A runway is what makes a defined area an airport. As the FAA’s exhaustive airport design and engineering standards suggest, it is more than a long, straight strip of dirt, gravel, grass, concrete, or, at Addison Airport, asphalt cut with shallow grooves to help dissipate water so the wheels of landing aircraft will not hydroplane. This asphalt is but the top layer of several on top of a substrate graded to a precise longitudinal crown that ensures water will run to its shoulders. Its thickness will support a 60,000-pound aircraft with a single wheel on each landing gear strut and 120,000 pounds with dual-wheel struts.
Addison’s Runway is designated 15/33 for each end’s magnetic heading to the nearest 10 degrees. (This is the approach end of 15; the approach end of 33 is below.) Using three digits, their headings are 155 and 355 degrees magnetic. The magnetic north pole moves with time, and if the runway’s headings gain a degree, to 156 and 356, Addison will have five years to renumber it to the closest 10 degrees, 16and 36. Painted markings not only “name” each end of the runway, they identify the centerline, threshold, touch down zone, which is right after the runway number, and the fixed distance marks, a diminishing number of longitudinal lines spaced 500 feet apart.
Surrounding the pavement is a runway safety area, a smooth graded area free of obstacles that would damage an airplane that inadvertently undershoots the threshold, over-runs the opposite end, or veers off the pavement to either side. The runway lights that parallel each edge are frangible, designed to break away from their mounts when hit. Because an airplane’s forward speed plays a significant part in undershooting the runway threshold or over-running its other end, the FAA requires runway safety areas to extend 1,000 feet beyond the pavement. Some airports meet this requirement EMAS, engineered material arresting system. Like funhouse foam pits that kids dive into, an EMAS is built of durable materials that reliably absorb high amounts of kinetic energy without excessive damage to the aircraft.
Addison’s runway is is 7,202 feet long and 100 feet wide, but its entire length is not available for landing. The threshold of Runway 15 is displaced 979 feet from the actual end of the pavement. Runway 33’s threshold is displaced 771 feet. Airports displace their thresholds for a number of reasons, from obstacle clearance and noise abatement or meeting the undershoot and over-run runway safety area requirements.
White arrows, like the ones here on Runway 33, designate the displaced threshold. Airplanes can taxi on and start their takeoff runs from a displaced threshold, they just can cannot land on them. This reduces the runway’s available landing distance. At Addison, that means 5,613 feet on Runway 15 and 6,341 feet on Runway 33. Available landing distance isn’t the only number important to jets; the accelerate-stop distance is another. It’s the distance a jet needs to reach V1, and then stop using maximum brakes, if an engine fails before or at this speed. If an engine fails after V1, there isn’t enough pavement to safely stop so the pilot continues the takeoff on one engine, which is a design requirement for commercial and corporate jets. In planning every flight, pilots look at their destination’s runway information to make sure the runway meets the airplane’s requirements.
A runway’s requirement can also extend off an airport’s property. At most airports, a 3-degree glideslope ensures that a landing airplane will have an obstacle-free approach slope between its final approach fix and the runway’s touch down zone. Several different light systems help pilots fly this approach path in good weather. Addison uses a PAPI, a precision approach path indicator composed of four lights that shine red or white depending on the airplane’s elevation. Four red is too low, four white is too high, and two of each is just right.
When the weather is bad, pilots follow their instrument landing system instruments, which align them with the runway centerline and keep them on glide path. Each instrument approach has weather minimums classified by ceiling and visibility. With its ILS, Runway 15 minimums are 200 feet and a half mile, which is why its served by an approach lighting system, a combination of light bars and strobes, that help pilots quickly make the transition from instruments to the runway and a safe landing.