Definition
A condition in which the wing flaps fail to extend or retract evenly on both wings, leaving more flap deployed on one wing than the other. The unequal lift and drag between the two wings produces a strong rolling and yawing tendency toward the wing with less flap, which the pilot must counter with aileron and rudder input.
Plain English
The flaps on one wing move but the flaps on the other wing don't, so the airplane wants to roll and turn toward the side with less flap because that wing is producing different lift and drag than the other.
Context Anchor
Encountered when using flaps before landing, including a water landing or snow landing where steady control and a predictable touchdown attitude are important.
Derivation
Asymmetrical comes from Greek roots meaning 'not the same on both sides.' Here it describes a flap system that has lost its symmetry between the left and right wings.
Why Pilots Care
Creates an immediate rolling tendency that must be countered to maintain control, especially hazardous close to the ground or during water landings.
Intuition Check
Do not read this as both flaps simply failing together. The key point is unevenness: one side is doing something different from the other side.
Example Sentence 1
After selecting flaps, the pilot felt the airplane roll sharply to the right and recognized an asymmetrical flap failure.
Example Sentence 2
During ditching, an asymmetrical flap failure requires prompt correction to keep the wings level for water contact.