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 the
Transport Canada Civil Aviation Directorate (TCCA), or the
European Aviation Safety Agency (EASA). Each operator prepares a Continuous Airworthiness Maintenance Program (CAMP) under its Operations Specifications or "OpSpecs". The CAMP includes both routine and detailed inspections.
Maintenance Review Board International approach (originated by the United States
FAA) is to establish initial
aircraft maintenance requirements for each
aircraft type in a Maintenance Review Board Report (MRBR). The MRBR is based on the analysis performed using
ATA "MSG-3 Operator/Manufacturer Scheduled Maintenance Development" document (MSG-3 is for Maintenance Steering Group – 3rd Task Force).
A check The A check is performed approximately every 400 to 600
flight hours, or every 200 to 300 flights, depending on aircraft type. It needs about 50 to 70
man-hours, and is usually performed in an airport hangar. The A check takes a minimum of 10
hours. Often airlines schedule these checks to occur overnight to minimize the impact of the plane being taken out of service. The actual occurrence of this check varies by aircraft type, the flight cycle count, 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 The B check is performed approximately every 6 to 8 months and typically takes 160 to 180 man-hours, depending on the aircraft. It is usually completed within 1 to 3 days in an airport hangar. However, modern aircraft no longer require stand-alone B checks. Instead, the necessary inspections and maintenance tasks have been integrated into successive A checks over the 6- to 8-month cycle, reducing aircraft downtime.
C check Boeing 757 undergoing C-check at the
British Airways Engineering maintenance base,
Heathrow (1996) The C check is performed approximately every 20 to 24 months, after a specific number of actual flight hours, or as defined by the manufacturer. This maintenance check is much more extensive than the B check, requiring a large majority of the aircraft's components to be inspected. This check puts the aircraft out of service for 1 to 4 weeks. The aircraft must not leave the maintenance site until it is completed. It also requires more space than A and B checks, therefore, it is usually carried out in a hangar at a maintenance base. The effort needed to complete a C check is up to 6,000 man-hours.
3C check Some authorities use a type of check, known as a 3C check or Intermediate Layover (IL), which typically includes light structural maintenance, including checks for corrosion, or on specific high-load parts of the airframe. The 3C check may also be used as the opportunity for cabin upgrades, e.g. new seats, entertainment systems, carpeting. This shortens the time the aircraft is out of service, by performing two distinct tasks simultaneously. As component reliability has improved, some MROs now spread the workload across several C checks, or incorporate this 3C check into D checks instead. is by far the most comprehensive and demanding check for an airplane. This check occurs approximately every 6 to 10 years. It is a check that more or less takes the entire airplane apart for inspection and overhaul. Even the paint may need to be completely removed for complete inspection of the fuselage metal skin. Such a check can generally take up to 50,000 man-hours, and six months to a year to complete depending on the number of technicians involved. It also requires the most space of all maintenance checks, and as such must be performed at a suitable maintenance base. The requirements and the tremendous effort involved in this maintenance check make it by far the most expensive, with total costs for a single D check in the million-dollar range. Because of the nature and the cost of a D check, most airlines plan D checks for their aircraft years in advance. Often, older aircraft being phased out of a particular airline's fleet are either stored or scrapped upon reaching their next D check, due to the high costs involved in comparison to the aircraft's value. On average, a commercial aircraft undergoes two or three D checks before being retired. Manufacturers often underestimate the cost of the D check. Boeing underestimates the cost for four of its aircraft, and the expectation is that it has underestimated it for the
Boeing 787-9 which in 2018 had not been in service for long enough to have been put through a D check. All amounts in millions of United States dollars, as of 2018. ==Comparison==