Several factors determine a runway’s length. (Hint: Flight volume isn’t one of them.)
The longest commercial service runway in the world is 18,045 feet long—that’s 3.4 miles or 5.5 kilometers—at the Qamdo Bamda Airport (BPX) in the mountains of Tibet. Contrast that with an airport near my neck of the woods: Machias Valley Airport along the coast of Maine. Its lone coastal runway is all of 2,900 feet—just over a half mile and 116 meters shy of a kilometer.
Why so different?
Airport runway length is contingent on a number of factors, including aircraft type. The larger the aircraft, the longer the runway required. This makes sense when you consider that a typical commercial jetliner needs to reach 150 to 180 mph (240 to 285 km/h) to take off, and the larger the aircraft, the longer it takes to reach these speeds. For the same reason, an aircraft’s maximum takeoff weight is also a factor. The more people and cargo on an aircraft, the longer the runway required. And the higher the aircraft’s engine thrust, the faster the aircraft can reach its take-off speed, and the shorter the runway required.
Many factors determine runway length including elevation above sea level.
Environmental factors also play a role. The higher the elevation, the lower the atmosphere pressure or air density. As wind flows over the wing, the air pressure on the bottom of the wing is greater than on top, thereby creating lift (known as Bernoulli’s principle). The “thin air” at high elevations results in less lift on the aircraft. This means the aircraft has to travel faster to take off and therefore needs a longer runway to do so. Qamdo Bamda Airport’s runway—which only handles a handful of flights per day—is so long because it sits 14,219 feet above sea level, making it the second highest airport in the world.
Decreased atmospheric pressure also results in reduced engine thrust because the engine has less oxygen to burn. This means the aircraft must travel at a greater speed and for a longer distance to achieve a safe takeoff. The same principles that apply to elevation also apply to temperature. The higher the temperature, the lower the atmospheric pressure, the longer the runway needs to be.
“I hope this runway’s long enough!”
Between these two runway extremes sits a “typical” commercial airline runway length: roughly between 8,000 feet (2,438 meters) and 13,000 feet (3,962 meters). Although most airline passengers don’t give much thought to runway length, I sometimes hear the stray passenger remark during takeoff or landing, “I hope this runway is long enough.”
Rest assured, it is! Commercial pilots are required to calculate the runway length required for their respective aircraft based on fuel, payload, temperature, and elevation to assure they are within runway tolerances. If the aircraft exceeds pre-flight tolerances, they pilot may need to reduce payload (freight, luggage, and/or passengers) before taking off.
If you’re interested in monitoring available runway length during takeoff or landing, there’s an easy method. Commercial service airports are equipped with “distance remaining markers.” These signs have a white number on a black background denoting the distance remaining to the end (threshold) of the runway. The number is a single digit; a “5” indicates 5,000 feet remaining to the end of the runway, a “4” indicates 4,000 feet, and so on. The signs are typically placed on the left side of the runway, as viewed from the most commonly used runway direction.
Runway “distance remaining markers” indicate how many thousand feet are left until the runway runs out.
When my daughter was young, she, along with her stuffed animals, would gaze out the window during take-off and count down the numbers on the "big black signs." Now she – and you – can do the same with the insider knowledge of what they actually stand for.
About the Author
David Dargie is the sector lead for our airports infrastructure group. He teams up with large and small airports on airfield improvement projects and collaborates with our architecture group for terminal expansions.More Content by David Dargie