ABC check system
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. Aircraft operators may perform some work at their own facilities, but often checks, and especially the heavier checks, take place at maintenance, repair and overhaul (MRO) company sites.
The A check is performed approximately every 400-600 flight hours, or every 200–300 flights, depending on aircraft type. It needs about 50-70 man-hours, and is usually performed in an airport hangar. The A check take a minimum of 10 man-hours. 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.
The B check is performed approximately every 6-8 months. It takes about 160-180 man-hours, depending on the aircraft, and is usually completed within 1–3 days at an airport hangar. A similar occurrence schedule applies to the B check as to the A check. B checks are increasingly incorporated into successive A checks, i.e. checks A-1 through A-10 complete all the B check items
The C check is performed approximately every 20–24 months, or a specific number of actual flight hours (FH), 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 1–2 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.
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.
The D check, sometimes known as a "heavy maintenance visit" (HMV), is by far the most comprehensive and demanding check for an airplane. This check occurs approximately every 6-10 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 2 months 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 — 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 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 they have underestimated it for the B787-9 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.