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A nearby airport has a single 05/23 runway (3900m long), where airplanes start and land in one direction during certain periods and in the opposite direction during other periods.

How is the direction determined -- I assume it's air traffic control which decides that but based on what ? (wind, sun ?)

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There are several factors:

  • Prevailing winds
  • Neighboring airport traffic patterns
  • Size of inbound aircraft
  • Where the aircraft is coming from
  • Final parking space
  • Congestion
  • Fuel status
  • Weather (thunderstorms)
  • Noise considerations

At small airports, it is usually just the runway that faces into the wind and can accommodate most of the traffic coming in.

At large airports, the wind dictates which flow is being used (like a "north flow" or "south flow"). From there, aircraft usually are assigned to land on which ever side of the airport their arrival procedure brings them to. Sometimes, it is also decided by which gate/terminal they'll be parking at. Lastly, the available approach procedures can change things if the weather is not good

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    $\begingroup$ At some small mountain airports, it's not physically possible to use both ends of the runway - airplanes must land one way, and take off in the other direction, due to nearby mountains. $\endgroup$
    – fluffysheap
    Commented Oct 1, 2014 at 6:34
  • $\begingroup$ @fluffysheap: And some airports have the same sort of restriction for other reasons; for instance, runway 14/32 at Logan. $\endgroup$
    – Vikki
    Commented Dec 17, 2018 at 4:29
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Wind direction mostly, planes take off best when facing in the wind as that means free airspeed so the takeoff roll is shorter.

In busier airspace (with multiple towered airports) each airport coordinates the corridors so they don't intersect depending on available runways, prevailing winds and relative demands on the airports.

It is possible that it is dictated by noise abatement measures so night time flights pass over a nearby forest rather than wake up the capital.

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    $\begingroup$ or the other way around. What with environmental regs ever more putting small furry creatures above and ahead of humans... $\endgroup$
    – jwenting
    Commented Sep 30, 2014 at 7:23
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    $\begingroup$ Early airfields had a number of intersecting runways so that the one most closely aligned with wind direction could be designated the active runway. An example is London Heathrow which briefly had six runways intersecting in a star of David pattern. Nowadays aircraft are better able to cope with cross-wind landings, but it is still advantageous to take-off and land into the wind. $\endgroup$
    – RedGrittyBrick
    Commented Oct 1, 2014 at 20:00
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Runway in use is based on wind, most of the time, with aircraft taking off into the wind. In some cases, when the wind is calm, airports have a default runway (it usually has better lighting, or approach systems for aircraft to use during conditions with poor visibility).

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Most aircraft have a max tailwind limitation (those I've flown had a limit of 10 knots). So for convenience, a pilot could take off with up to that 10 know tailwind, but not with a stronger tailwind. Depending on runway length, and aircraft gross weight, it might not be possible to takeoff with the full max tailwind, necessitating a takeoff into the wind anyway. Some runways are backed up against hills or mountains, and aren't safe to takeoff in that direction, so always land toward and takeoff away from the rising terrain.

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