A classification of maintenance inspections or checks used in airline operations and denoted by letters A to D. Check A is one in which visual inspection and an operational check of all major systems of the airframe and the engine are carried out. Check B involves a complete and more detailed check of certain specified systems for each type of aircraft. Check C requires completing the A and B checks, with a more in-depth access and inspection of certain primary structures, and usually involves a flight check if any primary flight control or engine is removed and replaced. Check D is the most in-depth inspection of the aircraft; it requires opening up and inspecting all primary structures and may include the overhaul or replacement of components such as the landing gear, depending on the interval specified by the manufacturer. The above checks generally are based upon the manufacturer's recommendations contained in the aircraft's MPD (Maintenance Planning Document).
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