Definition
The state of an aircraft in flight with the landing gear retracted, flaps and slats fully retracted, speed brakes stowed, and no other drag-producing devices deployed. In this configuration the airframe presents the smallest possible drag for a given airspeed, allowing the most efficient cruise performance and the best speed for a given power setting.
Plain English
The aircraft is flying with everything tucked in — no gear hanging down, no flaps out, nothing sticking into the airflow that doesn't have to be there. It is as smooth and slippery as the airplane gets.
Context Anchor
Seen in instrument flying when changing airspeed in straight-and-level flight, especially when comparing how the airplane behaves with and without added drag.
Derivation
Clean' here is used in its everyday sense of 'uncluttered' or 'with nothing extra attached.' The term took hold in early aviation to describe an airframe with no protrusions disturbing the airflow.
Why Pilots Care
Knowing when the aircraft is in a clean configuration is essential for accurate performance calculations, such as best glide speed and range.
Grounding Statement
Picture the airplane with its surfaces smoothed up for flight: fewer parts sticking into the airflow means less resistance and a slower rate of deceleration.
Intuition Check
Clean does not mean the airplane is physically clean. Minimum drag does not mean zero drag; it means the lowest drag condition normally available for that airplane at that moment.
Example Sentence 1
Once established in cruise, the pilot confirmed the aircraft was in a clean configuration before setting cruise power.
Example Sentence 2
In a clean configuration, the aircraft achieves its maximum glide distance for a given altitude loss.