 This ensures that the effective angle of attack is always lower at the wingtip than at the root, meaning the root will stall before the tip.This is desirable because the aircraft's flight control surfaces are often located at the wingtip, and the variable stall characteristics of a twisted wing alert the pilot to the advancing stall while still allowing the control surfaces to remain effective, meaning the pilot can usually prevent the aircraft from stalling fully before control is completely lost.Twist that decreases the local chord's incidence from root to tip is sometimes referred to as "washout".