clean
Allowing an uninterrupted flow over surfaces, without protrusions such as racks or landing gear.
clean: aerodynamic, with nothing sticking out
In aviation, a clean configuration means the aircraft presents the smoothest possible profile to the airflow: landing gear retracted, flaps stowed, speed brakes closed, external stores removed. Any protrusion, whether functional equipment or cargo pod, generates drag and disrupts laminar flow over the fuselage and wings. The term is not about surface dirt but about the mechanical state of the airframe.
The drag penalty of an unclean state is substantial. A deployed landing gear can increase drag by 40 to 60 percent depending on the aircraft type and airspeed. Deployed flaps at cruise add drag roughly proportional to their angle. This is why pilots must retract gear immediately after takeoff and why descent planning includes carefully sequenced extension of high-drag devices. Fuel burn and speed capability both degrade sharply when the airframe is not clean.
Maintenance technicians use the term when scheduling service and inspecting gear wells, actuators, and seals. A clean landing gear that retracts smoothly and seals properly prevents transonic buffeting and structural fatigue in the wheel bays. Corrosion, debris, or binding actuators mean the gear cannot be reliably clean in flight, a safety-critical defect. Similarly, flap track fairings and speed brake doors must close flush or the airframe remains aerodynamically dirty.
External tanks, weapons, or sensor pods can only be carried clean if they are faired: fitted with aerodynamic covers that make them part of the streamlined profile. A naked pylon or fuel tank creates a vortex generator, inducing yaw and consuming fuel. Military aircraft often fly in a clean configuration to minimize signatures and maximize endurance, accepting reduced payload for speed and efficiency.
The term extends to the design phase. Aircraft designed for sustained high-speed cruise, such as long-range transport or reconnaissance platforms, emphasize clean geometry in their baseline configuration. Those designed for short-field or carrier operations often must operate dirty: landing gear down, flaps extended, and accept the aerodynamic cost as a trade for slow-speed control authority.