Slats

Slats are small lifting surfaces located close to the leading edge of an aeroplane wing which, when extended, postpone the wing stall, allowing the aircraft to fly more slowly. They are usually used while landing or performing manoeuvres which take the aircraft close to the stall, but are usually retracted in normal flight to minimise drag.

Types include:

• Automatic - the slat lies flush with the wing leading edge until aerodynamic forces cause it to extend when needed. This type is typically used on light aircraft.
• Fixed - the slat is permanently extended. This is rarely used, except on specialist low-speed aircraft.
• Powered - the slat extension can be controlled by the pilot. This is commonly used on airliners.

The chord of the slat is typically only a few percent of the wing chord. They may extend over the outer third of the wing or may cover the entire leading edge. Slats work by increasing the camber of the wing, and also by opening a small gap (the slot) between the slat and the wing leading edge, allowing a small amount of high-pressure air from the lower surface to reach the upper surface, where it helps postpone the stall.

The slat has a counterpart found in the wings of some birds. The Alula is a feather or group of feathers which the bird can extend to under control of its "thumb".