Which of the following is NOT a part of the aircraft structure?

The rudder is indeed a critical component of an aircraft's control system, but it is not classified as part of the structural framework of the aircraft itself. Instead, it functions as a control surface that helps manage the aircraft's yaw by providing stability and maneuverability.

In contrast, the fuselage, longerons, and stringers are integral to the aircraft's structure. The fuselage serves as the main body, housing passengers and cargo while providing attachment points for other components. Longerons and stringers are structural elements that contribute to the airframe's strength and rigidity. Longerons provide longitudinal support, while stringers run along the fuselage to distribute loads and maintain the shape of the structure. Thus, the rudder stands out as the correct answer because, while vital for flight control, it does not contribute to the fundamental structural composition of the aircraft.