What defines a 'clean configuration' in flight?

A 'clean configuration' in flight refers to the state of an aircraft when all unnecessary drag-producing surfaces are retracted and configured to reduce aerodynamic drag. This generally means that the landing gear is up, and the flaps are retracted, which helps the aircraft achieve optimal performance, particularly in terms of speed and fuel efficiency.

When an aircraft is in a clean configuration, it maximizes its lift-to-drag ratio, allowing for smoother airflow over the wings and reduced resistance. This state is essential for cruising flight, where maintaining speed while minimizing drag is crucial.

In contrast, options that mention deploying lifting surfaces or speed brakes increase drag and are typically employed during different flight phases, such as takeoff, landing, or descent, rather than during cruising. Thus, understanding that the term 'clean configuration' specifically means having no unnecessary lift-generating or drag-producing surfaces deployed is key for assessing an aircraft's aerodynamic efficiency in flight.