How do you measure a runway?

Runways are essential for the operation of fixed-wing aircraft, providing the necessary space for takeoff and landing. However, determining if a runway is suitable for an aircraft is not as simple as measuring its surface length with a tape measure. The International Civil Aviation Organization (ICAO) requires ‘declared distances’ for runways, which are detailed in a country’s Aeronautical Information Publication (AIP). Let’s delve into these declared distances and their significance.

Understanding the Runway Components

First, it’s important to understand the three key terms defined by ICAO that describe different sections of the area commonly referred to as ‘the runway’:

The Runway

The runway is a rectangular area on a land aerodrome specifically prepared for the landing and takeoff of aircraft. This definition does not specify the surface material, though for simplicity, we’ll assume a paved runway. Runways are usually measured in meters, though feet are commonly used in North America. Both units are typically shown on airport charts.

The Stopway

A stopway is an area immediately following the runway, designed to be used in the event of a rejected takeoff. It must be strong enough to support the aircraft without damage. Stopways are constructed from the same material as the runway and must be at least the same width. They are marked with yellow chevrons. Due to their limited use and high construction costs, airports may opt to extend the full-strength runway instead, effectively integrating stopway functionality.

The Clearway

A clearway is a designated area beyond the runway and stopway, managed by the airport and typically on airport-owned land. Aircraft are not meant to touch the ground in this area during normal operations but will pass over it during initial climb. The clearway must be obstacle-free to allow for a specific rate of climb. According to the Federal Aviation Administration (FAA), clearways must be at least 150 meters (500 feet) wide, with a practical length limit of 300 meters (1,000 feet). Clearways can be on land or water.

Declared Distances

Understanding the runway components allows us to discuss how these areas are measured to provide the necessary takeoff, landing, and stopping distances for pilots.

Takeoff Run Available (TORA)

TORA is the total length of the runway suitable for an aircraft’s takeoff run. It is often equal to the Landing Distance Available (LDA), unless there is a displaced threshold, which landing aircraft must land beyond. TORA does not include the stopway or clearway and is typically 15% shorter than the entire runway length.

Takeoff Distance Available (TODA)

TODA includes the TORA plus the stopway distance. If no clearway exists, TORA and TODA are the same.

Landing Distance Available (LDA)

LDA is the portion of the runway surface declared suitable for landing. It begins at the runway threshold and ends before the stopway.

Accelerated Stop Distance Available (ASDA)

ASDA is relevant for rejected takeoffs or landing overruns. It combines the stopway with the TORA (for takeoff) or LDA (for landing), providing the total distance available to safely stop the aircraft.

Importance of Declared Distances

In summary, declared distances are critical for safety. Before takeoff and landing, pilots perform performance calculations considering factors such as the aircraft’s mass, wind, and outside air temperature. These calculations determine the required distances for safe takeoff and landing, which are then compared with the declared distances to ensure the runway is adequate for the operation.