading and standing water. The unit is load-rated for aircraft wheel traffic and is typically installed in an L-867-style base can recessed into the pavement, with stainless-steel or corrosion-resistant fasteners securing the top assembly. Internal wiring from all three lamp positions is bundled and terminates at a sealed L-823 primary connector, which mates to the secondary lead of the isolating transformer housed in a nearby base can, allowing the whole bar to be switched on and off (or flashed) as a single circuit element.