The ASK-21 is a two-seat, mid-wing glider developed by the German manufacturer Alexander Schleicher GmbH & Co. It was first introduced in 1979 and is widely used around the world for flight training, aerobatics, and general soaring. Constructed primarily from fiberglass-reinforced plastic, the ASK-21 offers a robust and durable design with forgiving flight characteristics, making it ideal for student pilots.

The glider features a T-tail configuration and side-by-side seating under a large, clear canopy that provides excellent visibility for both instructor and student. Its flight controls are responsive yet stable, and the aircraft is certified for basic aerobatic maneuvers, including loops and spins. With a wingspan of 17 meters (55.8 feet) and an efficient airfoil, the ASK-21 offers good glide performance while maintaining ease of handling.

Overall, the ASK-21 is known for its reliability, versatility, and effectiveness as a training platform in gliding clubs and flight schools worldwide.