dirty

In aviation, an aircraft is dirty when its undercarriage (landing gear) and wing flaps are extended from their stowed positions. This configuration increases aerodynamic drag significantly and reduces airspeed capability, making it the standard setup for approach and landing. The opposite state, with gear retracted and flaps flush, is called clean.

Pilots transition to a dirty configuration in stages during descent. Flaps are typically lowered in increments, such as 5 degrees, 15 degrees, and 25 degrees or full extension, with each extension requiring power adjustment to maintain descent profile. Landing gear is lowered last, usually around 2,000 feet on approach, because the drag penalty is severe but necessary only in the final minutes. On some aircraft, speed brakes or spoilers may also be deployed as part of getting dirty.

The term reflects the aerodynamic reality: protruding gear legs and extended flap surfaces create turbulence and interrupt smooth airflow over the wing. This extra drag is precisely what makes a dirty configuration desirable for landing, since it allows the aircraft to descend at a steeper angle and lose altitude while maintaining control. Without this drag, landing distances would be dangerously long.

Ground crews use the term routinely when inspecting or servicing aircraft between flights. A pilot might report that landing gear "won't go dirty" or flaps are "stuck clean," meaning the extension mechanism has failed. Maintenance technicians also speak of the dirty configuration when testing hydraulic systems or reviewing flight data for approach incidents.

The language is purely functional and carries no judgment. An aircraft must go dirty to land safely; a pilot who fails to do so on approach is committing a serious error. Weather, weight, and runway length all affect how early a crew will go dirty and how gradually they will extend flaps, but the principle remains unchanged across all fixed-wing aircraft types.