Slats are extendable and retractable aerodynamic devices located on the leading edge of an aircraft's wings, designed to increase lift during low-speed operations like takeoff and landing. These high-lift devices allow an aircraft to operate safely across a wide speed envelope, delaying stall and permitting lower approach speeds.
Most slats can be classified as automatic, powered, or fixed. Automatic slats deploy aerodynamically when the aircraft slows; powered variants use hydraulic, electrical, or pneumatic actuation commanded by the flight crew; fixed slats remain permanently positioned on slower aircraft or STOL models that demand continuous low-speed performance improvement.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What do slats do on an aircraft?
Slats are extendable high lift devices on the leading edge of wings that increase lift by increasing surface area and camber. Slats deployed allow aircraft to fly at slower speeds, which is used during takeoff and landing to operate within shorter distances. Slats are used during low-speed maneuvers that may take the aircraft close to a stall.
Slats are used on some fixed wing aircraft. They are used during approach and while performing low-speed maneuvers. Slats increase wing area when moved forward. Increasing the effective camber of the airfoil increases the lift, enabling it to generate more lift at a lower airspeed.
What are the types of slats on aircraft?
The types of slats on aircraft are outlined in the list below.
- Fixed Slats
- Automatic Slats
- Powered Slats
1. Fixed Slats
Fixed slats are the simplest type of slat and have no moving parts. They are permanently attached to the wing, remaining in a fixed position during flight so that the slot they create is always open. Because the slats are permanently positioned, they provide continuous airflow improvement over the leading edge. High-pressure air from beneath the wing flows through the gap, energizes the boundary layer on the upper surface, and improves lift at low speed while giving better low-speed control.
This straightforward, permanent design is common on STOL light aircraft, bush planes, and other slower aircraft that require specific low-speed performance. Well-known examples include the German Fieseler Fi 156 Storch and the Messerschmitt Me 163B Komet, both of which employed fixed slats or fixed slots built integrally with the wing panel's outer leading edges. Fixed slats are typically limited to smaller, low-speed airplanes because they create additional drag at cruise speeds.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
2. Automatic Slats
Automatic slats are spring-loaded leading-edge slats that move without pilot input to adapt to varying flight conditions. Activated by aerodynamic forces when angle of attack increases, they extend automatically and are thus characterized by their automatic method of operation. This type of high-lift device acts as a margin of safety on stall angle of attack and is a means to delay stall while it increases lift at low speeds.
Messerschmitt aircraft employed automatic slats as a general rule, so the Bf-109 slats were automatic. In cruise, automatic slats are held in the retracted position by aerodynamic forces. As the airplane slows down, the same forces diminish and the slats extend, so no cockpit action is required. Because auto slats deploy without pilot input, they provide low impact on pilot workload during takeoff, initial climb, approach and landing.
3. Powered Slats
Powered slats are the most common type found on modern aircraft. Powered slats are extended and retracted using hydraulically or electrically powered actuators, and pilots control powered slats from the cockpit. Most commercial jets use powered leading-edge slats because they offer pilots direct control over deployment and retraction according to flight phase needs. During low speed operations like takeoff, approach, and landing, pilots extend the powered slats to increase lift. Slats are powered by hydraulics or electricity, and post-World War II designs generally operated by hydraulics or electricity to assure consistent performance.
How many slats are on an aircraft?
The number of slats on an aircraft varies with size. A small airplane usually has four slats, one at each leading-edge section of the left and right wings. Large transport aircraft carry six to eight slats because the wing is longer and is divided into more panels. Each slat lines up with a flap so the number of slats follows the number of flap tracks. When every slat is deployed together, the stalling angle increases from 15 degrees to 22 degrees, giving the pilot a safer margin at low speed.
How do slats work on a plane?
Flaps work by increasing lift during takeoff and landing. They increase the effective camber of the wing thereby increasing wing area when extended, allowing the aircraft to fly slower without stalling. Flaps, when deployed, increase the angle of attack.
Once I move the slats forward, the stall diminishes, allowing easier ascent. When I retract the slats, the drag is reduced and speed is increased. Retracting slats pulls them flush to the contoured wing, allowing steady descent.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What was the first plane with leading edge slats?
The stall-related crash in August 1917 of a Rumpler C aeroplane prompted Gustav Lachmann to develop leading-edge slats in 1918. The heavily modified DH9a, fitted with full-length leading-edge slats that were closed in flight, became the first aircraft to demonstrate the concept when it first flew in March 1921.