Definition
The natural tendency of an airplane on the ground to pivot into the wind when a crosswind strikes it from the side. The fuselage, vertical stabilizer, and rudder act as a large surface behind the main wheels, so wind pressure on that rear surface pushes the tail downwind and swings the nose upwind, much like a weathervane on a roof.
Plain English
When the wind is blowing across the runway, the airplane wants to turn its nose into the wind on its own, just like a weathervane points into the wind.
Context Anchor
Seen in landing gear and ground-handling discussions, especially with tailwheel airplanes and crosswind takeoffs or landings.
Derivation
Weathervane comes from the old word ‘fane,’ meaning a flag or banner, combined with ‘weather.’ A weathervane is the small arrow-shaped device on top of a barn or church that swings to point into the wind. The airplane behaves the same way on the ground because most of its side area sits behind the main wheels, which act as a pivot.
Why Pilots Care
Uncorrected weathervaning can cause loss of directional control or a ground loop during takeoff and landing in crosswinds, especially in tailwheel aircraft.
Analogy
Like a weather vane on a roof, the airplane may try to line up with the wind. The difference is that the airplane is rolling on wheels, so the pilot must keep it tracking straight.
Intuition Check
Do not read “weathervaning” as something that only happens to a weather instrument. Here it means the airplane itself tends to turn into the wind while on the ground.
Example Sentence 1
During the landing rollout in a strong left crosswind, the pilot held right rudder to counter the airplane’s weathervaning tendency.
Example Sentence 2
In the tailwheel airplane, crosswind weathervaning tendencies become stronger as the tail rises during the takeoff roll.