A classification of maintenance inspections or checks used in airline operation.

A-check. A thorough visual inspection and an operational check of all major systems of the airframe and engine.

B-check. A complete A-check and a more detailed check of certain systems specified for the particular aircraft.

C-check. Complete A and B checks with more in-depth inspection of certain systems. A C-check may include a flight check.

D-check. The most in-depth inspection of the aircraft. It requires removal of such components as engines and landing gear.