flap

A flap is a hinged or sliding panel mounted on the trailing edge of an aircraft wing, typically spanning part of the wing's chord and positioned inboard of the ailerons. Its primary function is to increase both lift and drag by altering the wing's camber and effective area, allowing the aircraft to operate at lower airspeeds during takeoff and landing phases.

Most commercial aircraft use slotted or Fowler flaps, which not only change angle but also extend rearward on rails or tracks to increase wing surface area. Regional turboprops and older designs often employ simpler designs like plain or split flaps that rotate about a hinge point without extending. The degree of extension and deflection angle varies: takeoff flap settings typically range from 5 to 15 degrees, while landing configurations use 25 to 40 degrees depending on aircraft type and weight.

Flap position is controlled from the flight deck via a selector handle that commands hydraulic actuators or electric motors. Pilots deploy flaps in staged increments as the aircraft slows and enters the approach phase. Each increment requires aerodynamic trim adjustment and a brief pitch change before stabilizing. The flap extension speed, or Vfe (velocity flap extended), varies by flap position; exceeding this speed risks structural damage to the flap mechanism and support structure.

Common maintenance issues

Worn actuator seals cause flap extension asymmetry, where one wing's flaps extend slower or less fully than the other, creating uncommanded roll tendency. Corroded or contaminated flap tracks restrict smooth motion and increase actuator loads. Loose or cracked ribs and spar attachments in the flap structure itself are caught during scheduled structural inspections. Limit switches that sense fully extended and retracted positions occasionally fail, preventing crews from confirming flap position independently of the position indicator.

The name derives from the flapping motion created when these surfaces deflect, mimicking the wing adjustments of birds. Understanding flap operation is essential for maintenance technicians because flap-related defects directly impact dispatch reliability and safety margins during low-speed flight phases.