Airplanes are one of the most complicated machines ever built by humans. People often overlook the sheer level of engineering that goes into manufacturing a metal machine that can sustain flight for several hours, cross oceans, land countless times and repeat the same procedures the next day. When boarding a modern jet airplane, it’s easy to forget that you’re boarding one of the most sophisticated machines ever built. For instance, the Boeing 747 has about 1,000 wire bundles, ranging in length from 2 feet (inside electrical panels) to 120 feet, and comprises more than a million parts. As sophisticated and mesmerizing these machines get, there is a need to take care of them. The maintenance of an airplane is one of the most expensive things to do out there. At times, the maintenance costs can exceed the price of the airliner itself. Crazy right? Let us divide the different checks required while performing maintenance on an aircraft.

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JMC performs first C-Check on a Boeing 767 (JMC)
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Aircraft maintenance checks are periodic inspections that have to be done on all commercial and civil aircraft after a certain amount of time or usage. Military aircraft normally follow specific maintenance programs which may, or may not, be similar to those of commercial and civil operators. They are divided into three types of “checks” that bundle together hundreds of tasks: A Checks, B Checks, C Checks, and D Checks. Here’s a quick summary of what checks happen, and when:

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Line Maintenance: This type of maintenance is the most common. During transit checks, things like wheels, brakes, and fluid levels (oil, hydraulics) are inspected. Plus, any ongoing maintenance that the aircraft indicates it needs via thousands of onboard sensors. Line maintenance would take roughly 12 hours per week for most airplanes. These occur all around the world and at all times.

Photo Credits: SR Technics
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A-Check: Filters will be changed every eight to ten weeks, important systems (such as hydraulics in the aircraft’s ‘control surfaces’) will be greased, and a thorough inspection of all emergency equipment (such as inflatable slides) will be conducted. A normal “A-Check” on B737 takes anywhere from six to twenty-four hours. This type of 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” takes 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.

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A-Check being performed on a Boeing 747 (Aviation Enthusiasts)

B-Check: 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.

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An Airbus A330 under maintenance (Reddit)

C-Check: 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.

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A DHL Boeing 737 undergoing C-Checks (Jakub Wrzesien)

D-Check: The “D-check”, sometimes known as a “heavy maintenance visit”, 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! This is also known as a C4 or C8 check depending on the aircraft type. the entire aircraft is basically dismantled and put back together. Everything in the cabin is taken out (seats, toilets, galleys, overhead bins) so engineers can inspect the metal skin of the aircraft, inside out. The engines are taken off. The landing gear is removed and overhauled with the aircraft supported on massive jacks.  All of the aircraft systems are taken apart, checked, repaired, or replaced and reinstalled.

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Iberia Air’s Airbus A321 undergoing heavy maintenance (Wikipedia)

To put the cost in perspective, on average, it costs up to 5 million dollars to perform a D-Check on a Boeing 777-300ER and about 6 million dollars for a Boeing 747 family. Maintenance may seem like a hassle but this is the only way many airplanes are still flying despite being 25-30 years old.

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Sources

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