# How Airports Work

Science | Modern

## Airport Runways: Entering and Exiting the Friendly Skies

At its most basic level, an airport has just two parts -- an airfield and a terminal. The airfield consists of runways and ramps, known as taxiways, that connect to the terminal. When laying out runways, engineers borrow conventions used in navigation and surveying, fields that indicate direction with a compass reading. For example, the 0/360 position on a compass marks north; 90 degrees marks east; 180 degrees marks south; and 270 degrees marks west. Every runway has a different number, expressed in a shorthand format, painted at each end to designate its orientation. A runway facing due west on one end and due east on the other would be marked 27 and 9 respectively. A runway with a northwest/southeast orientation might be marked 31 on one side (short for 310 degrees), 14 on the other (short for 140 degrees).

The U.S. Federal Aviation Administration (FAA), which provides guidelines for airport layout, specifies 20 different ways to arrange runways, although many of these are variations on four basic configurations:

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• Single runway -- When using this layout, airport engineers will orient the runway so aircraft can take advantage of prevailing winds.
• Parallel runways -- The distance between the two runways depends on the size and number of aircraft using the airfield. Close parallels have less than 2,500 feet between runways; intermediate parallels are between 2,500 and 4,300 feet (762 and 1,310 meters) apart; and far parallels are greater than 4,300 feet apart. Dual-line layouts feature two sets of parallels spaced 4,300 feet apart.
• Open-V runways -- Two runways that converge but don't intersect make an open-V layout. This arrangement gives air traffic controllers greater flexibility as they maneuver planes on the runways. For example, if no wind is blowing, they'll use both runways. But if the wind becomes strong in one direction, they'll shut down one runway and use the one that allows planes to take off into the wind.
• Intersecting runways -- Runways that cross each other are common at airports where the prevailing winds can change throughout the year. The intersection point can be in the middle of each runway, near the threshold (where aircraft touch down) or far from the threshold (where aircraft lift off).

To determine the length of a runway, airport planners consider a number of factors, including airport elevation, temperature, wind velocity, airplane operating weights, takeoff and landing flap settings and runway surface condition (dry or wet). A typical runway at a commercial airport, which must be able to accommodate jumbo jets, is between 10,000 and 12,000 feet (3,048 and 3,658 meters) long. Runways at small community airports, which serve aircraft with 20 or fewer passenger seats, may only be 3,000 to 4,000 feet (914 to 1,219 meters) long [sources: Bennett, Landrum & Brown].

Length isn't the only consideration. The runway surface itself must be much thicker than a highway to handle the extra weight of modern aircraft. Imagine a fully loaded Boeing 777 touching its wheels to the ground. That's 660,000 pounds (299,370 kilograms) coming down hard on the runway surface! Runways have to be specially constructed to take that strain without cracking or, worse, buckling. When the Denver International Airport was built, it took 2.5 million cubic yards (1.9 million cubic meters) of concrete to create five 12,000-foot (3,658-meter) runways, plus taxiways and aprons. First, 6 feet (1.8 meters) of compacted soil was put down; then, a 1-foot-deep (0.3-meter) layer of soil was spread, topped by an 8-inch- (20-centimeter-) thick cement-treated base; that was followed by 17 inches (43 centimeters) of concrete paving [source: Denver International Airport].

After landing, pilots use taxiways to get from a main runway to the terminal area, with its many gates. That's where we're headed next.