Which of the following is NOT a type of turn in aviation?

A flat level turn is not recognized as a standard type of turn in aviation terminology. In aviation, turns are typically categorized based on their steepness and the aircraft's altitude configuration. A steep turn involves a bank angle usually greater than 45 degrees, allowing for tighter turning radius but requiring careful handling due to increased load factors. A medium-level turn is generally performed with a bank angle that is less steep than a steep turn, offering a balance between agility and safety.

A climbing turn refers to a maneuver where the aircraft ascends while turning, which is crucial during takeoff and certain flight operations. It involves managing both altitude gain and direction simultaneously. In contrast, the term "flat level turn" does not align with established aviation practice, as it may imply a broad or shallow turn that doesn't appropriately describe the dynamics involved in turning an aircraft at a constant altitude. This terminology is not commonly used or recognized in aviation training or manuals. Therefore, among the choices listed, a flat level turn stands out as the option that is not a defined type of turn in aviation.