What is included in the definition of a complex airplane as per FARs?

A complex airplane, as defined by the Federal Aviation Regulations (FARs), specifically includes two significant features: retractable landing gear and a controllable-pitch propeller. This definition is important because it indicates a higher level of operational complexity and requires additional training for pilots, particularly in the handling and operation of these systems.

Retractable landing gear allows for reduced drag during flight, enhancing aerodynamic efficiency, while controllable-pitch propellers provide better performance, particularly in varying flight conditions. Pilots operating complex airplanes must understand the mechanics and operational procedures associated with these systems, including how to handle potential emergencies related to gear deployment and propeller pitch adjustments.

The other possible choices do not meet the full criteria established by the FARs for a complex airplane. For instance, fixed landing gear does not contribute to the classification of an airplane as complex, nor do features like folding wings, which pertain to a different category of aircraft design.