Taxiway

A Taxiway is a defined paved path on an aerodrome used for the ground movement of aircraft between runways, aprons, hangars, and other facilities. Taxiways are essential for the efficient flow of air traffic on the ground, allowing aircraft to move safely without interfering with runway operations. They are typically constructed of asphalt or concrete and are marked with standardised visual guidance systems including edge markings, centreline markings, and taxiway guidance signs.

Key characteristics of a taxiway include:

• Width: Generally 7.5 m – 25 m depending on the aircraft category served (ICAO code letter A through F), with adequate clearance for wingtips and engine nacelles.
• Pavement Type: Flexible (asphalt) or rigid (concrete) pavement, designed for lower loads than runways but still capable of supporting heavy aircraft.
• Markings: Yellow centreline (solid or dashed), edge lines (double yellow), and holding position markings (solid and dashed yellow lines).
• Lighting Systems: Blue edge lights and green centreline lights for night and low‑visibility operations, plus stop bars and clearance bars at intersections.
• Signage: Location, direction, and mandatory instruction signs that provide critical guidance to pilots.
• Holding Positions: Designated points where aircraft must stop and await clearance to enter a runway or cross another taxiway.

The taxiway's purpose is to let an aircraft move under its own power between the apron and the runway without needing anything like the precision instrument guidance used for takeoff and landing, relying instead on a defined path, visual markings, and lighting that a pilot can follow reliably even at night or in reduced visibility. Because ground movement happens far more frequently than takeoffs and landings and involves aircraft, at various points, sharing the network with other aircraft and occasionally vehicles, the taxiway's markings, signage, and holding positions exist to keep that movement orderly and predictable, so an aircraft always knows both where it is permitted to go and where it must stop and wait for clearance before proceeding further, particularly at any point where its path would otherwise cross an active runway.

Taxiways are found at all airports where aircraft need to move between the runway and other areas:

• Commercial Airports: Complex networks of parallel, connecting, and apron taxiways to handle high volumes of aircraft movement, often with rapid exit taxiways to clear runways quickly after landing.
• General Aviation Airports: Simpler taxiway layouts connecting runways to hangars and fuel facilities.
• Military Airfields: Taxiways designed for tactical aircraft, often with blast pads and hardened shelters.
• Heliports: Smaller taxiway or taxi‑route systems for helicopter ground movement.
• Cargo and Maintenance Areas: Specialised taxiways leading to cargo terminals, maintenance hangars, and de‑icing pads.

Taxiways serve multiple operational functions:

• Runway Access/Exit: Connecting aircraft to and from the runway for take‑off and landing.
• Apron Access: Guiding aircraft to and from parking stands, gates, and servicing areas.
• Cross‑field Movement: Allowing aircraft to move between different parts of the airport without using the runway.
• Holding and Waiting: Providing areas where aircraft can hold before entering a runway (holding points).

Taxiway alignment and width are set by the code letter of the largest aircraft expected to use it, with curves, fillets, and junction geometry designed around that aircraft's wheelbase and turning performance so the wingtip and gear clearances required by ICAO Annex 14 are maintained throughout. Pavement structure is built to a lighter design loading than the runway, since taxiing is a rolling, low-speed load rather than the touchdown or take-off roll stresses the runway sees, but it still ties structurally into the runway pavement at each connecting point so there's no differential settlement at the junction. Centreline and edge lighting, signage, and holding position markings are surveyed and set out together with the pavement during construction, and the taxiway's route is planned to keep separation from the runway's obstacle-limitation surface and safety areas along its full length.

A taxiway operates through an integrated system of design, marking, lighting, and control:

• Geometric Design: Taxiways are designed with appropriate radii at turns to accommodate the turning circle of the largest aircraft type expected. Fillet curves at intersections ensure wingtip clearance.
• Pavement Strength: Taxiways are designed to support the static and dynamic loads of taxiing aircraft, using the ACN‑PCN system to communicate pavement strength to pilots and ground handlers.
• Surface Friction and Drainage: Like runways, taxiways are textured and sloped for adequate friction and water runoff to prevent hydroplaning during ground movement.
• Visual Markings: Taxiways use a standardised system of painted markings:
  • Centreline: A single solid yellow line, with dashed segments for turns and intersections.
  • Edge Lines: Double yellow lines defining the lateral limits of the taxiway.
  • Holding Position Markings: Two solid and two dashed yellow lines perpendicular to the taxiway centreline — FAA and ICAO both use yellow for this marking — indicating where aircraft must stop before entering a runway.
• Lighting Systems: Taxiways are equipped with:
  • Blue Edge Lights: Omni‑directional blue lights defining the taxiway edge, spaced at intervals not exceeding 60 m on straight sections and closer on curves.
  • Green Centreline Lights: Inset green lights along the centreline, spaced at 7.5 m or 15 m intervals.
  • Stop Bars: Red lights across the taxiway at runway holding positions, indicating the point where aircraft must stop.
  • Clearance Bars: Alternating yellow and green lights indicating that a crossing is cleared.
• Guidance Signage: Taxiway signs provide location and directional information:
  • Location Signs: Black background with yellow text and border indicating the current taxiway.
  • Direction Signs: Yellow with black text and arrows indicating direction to other taxiways or runways.
  • Mandatory Instruction Signs: Red with white text (e.g., "RUNWAY 09", "STOP") indicating mandatory hold points.
• Control and Monitoring: Taxiway traffic is managed by Air Traffic Control (ATC) through ground radar, visual observation, and communication with pilots. Advanced airports use Surface Movement Radar (SMR) and Airport Surface Detection Equipment (ASDE) to track aircraft and vehicles on taxiways.

Blue edge and green centreline lights run on constant-current series circuits fed by dedicated taxiway CCRs, commanded by ALCMS at loop level exactly as for runway lighting — every fixture on a given loop switches and dims together. Stop bars and clearance bars sit in an additional interlock layer above this: ALCMS or tower-controlled stop bar logic can override the local guidance loop segment at a holding position, forcing the red stop bar on and, where clearance bars are fitted, coordinating the yellow/green indication, independent of whether the adjoining centreline/edge loop is otherwise lit. This layered architecture — loop-level intensity control plus point-level interlock overrides at holding positions — is what lets a single taxiway circuit both provide continuous guidance lighting and enforce hard stop points.

A taxiway is built as a full structural pavement, constructed from a compacted subgrade overlaid with graded sub-base and base courses, topped with either a flexible asphalt wearing course or a rigid reinforced concrete slab, with the total pavement thickness sized to the weight and frequency of the aircraft the taxiway is designed to serve. The pavement is graded with a longitudinal slope kept low to ease aircraft ground handling and a transverse crown or slope to shed surface water into adjacent drainage, and at turns and intersections the pavement is widened into fillets to maintain wingtip clearance as aircraft manoeuvre through the curve. Centreline and edge markings are applied to the finished surface using reflective, durable paint or thermoplastic material rated for aircraft tyre contact and de-icing fluid exposure, while centreline and edge lights are set into the pavement in cored recesses (inset fixtures) or mounted on frangible stakes alongside the edge, wired back through underlying conduit runs installed during pavement construction. Buried cable ducting, drainage structures, and any embedded sensor loops are laid beneath the base courses before the final wearing course is placed, so the finished pavement presents a continuous, unobstructed surface to aircraft.