Why do planes land sideways?

Runways are oriented according to the prevailing winds at each location, but wind rarely blows exactly along their length. The crosswind component pushes the aircraft sideways during approach. If the pilot kept the nose pointed along the runway centerline, the aircraft would drift off to one side and land off-center or even off the pavement. To compensate, the pilot angles the aircraft into the wind by a drift angle, so that the sum of its own velocity and the wind's carries it in a straight line toward the runway threshold. From the ground, the aircraft appears to be moving sideways: this is the crab technique, and it's what produces those striking images at airports like Bilbao, Madeira or Birmingham.

An aircraft cannot land with its fuselage axis crossed relative to the runway; the landing gear would bear lateral loads it isn't designed for, and the tires would suffer. That's why, in the final moments, the pilot has two options usually combined. The first is the decrab: just before flare, the pilot presses the rudder pedal to align the nose with the runway, and in that instant the aircraft touches down before the wind can push it off again. The second is the wing-low method: the pilot banks the aircraft into the wind with the ailerons to counter the drift and uses the rudder to keep the nose straight; the aircraft touches down first on the wheel on the windward side. On large-wingspan jets the bank angle is limited to avoid scraping an engine nacelle on the ground, so combining both techniques is the norm.

Manufacturers publish, for each aircraft, a maximum demonstrated crosswind component from certification, typically around 30 to 40 kn (55 to 75 km/h) for airliners, with lower values for wet or contaminated runways. Airlines usually set more conservative operating limits and add restrictions for pilots with little experience on the type. If the wind exceeds those values, the approach is either not started or is aborted with a go-around, and the aircraft diverts to another airport. Automatic landing systems also have their own crosswind limits, generally lower than those for manual landing.

Landing in a crosswind is a maneuver trained from the earliest hours of flight school and repeated in the simulator at every rating renewal. The sensation that the aircraft is arriving crooked is unsettling for passengers, but the geometry is simple: the track over the ground is a straight line along the runway centerline, and only the fuselage, not the motion, is turned.