Airport Landing Strips: Design And Functionality Explored

Airport landing strips, or runways, are designed with specific materials and markings to facilitate the safe takeoff and landing of aircraft. The design of a runway can vary depending on the size and location of the airport, with some made from asphalt, concrete, grass, gravel, or even ice and sand. Runways may also have different equipment and markings at each end, with precision guidance equipment often installed on only one end to reduce costs. Visual runways, commonly found at small airstrips, may lack any markings, while precision instrument runways at larger airports provide both horizontal and vertical guidance for instrument approaches. The maintenance and pavement conditions of runways are crucial for safety and economic outcomes, with inspections conducted regularly to ensure proper functionality.

Runway types: visual, non-precision instrument, precision instrument

There are three types of runway markings: visual, non-precision instrument, and precision instrument.

Visual runways are typically found at small airstrips and are usually a strip of grass, gravel, ice, asphalt, or concrete. They may have threshold markings, designators, and centerlines, but they do not provide an instrument-based landing procedure. Pilots must be able to see the runway to use it and must be self-reliant as radio communication may not be available.

Non-precision instrument runways are found at small to medium-sized airports. They may be marked with threshold markings, designators, centerlines, and sometimes a 1,000 ft (305 m) mark (known as an aiming point, sometimes installed at 1,500 ft (457 m)). The centerlines provide horizontal position guidance, while the aiming point markers provide vertical position guidance to planes on visual approach. Non-precision approaches are instrument approaches and landings that utilise lateral guidance but not vertical guidance. They make use of ground beacons and aircraft equipment such as VHF Omnidirectional Radio Range (VOR), Non-Directional Beacon, and the LLZ element of an Instrument Landing System (ILS).

Precision instrument runways are found at medium and large-sized airports. They consist of a blast pad/stopway (optional, for airports handling jets), threshold, designator, centerline, aiming point, and touchdown zone marks. Precision runways provide both horizontal and vertical guidance for instrument approaches.

Runway markings: threshold, designator, centreline, aiming point

There are three types of runway markings: visual, non-precision instrument, and precision instrument. Visual runways are usually just a strip of grass, gravel, ice, asphalt, or concrete with no markings. However, they may have threshold markings, designators, and centrelines. Non-precision instrument runways are found at small- to medium-sized airports and may be marked with threshold markings, designators, centrelines, and sometimes a 1,000 ft (305 m) mark (known as an aiming point). Precision instrument runways are found at medium- and large-size airports and consist of a blast pad/stopway, threshold, designator, centreline, aiming point, and touchdown zone marks.

Threshold Markings

Threshold markings are a series of bold, long, wide stripes painted between the start of the runway and the numbers. They mark the beginning of the space available for landing. The number of stripes corresponds to the width of the runway. For example, a 60-foot-wide runway would have four stripes (two on each side), while a 200-foot-wide runway would have 16 stripes. These stripes are usually 20 feet beyond the beginning point of the runway surface usable for landings.

Designators

Runways are designated using the magnetic direction that they point towards, rounded to the nearest 10 degrees. For example, a runway pointing in a 063 magnetic heading will be designated as "Runway 6" ("Runway 06" outside of the US). The runway number is determined from the approach direction and is the whole number nearest one-tenth the magnetic azimuth of the centreline of the runway, measured clockwise from the magnetic north.

Centreline

The runway centreline identifies the centre of the runway and provides alignment guidance during takeoff and landings. The centreline consists of a line of uniformly spaced stripes and gaps. Each stripe is 120 ft in length, and the gap between markings (stripes) is 80 ft (so 200 ft between the start of one stripe and the start of the next).

Aiming Point

The aiming point marking serves as a visual aiming point for a landing aircraft. It consists of two thick white stripes on each side of the centreline, usually 1,000 ft (305 m) from the runway threshold. Aiming point markers are used as a target on the runway to fly towards and provide vertical position guidance to planes on a visual approach. They should not be confused with touchdown zone markings, which are thin white stripes that identify the touchdown zone of the runway.

Runway surfaces: grass, gravel, asphalt, concrete

Runway surfaces are constructed from a variety of materials, including grass, gravel, asphalt, and concrete. Each surface has its own advantages and disadvantages, and the choice of material depends on various factors such as cost, aircraft weight, weather conditions, and maintenance requirements.

Grass runways, commonly found at small airstrips, are typically only suitable for small, light aircraft models or those specially designed for grass surfaces. Before the introduction of heavy monoplane aircraft in the 1930s, civil air-transport aircraft could operate from grass runways with takeoff distances of less than 600 metres (2,000 feet). However, with the advent of heavier aircraft, paved runways became necessary to accommodate longer takeoff distances.

Gravel runways are less common than concrete or asphalt, usually found at smaller airfields. Gravel runways have limited versatility, as planes often require special modifications or design considerations to land on them. For example, Boeing offers a commercial kit with abrasion-resistant paint for the underside of wings and fuselage, as well as metal shields for brake cables and hydraulic tubes.

Asphalt runways are made from aggregates like sand, gravel, and crushed stone, bound together with bitumen, a sticky, black, highly viscous liquid derived from petroleum. Asphalt provides a smooth surface, reducing wear and tear on aircraft tires and landing gear. It is also flexible, accommodating slight ground movements without cracking, making it suitable for areas with variable temperatures. However, asphalt requires more frequent maintenance and resurfacing than concrete due to wear and degradation over time. It is sensitive to temperature, with heat causing softening and potential surface deformation, and cold causing brittleness and cracking. Special mixtures are available to improve durability, such as the Marshall mixture, which includes a higher proportion of bitumen, and blends that are more resistant to airplane fuel and hydraulic fluids.

Concrete runways, composed of coarse aggregate (gravel or crushed stones), fine aggregate (sand), and a fluid cement that hardens over time, offer extreme durability and can last several decades with minimal maintenance. Reinforced concrete incorporates steel bars or mesh to increase tensile strength and load-bearing capacity, making it capable of withstanding heavy aircraft loads. Concrete runways are more resistant to cracking and damage than non-reinforced concrete, particularly in high-stress areas. However, the addition of steel reinforcement increases the initial construction cost and complexity. Grooving is often applied to concrete runways to enhance aircraft safety and improve braking performance during wet conditions.

Runway maintenance: inspections, repairs, and renovations

Runway maintenance is critical to ensuring the safety of aircraft operations and can also impact the economic outcomes of an airport. Regular inspections are necessary to maintain runway surfaces in good condition and to prevent costly repairs. The Federal Aviation Administration (FAA) in the United States, for example, conducts yearly inspections.

Runway pavement maintenance requires the periodic renewal of the top or wearing surface. The interval between surface renewal depends on the type of surface. The most common hard surface types used in runways are concrete and asphalt. Concrete surfaces are often grooved laterally to aid in surface water dispersal, while asphalt surfaces may employ a porous top layer to allow water to run off below the surface. Certain types of asphalt can also be grooved.

Minor repairs such as joint re-sealing, crack stopping, and rubber deposit removal from the touchdown zone (TDZ) can be completed with relatively little runway downtime. However, major repairs or renovations will require either complete or partial runway closure for an extended period. To minimise the impact on airport operations, some repairs may be carried out at night or during periods of low air traffic.

Runway renovations can involve resurfacing or replacing the pavement. For example, during the renovation of the runway at Madrid Barajas Adolfo Suárez Airport, 70,000 tons of flexible pavement were laid in just 24 hours. To minimise the closure time, runway renovations must be carried out at a much faster pace than roadway construction.

To ensure the safety of aircraft operations and to avoid incidents, it is crucial to properly brief flight crews on runway availability and closures due to maintenance or repairs. This includes providing accurate and up-to-date information during pre-flight briefings and ensuring clear runway identification.

Runway lighting: controlled by tower or station, or pilot-controlled

Runway lighting is crucial for pilots to navigate their aircraft safely, especially during low-visibility conditions like nighttime operations. These lights are typically controlled by air traffic controllers in a control tower or flight service station, who can adjust their intensity and settings as needed. However, some airports, particularly smaller or uncontrolled airfields, may have pilot-controlled lighting systems. This allows pilots to temporarily activate the lights when no other authority is available, avoiding the need for automatic systems or staff to manually turn on the lights.

Pilot-controlled lighting systems offer several advantages, including cost savings by eliminating the need for permanent lighting systems or staff to operate them. These systems are typically found at smaller airports or private airfields, where they may be the only source of illumination for runways during low-visibility conditions. The lighting systems can be activated by pilots using specific frequencies provided in the Chart Supplement and standard instrument approach procedures publications.

In contrast, at controlled airports with air traffic services, runway lighting is an essential tool for controllers to manage traffic and maintain safe operations. Controllers can adjust the lighting intensity to one of five brightness levels, ensuring optimal visibility under various conditions. This flexibility allows controllers to provide the best visual guidance to pilots during takeoff, landing, and taxiing.

While runway lighting is crucial for safe operations, it is just one component of a comprehensive system. Airports also employ other lighting aids, such as taxiway centerline lights and clearance bar lights, to facilitate ground operations and enhance safety during low-visibility conditions. These lights aid pilots in navigating their aircraft to and from runways and maintaining proper spacing between aircraft.

Additionally, runway status lights, such as Runway Entrance Lights (REL) and Takeoff Hold Lights (THL), provide critical safety information. These lights are automated and signal to pilots and vehicle operators when it is unsafe to enter, cross, or begin takeoff on a runway. They respond to traffic conditions, providing immediate alerts without requiring input from controllers, thus adding an extra layer of safety to runway operations.

Frequently asked questions