A slat is an aerodynamic surface mounted on the leading edge of a fixed-wing aircraft's wing. Extending forward, it opens a narrow slot between itself and the wing, increasing camber and the effective angle of attack. This action turns slats into high-lift devices that let an airplane generate extra lift during the low-speed, high-demand phases of takeoff and landing.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are slats on an aircraft?
Slats are extendable, retractable leading-edge devices mounted on curved tracks within the wing. A slat is a high-lift device situated at the front of the airfoil. When the designer tries to change airfoil shape by putting moving parts on the leading edge, the slat is the part that moves forward and slightly downward along metal tracks built into the wings.
By creating a duct-like slot between itself and the main wing, the slat forces incoming air down onto the top surface and this gap increases lift by delaying airflow separation. The result is that the effective camber of the airfoil increases, enabling more lift at lower airspeed and reducing stall speed. Thus, during low-speed operations - takeoff, initial climb, approach, and landing - the airplane can generate the necessary lift without demanding excessive speed from the engines.
What is the purpose of slats on a plane?
Slats are vital in boosting flight performance and safety by delaying flow separation and energizing the boundary layer. The slot allows high-pressure air to flow from the bottom of the wing to the top, energizing the boundary layer so that the airflow stays attached at higher angles of attack. Delayed flow separation results in increased lift, so slats are used during low-speed operations like takeoff, initial climb, approach, and landing when high-speed planes like airliners and fighter jets need more lift. The automatic extension of slats provides a stall margin, while moving the slats forward increases the camber of the wing. Increased camber enables the wing to generate more lift at lower airspeed, giving the airplane the lift it needs without having to raise the nose.
What is a slat failure on an airplane?
A slat failure can happen due to different reasons. Slats are segmented and are normally operated hydraulically, electrically, or pneumatically, but a slat drive failure leaves the slats unavailable to the crew and is indicated by a SLATS DRIVE message. Such a failure occurs during heavy-weight takeoff and leaves the slats stuck in the current position, producing asymmetric extension that results in disaster. The crew is then confronted with unusual flight characteristics on go-around and must use checklists for abnormal slat configurations, because flap retraction becomes tricky and the combined flap/slat limit speed leaves no room to manoeuvre. A no-slat landing requires a higher approach speed and a longer landing roll.
Physical damage is another issue. A broken, delaminated, or shredded slat causes extra drag - about 2% extra drag on that side - yet the aircraft's automated systems can compensate for the imbalance. Whether the failure is a drive fault or visible damage that leaves the upper part of the slat appearing shredded and chewed, pilots train for these conditions and apply the same checklists and procedures that exist for abnormal flap and slat configurations.
