Krueger flap

A Krueger flap is a hinged panel mounted on the lower surface of an aircraft wing's leading edge that deploys downward and forward to increase wing camber during takeoff and landing. Unlike a slat, which opens a slot in the leading edge, a Krueger flap pivots from a point aft of the leading edge itself, exposing the undersurface of the fixed wing structure and creating additional curvature without creating a pressure-side gap.

The mechanism works by altering the effective shape of the airfoil at low airspeeds. When extended, typically 5 to 25 degrees depending on aircraft design, it increases the wing's maximum lift coefficient by 15 to 30 percent. This allows the aircraft to fly safely at lower speeds during approach and climb-out, reducing takeoff distance and improving climb performance with heavy loads or from high-altitude airfields. The flap is usually deployed automatically or by pilot selection during the landing gear extension phase.

Krueger flaps are commonly found on large transport aircraft and some business jets, including the Boeing 737 and certain Airbus models. The flap is actuated hydraulically and must be fully retracted before reaching cruise speed, typically around 250 knots indicated airspeed or higher, to avoid aerodynamic buffeting and structural stress. Position sensors confirm deployment and retraction for flight management systems and crew indication.

Maintenance technicians inspect the flap actuators, hinges, and seals for wear and corrosion, as salt spray and thermal cycling can degrade hydraulic integrity. The deployment mechanism must operate smoothly without hesitation, and any asymmetry between left and right flaps during extension can cause unwanted rolling moments requiring immediate corrective action. The linkage and rigging must be checked for free play and proper centering throughout the full range of motion.

The term comes from the flap's inventor or primary developer in German aircraft design tradition. It remains distinct from more common trailing-edge high-lift devices in that it operates from the leading edge, making it a critical element in the overall wing system for safe operation at the low-speed flight envelope.