Definition
A permanent twist built into a wing so that the angle of incidence is greater at the wingtip than at the wing root, causing the tip to produce more lift than it otherwise would.
Plain English
The wing is twisted slightly so the outer end meets the air at a steeper angle than the inner end, making the tip lift harder.
Context Anchor
Seen in aircraft maintenance when checking or adjusting wing alignment and flight control handling problems.
Derivation
The term comes from old rigging language used by aircraft builders. 'Wash' referred to a deliberate twist or change along the length of the wing, and 'in' indicated the leading edge was angled upward (into the wind) at the tip. Its opposite, 'wash out,' twists the leading edge downward at the tip.
Why Pilots Care
Wash-in alters stall progression and roll behavior; improper rigging can produce unexpected handling qualities or asymmetric stall.
Intuition Check
Wash in does not mean cleaning the airplane. Here it means a wing twist that increases the wingtip’s angle into the airflow.
Example Sentence 1
During the wing repair, the technician checked the rigging to make sure the original wash in at the tip had not been altered.
Example Sentence 2
The left wing showed slight wash-in that caused the aircraft to roll left at the onset of stall.