Aircraft maintenance checks are periodic checks that have to be done on all aircraft after a certain amount of time or usage. Airlines and other commercial operators of large or turbine-powered aircraft follow a continuous inspection program approved by the Federal Aviation Administration (FAA) in the United States[1], or by other airworthiness authorities such as Transport Canada or the European Aviation Safety Agency (EASA). Under FAA oversight, each operator prepares a Continuous Airworthiness Maintenance Program (CAMP) under its Operations Specifications or "OpSpecs".[2] The CAMP includes both routine and detailed inspections. Airlines and airworthiness authorities casually refer to the detailed inspections as "checks", commonly one of the following: A check, B check, C check, or D check. A and B checks are lighter checks, while C and D are considered heavier checks.

A Check

This is performed approximately every 500 - 800 flight hours. This check is usually done overnight at an airport gate. The actual occurrence of this check varies by aircraft type, the cycle count (takeoff and landing is considered an aircraft "cycle"), or the number of hours flown since the last check. The occurrence can be delayed by the airline if certain predetermined conditions are met.

B Check
This is performed approximately every 3 months. This check is also usually done overnight at an airport hangar. A similar occurrence schedule applies to the B check as to the A check. B checks may be incorporated into successive A checks, ie: A-1 through A-10 complete all the B check items.

C Check
This is performed approximately every 12–18 months or a specific amount of actual Flight Hours (FH) as defined by the manufacturer. This maintenance check puts the aircraft out of service and requires plenty of space - usually at a hangar at a maintenance base. The schedule of occurrence has many factors and components as has been described, and thus varies by aircraft category and type.

D Check
This is the most comprehensive check for an airplane. It is also known as a Heavy Maintenance Visit (HMV). This check occurs approximately every 4–5 years. This is the check that, more or less, takes the entire airplane apart for inspection. This requires even more space and time than all other checks, and must be performed at a maintenance base. Often, older aircraft being phased out of a particular airline's fleet are stored or scrapped upon reaching their next check, due to the high costs involved in comparison to the aircraft's value. Many Maintenance, Repair and Overhaul (MRO) shops believe it is virtually impossible to perform a D check profitably at a shop located within the United States, and thus do not offer D checks.

Maintenance Review Board
Initial aircraft maintenance requirements are proposed in a Maintenance Review Board (MRB)[3] report based on Air Transport Association (ATA) publication MSG-3.

Modern transport category airplanes with MSG-3 derived maintenance programs employ usage parameters for each maintenance requirement such as flight hours, calendar time, or flight cycles. Maintenance intervals based on usage parameters allow more flexibility in scheduling the maintenance program to optimize aircraft utilization and minimize aircraft downtime.

References
^ AFS-600 (2008). "Chapter 8. Inspection Fundamentals" (pdf). Aviation Maintenance Technician Handbook. Federal Aviation Administration. FAA-H-8083-30. http://www.faa.gov/library/manuals/aircraft/amt_handbook/media/FAA-8083-30_Ch08.pdf. Retrieved 2010-01-12. 
^ AFS (2009). "Vol. 3 Chapters 18 & 43". Flight Standards Information Management System. CHG 80. Federal Aviation Administration. Order 8900.1. http://fsims.faa.gov/PICResults.aspx?mode=EBookContents. Retrieved 2010-01-12. 
^ AFS-330 (1997) (pdf). Maintenance Review Board Procedures. Federal Aviation Administration. Advisory Circular AC 121-22A. http://rgl.faa.gov/regulatory_and_guidance_library/rgadvisorycircular.nsf/key/ac%20121-22a. Retrieved 2010-01-12.