Line Checks
After every flight the airplane's engine, tires, oil level and brakes are given a quick but thorough examination to make sure that they weren't damaged on the past flight. The passenger areas and galley are cleaned and restocked.

A Checks
These are performed once each month. They usually involve a thorough inspection of the movable surfaces, electronics, landing gear and engines. Various parts of the plane are also checked for corrosion and other signs of wear and tear. This kind of check can be done at almost any airport. A Checks are often performed overnight.

B Checks
These are performed about once every three months. They are slightly more in-depth than A Checks. B Checks might involve testing the emergency slide rafts and recalibrating the navigation equipment.

C Checks
These are performed about every fifteen months. They can involve a structural inspection of the plane's wings and fuselage. Some pieces of the plane are actually removed so that they can be studied by themselves. Any damaged parts are replaced. Sometimes airlines replace perfectly good parts if there is something better or more efficient on the market. For example, an airline wishing to install hush kits to make its airplanes' engines quieter might wait for a C Check. C Checks can take a couple of weeks to complete and require a fully equipped maintenance base facility.