slat
A control surface that extends forwards and downwards from the leading edge of a wing, leaving a gap between it and the leading edge, in order to modify the airflow around the wing so as to allow flight at a higher angle of attack without stalling, lowering the aircraft's stall speed.
slat: leading-edge flap that delays the stall
A slat is a hinged or sliding device mounted on the forward edge of an aircraft wing. When deployed, it extends downward and forward, creating a narrow gap (typically 1 to 3 inches) between itself and the fixed leading edge. This gap accelerates air over the upper wing surface, energizing the boundary layer and allowing the wing to generate lift at angles of attack that would otherwise cause separation and stall.
Slats appear on most commercial transport aircraft and many business jets. They are spring-loaded or motor-driven, deploying automatically during approach and landing when pilots lower the flap handle, though they can also be deployed independently. The slat extends further on outer wing sections than inboard, optimizing spanwise flow patterns. In cruise, they retract flush against the leading edge to minimize drag. Aircraft like the Boeing 737 and Airbus A320 use hydraulically actuated slats across most of the wing span.
Maintenance technicians monitor slat condition closely because gaps, dents, or misalignment create airflow disruptions and drag penalties. Contamination, ice accretion, or insect remains on the slat surface degrade performance significantly. Slat tracks and roller assemblies wear and require periodic lubrication and wear limit checks. Extended cracks in the slat skin or failure of the deployment mechanism requires replacement or repair before flight release.
Slats versus flaps
Slats differ fundamentally from trailing-edge flaps. While flaps increase wing area and change camber by deflecting downward at the rear, slats modify the pressure distribution at the leading edge and provide vortex shedding that delays flow separation. An aircraft may deploy slats while holding flaps retracted, or extend both together for maximum lift generation during takeoff and landing. Some older aircraft used leading-edge slats without powered flaps.The term 'slat' likely derives from the narrow, slot-like gap created between the deployed device and the wing surface. Aerodynamicists call this the 'slot effect,' a principle discovered in the 1920s. The word appears in technical literature by the 1930s as powered flight matured. Modern automatic slat systems represent decades of refinement aimed at reducing pilot workload during critical flight phases while maintaining margin to stall.