Runway orientation

Aircraft take off and land into the wind, because a headwind shortens the ground run. A crosswind beyond a limit makes operations unsafe. Runway direction is therefore chosen from wind records so that the runway can be used for a high share of the time.

Wind data, direction and speed over several years, are plotted on a wind rose. A template representing the runway with a band whose half-width is the permissible crosswind is rotated over it to find the orientation that gives the highest usability. ICAO Annex 14 recommends a usability factor of at least 95 percent, with allowable crosswind depending on aircraft size.

Other factors also shape the choice: obstacles and terrain on the approach paths, noise over towns, airspace, land and soil conditions. Where one direction cannot reach the target usability, a crosswind runway is added.

Runways are numbered by their magnetic heading divided by ten and rounded, so a runway pointing 087 degrees is runway 09, and its other end, 267 degrees, is runway 27.

Changing a runway's orientation later is almost impossible, so the decision deserves long wind records and careful study.