Definition
The tendency of an airplane to drift sideways and to weathervane into the wind during takeoff and landing when the wind has a component blowing across the runway rather than directly down it. The drift component pushes the airplane sideways through the air, while the weathervaning component yaws the nose toward the wind because the larger surface area behind the main gear (fuselage and tail) acts like a weather vane.
Plain English
When the wind blows across the runway instead of straight down it, two things happen on takeoff and landing: the airplane gets pushed sideways, and its nose tries to swing into the wind. The pilot has to actively counter both with the controls.
Context Anchor
Encountered during crosswind takeoffs, especially as the airplane transitions from rolling on the runway to flying just above it.
Why Pilots Care
Uncorrected crosswind effect can cause the aircraft to drift off the runway centerline and result in loss of directional control or a runway excursion.
Grounding Statement
Picture the airplane becoming light on its wheels while a side wind is pushing on it; as soon as it starts flying, that sideways push can move it off the runway path quickly.
Intuition Check
Do not think of crosswind effect as only a problem after the airplane is fully airborne. It starts during the takeoff roll and becomes especially important at lift-off, when the airplane is light and easiest for the wind to move.
Example Sentence 1
During the takeoff roll, the pilot held aileron into the wind and used rudder to counter the crosswind effect and keep the airplane tracking straight down the centerline.
Example Sentence 2
Once the nosewheel lifted, the pilot used a slight aileron deflection to prevent the crosswind effect from lifting the upwind wing.