How runway selection works ​

Aircraft land and take off into the wind. That's the single most important rule driving runway selection. Everything else is secondary.

A headwind reduces the ground speed during landing and takeoff, shortening the runway distance needed. A tailwind does the opposite - it increases ground speed and requires more runway. Most aircraft have a tailwind limit of about 10 knots for landing and 15 knots for takeoff.

Wind determines direction ​

If the wind is blowing from the west at 270 degrees, west-facing runways are active. Runway 27L and 27R would be in use. If the wind shifts to come from the east at 090 degrees, the airport flips to runway 09L and 09R.

The number on a runway is its magnetic heading divided by 10 and rounded. Runway 27 points at 270 degrees. Runway 09 points at 090 degrees. They're the same strip of pavement, just used in opposite directions.

When wind is calm or light and variable, airports pick a "calm wind" configuration based on noise abatement, nearby traffic, terrain, or operational preference. At JFK, that's usually landing runway 31L. At LAX, it's westbound operations almost always.

Crosswind component ​

Wind rarely blows straight down the runway. A 15-knot wind at 30 degrees off the runway heading creates a crosswind component. Most commercial aircraft can handle crosswinds up to about 30-35 knots, but it gets uncomfortable well before that.

When crosswinds get strong, controllers might select a runway more aligned with the wind even if it's less efficient for traffic flow.

Parallel runway operations ​

Big airports have parallel runways specifically so they can handle more traffic. But not all parallel operations are the same.

Independent parallel approaches require runways spaced at least 4,300 feet apart (with certain radar systems, sometimes less). Aircraft land on both runways simultaneously without coordination between the two approach streams. JFK runs independent parallels on 31L and 31R.

Dependent parallel approaches are for runways closer together. Aircraft on adjacent runways must be staggered - you can't have two aircraft at the same distance from the threshold. This cuts capacity compared to independent operations.

Single runway operations are what you get at smaller airports or when weather forces it. One runway for both arrivals and departures. The controller has to sequence departures between arriving aircraft, which limits throughput to about 30-40 operations per hour.

Runway configuration ​

A runway configuration is the specific combination of active runways. At a big airport, this is a detailed plan:

Atlanta (KATL) has five parallel runways. A typical south-flow configuration uses:

• Runways 26L and 27R for arrivals
• Runways 28L and 28R for departures
• Runway 26R available for either

That's five runways working simultaneously. Switching to north-flow means flipping all of them - 08L, 08R, 09L, 09R, 10. The changeover takes time, and every aircraft in the arrival sequence needs to be repositioned.

LAX (KLAX) typically runs inbound on 24L and 25L and outbound on 24R and 25R. The north complex and south complex each get an arrival and departure runway.

When runways change ​

Runway changes happen when the wind shifts enough to exceed tailwind limits or when the crosswind component gets too high. They also happen for noise abatement (some airports switch configurations at night) or construction.

A runway change is disruptive. Arrivals that were lined up for one end of the airport now need to approach from the other side. STARs change. Approach types change. The controller basically resets the entire operation.

Smart controllers anticipate changes by watching the METAR and wind trends. Starting to set up the new flow before the official changeover reduces the chaos.

In radarcontrol.io ​

radarcontrol.io uses live METAR wind data to automatically select runway configurations. The wind at the real airport right now determines which runways you'll use in the sim.

You can see the effect of wind on operations at these airports:

• Atlanta (KATL) - Five runways, configuration changes are a major event. Play now.
• JFK (KJFK) - Two parallel pairs, frequent wind shifts along the coast. Play now.
• LAX (KLAX) - Almost always westbound, but watch what happens when the Santa Anas blow. Play now.

Related: What is ATIS? | What is a METAR? | How arrivals are sequenced

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