threshold
The start of the landing area of a runway.
threshold: where the runway officially begins
In aviation, the threshold is the first point of the runway surface where an aircraft is permitted to touch down during landing. It is marked by a white line painted perpendicular to the runway centerline, and often accompanied by a series of white stripes called threshold markings that extend into the runway. The threshold does not necessarily coincide with the physical start of the runway pavement; there may be overrun areas, blast pads, or clearway zones before it.
The threshold location matters because it defines the usable landing distance available to a pilot. Runway length is measured from the threshold to the far end, not from the actual beginning of the pavement. Obstacles such as buildings, terrain, or radio navigation equipment near an airport may force the threshold to be displaced, or moved inward from the physical runway start. A displaced threshold reduces the available landing distance, which limits the weight and fuel load of aircraft that can safely land there.
Threshold identification and markings
The threshold is identified by its designation number, which corresponds to the magnetic heading of the runway rounded to the nearest ten degrees. A runway oriented toward 090 degrees magnetic is labeled runway 09; the opposite direction is runway 27. The threshold area includes approach lighting systems and precision approach path indicators (PAPI) or similar visual guidance aids positioned to the side of the runway to help pilots judge descent angle and height during final approach.
Maintenance of the threshold involves keeping markings visible and the surrounding surfaces free of debris that could damage aircraft tires or engines. The grass or paved areas immediately before the threshold, called the approach surface, must be maintained to prevent foreign object damage (FOD). Displaced thresholds require special attention because the unusable runway area behind them can accumulate standing water or become a trap for wildlife during certain seasons, creating hazards during crosswind landings.