Definition
The tendency of an airplane on the ground to turn its nose into the wind when taxiing in a crosswind. It happens because the vertical tail surfaces present a larger side area behind the main wheels than the structure ahead of them, so the wind pushes the tail downwind and swings the nose upwind.
Plain English
When the wind is blowing from the side while you're taxiing, the airplane wants to turn and point itself into the wind, like a wind vane on a roof.
Context Anchor
Seen during taxiing, especially when wind is coming from the side and the airplane starts turning more than the pilot intended.
Derivation
From 'weather vane' -- the rooftop arrow that pivots so its pointer faces into the wind. The airplane behaves the same way on the ground, so pilots use the verb 'to weathervane' to describe the motion.
Why Pilots Care
Uncorrected weathervaning can pull the airplane off the taxiway or runway centerline and requires prompt rudder input to maintain directional control.
Grounding Statement
Picture the wind pushing the tail around until the nose swings toward the wind.
Intuition Check
Weathervane does not mean the airplane is checking the weather. It means the airplane is being turned by the wind, like a weather vane pointing into the wind.
Example Sentence 1
Taxiing in a strong left crosswind, the pilot anticipated the airplane's tendency to weathervane and applied right rudder to hold the centerline.
Example Sentence 2
The student pilot practiced correcting for the weathervane effect while taxiing back to the ramp after landing.