Aircraft maintenance checks are periodic checks that have to be done on all aircraft after a certain amount of time or usage. Airlines casually refer to these checks as one of the following:

A Check
This is performed approximately every month or 500 Flight Hours (FH). The actual occurrence of this check varies by aircraft type, the cycle count or the number of hours flown since the last check.

B Check
This is performed approximately every 3 months. A similar occurrence schedule applies to the B check as to the A check.

C Check
This is performed approximately every 12–18 months or 2500 Flight Hours (FH). This maintenance check puts the aircraft out of service.

D Check
This is the heaviest check for the airplane, also known as a Heavy Maintenance visit (HMV). This check occurs approximately every 4–5 years. This is the check that, takes the entire airplane apart for inspection.