Definition
An aircraft configuration in which the flaps, landing gear, spoilers, and any other high-drag devices are retracted or stowed, producing the lowest drag profile for that airframe.
Plain English
The airplane is in its smoothest shape — gear up, flaps up, nothing sticking out into the airflow.
Context Anchor
Seen in discussions of wake turbulence and vortex strength, especially in the phrase “heavy, clean, and slow.”
Derivation
From the everyday sense of 'clean' meaning unobstructed or uncluttered. In aviation it describes an aircraft whose outer shape is uncluttered by extended surfaces, so the air flows around it with minimum disturbance.
Why Pilots Care
A clean aircraft behaves very differently from a dirty one — it accelerates faster, glides farther, stalls at a lower speed, and produces stronger wingtip vortices. Knowing the configuration matters for performance numbers, wake turbulence avoidance, and emergency planning.
Intuition Check
Clean does not mean the airplane is washed or free of dirt here. It means the aircraft is in a smooth, low-drag flying configuration.
Example Sentence 1
Once the gear and flaps were up, the aircraft was clean and accelerated quickly through 120 knots.
Example Sentence 2
In the clean configuration, the wing generates stronger vortices at a given angle of attack.