Definition
Sideways forces transmitted into the landing gear when the airplane's wheels contact or roll on the runway while the airplane is moving in a direction not aligned with the direction the wheels are pointing. In a crosswind takeoff or landing, side-loads occur if the airplane lifts off or touches down while drifting sideways across the runway, forcing the tires and gear structure to absorb a lateral load they were not designed to carry continuously.
Plain English
Sideways stress on the wheels and landing gear caused by the airplane moving sideways while the wheels are rolling straight ahead. The tires get scrubbed sideways and the gear legs get bent sideways instead of just up and down.
Context Anchor
Seen during crosswind takeoff and landing discussions, especially while the wheels are still carrying weight on the runway.
Derivation
From 'side' (lateral direction) and 'load' (a force or stress placed on a structure). In engineering, a 'load' is any force a structure must carry. A side-load is therefore a force pushing on a structure from the side rather than along its intended axis. Landing gear is built to carry vertical loads well and side loads poorly.
Why Pilots Care
Excessive side-loads can damage tires, wheels, or landing-gear structure and may cause loss of directional control.
Intuition Check
Do not read “load” here as weight in the airplane. In this context, a load is a force on the airplane, and side-loads are forces from the left or right.
Example Sentence 1
During a crosswind takeoff, the pilot held the upwind wing down and used rudder to keep the nose straight, lifting off cleanly without imposing side-loads on the gear.
Example Sentence 2
Releasing back pressure too early in a gusty crosswind can let the airplane drift and impose side-loads at liftoff.