Definition
In aerodynamic terms, an aircraft configuration in which the landing gear, flaps, and/or other high-lift or drag-producing devices are extended, increasing both lift and drag and reducing the airspeed at which the aircraft can fly. The opposite configuration — gear and flaps retracted — is referred to as 'clean.'
Plain English
The aircraft is set up for slow flight, with the wheels down and the flaps out. This adds drag and lets the plane fly more slowly, which is what you want for landing.
Context Anchor
Seen in discussions of aircraft configuration, landing, takeoff, and wake turbulence, especially when comparing clean and dirty aircraft.
Derivation
From the everyday sense of 'dirty' meaning 'not clean.' Pilots and engineers borrowed it as the natural opposite of 'clean' — a smooth, streamlined airframe. When gear and flaps hang out into the airflow, the aircraft is no longer aerodynamically clean, hence 'dirty.'
Why Pilots Care
A dirty configuration reduces the strength of wingtip vortices compared with a clean configuration, affecting separation requirements behind large aircraft.
Intuition Check
Dirty does not mean the airplane is physically unwashed or poorly maintained here. It means the aircraft has parts extended into the airflow, such as flaps or landing gear.
Example Sentence 1
On final approach the airplane was fully dirty — gear down, full flaps — and trimmed for the target approach speed.
Example Sentence 2
Pilots extend flaps and gear to fly dirty during the landing phase of flight.