Runway numbers describe magnetic direction
A runway designation is based broadly on its magnetic heading rounded to the nearest ten degrees. The opposite end carries the reciprocal number, so the same strip is identified differently depending on the direction in use.
Wind strongly influences runway choice
Aircraft generally prefer to take off and land into the wind because it improves performance relative to the ground. Airports may still use another runway because of traffic flow, noise procedures or operational constraints.
Takeoff distance changes with weight and conditions
More weight usually requires more runway. High temperature and high elevation reduce air density, which can also increase required distance or force payload restrictions at airports where runway length is limited.
Landing performance has its own constraints
Pilots consider runway length, braking action, wind, slope and contamination such as water, snow or ice. A runway that is perfectly adequate when dry may become restrictive in poor conditions.
Some aircraft need stronger pavement as well as more length
Large aircraft place substantial loads on runways and taxiways. Airports publish pavement strength information so operators can determine whether regular use is suitable.
Terrain and obstacles affect usable performance
Mountains, buildings and departure procedures can limit how an aircraft climbs away from the airport. That is why runway length alone does not tell the full story of whether a particular airplane can operate safely from a field.
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