Runways – an essential part of the equation when it comes to operating fixed wing aircraft. Not only do aircraft need a runway to operate at all, but they also need a runway long enough to take off or land. As such, we need to know how long that runway is. This is not as simple as getting your tape measure out and measuring the entire length of the surface. The International Civil Aviation Organization (ICAO) requires ‘declared distances’ for runways, which are published in the county’s Aeronautical information Publication (AIP). Let’s take a look at those declared distances and what they mean.
There are three key terms that define the different sections of the area we generally think of as ‘the runway’ that we need to understand first, as defined by ICAO.
The Runway
The Runway is defined as a “Rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft”. Notice that this doesn’t actually specify tarmac. Runways can vary vastly in length and surface material, but we will assume a paved runway for the sake of this explainer.
Runways are typically measured in meters, though in North America it is more common to use feet. That said, both of these are printed on airport charts.
The Stopway
The Stopway is an area placed immediately after the runway that is used by the aircraft in the event of a rejected takeoff, and as such it must be strong enough to support the aircraft without damaging it. The Stopway area is built of the same material and has to be at least the same width as the rest of the runway.
As Stopways have limited use and are expensive to construct, it can prove more cost effective for an airport to simply build a full-strength runway that functions with Stopway space in both directions. Stopways are marked with a series of yellow chevrons.
The Clearway
The Clearway is a defined area beyond the paved surface of the Runway and Stopway. This area must be managed by the airport and is typically on airport-owned land.The aircraft is never intended to be on the ground in this area under normal circumstances, but will be in its initial climb when passing over it. It must be free of obstacles and an aircraft must be able to reach a certain rate of climb over it. The size of the unobstructed area at the end of a runway may influence a pilot’s decision on how much flap to use (as a takeoff with flaps extended reduces the takeoff roll but also reduces the rate of climb).
According to The Federal Aviation Administration (FAA), Clearways must be at least 150 meters (500 ft) wide and the practical limit for clearway length is 300 meters (1,000 ft). It is interesting to note that the clearway can be on land or water.
A recent example of an aircraft entering the Clearway during a landing occurred on October 23, 2022, when Korean Air Flight 631 suffered a landing overrun. The image below shows the approximate location of the aircraft when it came to a stop in the Clearway.
Declared distances
Now that we understand the runway areas, let’s have a look at how we measure them to calculate the available takeoff, landing, and stopping distances that pilots need to know.
Takeoff Run Available (TORA)
The TORA is the total length of runway suitable to handle the aircraft’s takeoff run. This is often equal to the Landing Distance Available (LDA), unless there is a displaced threshold, which landing aircraft must land beyond. The TORA length cannot include the stopway or the clearway, and will be 15% shorter than the entire runway length.
Takeoff Distance Available (TODA)
Takeoff Distance Available (TODA) is a measurement that adds the Stopway to the Takeoff Run Available distance. If the airport does not have a clearway, the TORA and TODA are the same.
Landing Distance Available (LDA)
The Landing Distance Available (LDA) is the portion of the surface that is declared fit for the normal landing of an aircraft. This begins at the runway threshold and ends before the stopway.
Accelerated Stop Distance Available (ASDA)
This is the measure we are interested in for the event of a rejected takeoff or a landing overrun. The ASDA distance adds the Stopway to the TORA (during takeoff) or LDA (during landing) distance, telling us how far we have to safely stop.
Why are these important?
In short, safety. Before takeoff and landing, pilots conduct performance calculations that take into account factors such as the aircraft’s mass, wind and outside air temperature to calculate the distances required to safely takeoff and land. These are compared with declared distances above to ensure the runway is long enough to do so.
Cover photo: Andrea Spoladore, JetPhotos
