Runway Design: Engineering Safe Airport Landing Strips

Airport runways are defined by a number of factors, including their length, width, and orientation, as well as the location, number, and direction of the runways themselves. The layout of an airport runway is influenced by the number of runways, their orientation, the shape and constraints of the site, and the need to avoid obstacles during takeoff and landing. Airports with a single runway are often constructed to align with the prevailing wind, while larger airports may have multiple runways in different directions to accommodate changing wind patterns. Runways are typically long, paved strips with markings and lighting to indicate their dimensions and direction, and they are designed to accommodate various aircraft types and sizes.

Runway construction and maintenance

One of the preliminary steps in constructing a runway is compiling a wind rose to determine the wind direction. Airports with a single runway are often built to align with the prevailing wind, reducing the runway distance required during takeoff and landing. The length of the runway is influenced by factors such as aircraft weight, airport elevation, and air temperature, with specific "declared distances" defined for operational capability.

Runway maintenance is essential to minimise repair costs and maintain safe operations. Inspections play a vital role in safety and economic outcomes, and they can lead to temporary runway closures to ensure the safety of construction crews and aircraft. Regenerative construction on runways must be expedited to minimise downtime, often requiring continuous 24-hour shifts.

The maintenance of runway safety areas is crucial. These areas are designed to mitigate the risk of aircraft damage in the event of an excursion from the runway. They must be kept clear of debris and properly drained and graded to support aircraft and emergency operations. Various companies offer specialised services and technologies for runway maintenance, including asphalt preservation, line marking, grass management, and ground support equipment.

Runway markings and designations

The numbers painted at the end of runways are not sequential but are based on the magnetic azimuth, or compass bearing, of the runway's orientation. With 360 degrees on a compass rose, runway numbers are determined by rounding the compass bearing of one end to the nearest 10 degrees and removing the last digit, resulting in runway numbers ranging from 1 to 36. For instance, a runway numbered 9-27 indicates an east-west orientation, with 9 representing east and 27 representing west.

When referring to runway number designations during airport operations, air traffic control pronounces each number individually. For example, Runway 4L-22R would be pronounced as "Runway Four Left" or "Two Two Right," depending on the direction of landing clearance. This clear enunciation enhances communication and safety.

In addition to the numerical designations, runways may also use compass point naming, especially those with non-hard surfaces, such as small turf airfields and waterways for seaplanes. For instance, the Ketchikan Harbor Seaplane Base uses the designation "Waterway E/W." Airports with unpredictable water currents may use the designation "Waterway ALL/WAY" to indicate the lack of a designated landing direction.

Runway length and elevation

The length of a runway can vary significantly depending on various factors. Runways can be as short as 804 feet (245 meters) at smaller general aviation airports and as long as 18,045 feet (5,500 meters) at large international airports. The lake bed runway 17/35 at Edwards Air Force Base in California is a massive 39,098 feet (11,917 meters) long and was developed as a landing site for the Space Shuttle.

The length of a runway is determined by several factors, including the type of aircraft it will accommodate, the aircraft's maximum takeoff weight, the number of people and cargo on the aircraft, and the aircraft's engine thrust. The larger the aircraft and the more people and cargo it carries, the longer the runway needs to be. On the other hand, higher engine thrust can reduce the length of the runway required as it allows the aircraft to reach takeoff speed faster.

Another critical factor that affects runway length is the airport's elevation above sea level. Airports located at higher elevations experience lower atmospheric pressure and air density, which results in reduced lift for aircraft. Consequently, aircraft need to travel faster to achieve takeoff, requiring a longer runway. For example, the Qamdo Bamda Airport in Tibet has a runway length of 18,045 feet (5,500 meters) to accommodate for its high elevation of 14,219 feet above sea level.

In addition to the physical length of the runway, there are several other distance considerations that come into play during takeoff and landing. These include:

• Takeoff Run Available (TORA): The length of the runway declared available and suitable for the ground run of an aircraft taking off.
• Takeoff Distance Available (TODA): The length of the takeoff run available plus the length of the clearway, if provided.
• Landing Distance Available (LDA): The length of the runway declared available and suitable for the ground run of an aircraft landing.
• Emergency Distance Available (EMDA): LDA (or TORA) plus a stopway.

Obstacle limitation surfaces

There are several types of OLS, including the approach surface, the inner horizontal surface, the conical surface, and the take-off climb surface. The approach surface is a wedge-shaped surface that slopes upward in stages from the end of the runway. The inner horizontal surface is an imaginary flat plane established around licensed aerodromes, which can be circular or 'racetrack' shaped. The height of this surface is defined relative to the lowest runway threshold. Extending outwards from the inner horizontal surface is the conical surface, which slopes upwards away from the runway at a uniform angle. The take-off climb surface is also set at a uniform angle, sloping upwards away from the runway, and is established for each runway direction intended for takeoff.

The Outer Horizontal Surface is another type of OLS. For a civil aviation runway longer than 1,860 meters, the Outer Horizontal Surface is set at 150 meters above the lowest runway threshold. For military aircraft using a similar runway, the height is 5 meters lower, at 145 meters.

Understanding OLS is crucial for any construction projects near aerodromes. Failing to consider these surfaces can jeopardize a project's success. Different countries and organizations have their own variations of OLS guidance, and it is important to refer to the relevant guidelines for specific requirements.

Runway orientation and wind direction

The orientation of a runway is defined by its magnetic azimuth, or compass bearing, and is numbered from 1 to 36 based on the nearest 10-degree increment. For example, a runway numbered 9-27 is oriented east-west. The opposite end of the runway always differs by 180 degrees, so it is numbered 18 higher or lower. Airports with one runway are often constructed to be aligned with the prevailing wind direction, as it is advantageous for planes to take off and land into the wind.

When constructing airport runways, one of the preliminary steps is to compile a wind rose, which shows the wind direction and helps determine the runway orientation. Originally, in the 1920s and 1930s, airports were built with three runways in a triangle-like pattern at 60-degree angles to each other. This was due to the limited knowledge of wind behaviour at the time, and the understanding that wind affected runway distance requirements. As a result, the runway with the most traffic would become the main runway, while the others would be converted or abandoned.

Today, larger airports have several runways in different directions to accommodate varying wind conditions and traffic patterns. These runways are identified by appending left (L), centre (C), or right (R) to the end of the runway number, indicating their position when facing the direction of the runway. For example, runways one-five-left (15L), one-five-centre (15C), and one-five-right (15R). Some airports with multiple parallel runways may shift the runway identifiers by one to avoid ambiguity, as in the case of Los Angeles International Airport, which has runways 6L, 6R, 7L, and 7R, despite all four runways being parallel.

Runway orientation is also influenced by the need to avoid obstacles during landing and takeoff procedures. Additionally, factors such as airport elevation, air temperature, aircraft weight, and take-off mass impact the runway length requirements and overall runway design.

Frequently asked questions