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, 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". 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.
This is performed approximately every 500 - 800 flight hours. It 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.
This is performed approximately every 3-6 months. It is usually done in 1-3 days 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.
This is performed approximately every 15–21 months or a specific amount of actual Flight Hours (FH) as defined by the manufacturer. This maintenance check is more extensive than a B Check, as pretty much the whole aircraft is inspected. This check puts the aircraft out of service and until it is completed, the aircraft must not leave the maintenance site. It also requires more space than A and B Checks - usually a hangar at a maintenance base. The time needed to complete such a check is generally 1-2 weeks. The schedule of occurrence has many factors and components as has been described, and thus varies by aircraft category and type.
This is - by far - the most comprehensive and demanding check for an airplane. It is also known as a Heavy Maintenance Visit (HMV). This check occurs approximately every 5–6 years. It is a check that, more or less, takes the entire airplane apart for inspection and overhaul. Such a check can generally take from 3 weeks to 2 months, depending on the aircraft and number of technicians involved (it is not uncommon to have as many as 100 technicians working on a Boeing 747 at the same time). It also requires the most space of all maintenance checks, and as such must be performed at a suitable maintenance base.
Because of the nature and the cost of such a check, most airlines - especially those with a large fleet - have to plan D Checks for their aircraft years in advance. Often, older aircraft being phased out of a particular airline's fleet are stored or scrapped upon reaching their next D Check, due to the high costs involved in it in comparison to the aircraft's value. Many Maintenance, Repair and Overhaul (MRO) shops state that it is virtually impossible to perform a D Check profitably at a shop located within the United States. As such, only few of these shops offer D checks.