The ABCs of Aircraft Maintenance Checks, Simplified

Modern aircraft operate under extremely strenuous conditions for long hours with maximum efficiency to ensure a safe flying experience for everybody on board. Hence, all civil, military, and commercial aircraft must undergo inspections to optimize their longevity and safety after specific periods. Most inspection programs in the United States are approved and regulated by authorities such as the Federal Aviation Administration (FAA) in compliance with the standards set by Title 14 of the Code of Federal Regulations, better known as the Federal Aviation Regulations.

Aircraft inspections encompass repairing and replacing components exhibiting less than satisfactory performance. Inspections are generally divided into four levels, namely A, B, C, and D. Such inspections are performed at prescribed intervals depending on the average flight time and flight cycles (where one flight cycle equals one landing and takeoff.) Moreover, several airworthiness authorities establish their own regulations to ensure aircraft safety and preserve capital and energy optimization while maintaining rigorous maintenance under their aegis. In this blog, we shall delve into the detailed requirements of all levels of aircraft inspection performed today.

Understanding Airworthiness

For an aircraft, possessing airworthiness as recognized by a legal, regulatory authority is crucial in determining its safety for flight. Airworthiness requirements consider predefined standard parameters of all aircraft components to deem them safe for regular operation. Some parameters include the structural integrity of the design, provisions for a crash landing, presence of hydraulic systems, and overall aerodynamic efficiency. Determining the range of the selection criteria for all inspections is the onus of different airworthiness authorities worldwide.

The Different Components of Aircraft Maintenance Schedules

The FAA mandates four components of aircraft checks (A, B, C, and D checks), each increasing in its level of complexity. Each of these levels is concerned with the rates of inspection, specific aircraft parts to check, and duration between checks. These inspection levels were designed according to the standardized maintenance programs built off the schedule of Boeing’s Maintenance Steering Group (MSG) for improving and maintaining the safety of their B747-100 aircraft.

Tasks are often categorized into ‘blocks,’ as well as are referred to as an ‘overhaul cycle’ or progressive maintenance for coordinating various activities. While A & B checks are dubbed ‘lighter checks,’ C & D checks are called ‘heavy checks,’ and a detailed description of each is given below.

  • Daily checks: This level of inspection is also known as a service check, pre-flight maintenance, or overnight check. This level involves a cursory examination by the crew, without any specialized assistance or tools, to look for any prominent signs of post-flight damage. Furthermore, the aircraft log sheet is also closely examined to find any discrepancies in general daily activity or performance.

  • A check: A-level inspections depend on the aircraft type, the number of hours it has been in flight since the last review, and the cycle count. This check is performed every 200-300 flight cycles or 400-600 flight hours. Usually, it involves a superficial examination of the avionics, powerplant, accessories, and the airframe to ascertain the quality of the aircraft’s general working condition while replacing any missing parts. Usually, the working conditions of the crew oxygen system, emergency lights, nose gear retract actuator, and the pressure of the parking brake actuator are checked. This level involves nearly 50-70 man-hours and 8 hours of ground time to complete, and it is usually performed in a hangar.

  • B check: This level of inspection is rated higher in complexity than the previous level, and it involves a slightly more detailed examination of aircraft systems under operation. Although special tests and equipment may be mandated under this level, there is no disassembly or removal of components. B-level checks are conducted after every 6-8 months, and they require 160-180 man-hours and 1-3 days at the hangar for completion. Some authorities delegate and distribute the tasks of B level checks to the A and C levels for more efficiency.

  • C check: Traditionally falling under the ambit of heavy maintenance checks, this level is conducted every 20-24 months, which generally equates to 3000 flight hours. C-level checks involve the disassembly of critical aircraft parts, and hence they are more comprehensive in examining aircraft health than the previous levels. Thus, this level of inspection takes 1-3 weeks for completion at aircraft hangars or a special maintenance base. C checks can be further divided into 3C checks or Intermediate Layover (IL) checks involving light structural maintenance and upgrades (like installing seat covers and new carpets). Furthermore, checks for corrosion and high load parts are performed under 3C inspections. 

  • D check: D checks are highly extensive in their scope and are the ultimate aircraft inspection level. Better known as ‘heavy maintenance visits’ or ‘overhaul checks,’ D checks are undertaken every 6-10 years, requiring nearly 50,000 man-hours. To complete this level mandates the complete disassembly of the plane by highly-skilled technicians and takes almost two months to complete in total. Therefore, only designated maintenance units are utilized for this purpose. Some of the activities falling under the D check are inspections of stabilizer attach bolts, floor beams, and the wing box structure.

Variations in Regular Maintenance: Phase Checks

Traditional heavy-level checks can render an aircraft inactive for standard service for several weeks, or even months, at a time. This is why heavy-level checks are segmented into phase checks which involve the inspection of specific components or systems after every 200-800 flight hours. Such phase checks incorporate maintenance for older aircraft or VIP planes and involve corrosion control, aging system checks, and additional structural inspections. Such phase checks save time and lend flexibility to the maintenance of aircraft.

Wrapping Up

Nobody understands the need for regular aircraft maintenance more than NSN Stocks. We have an exhaustive inventory of new, obsolete, and hard-to-find aircraft parts that can serve as perfect replacements for damaged parts discovered during routine aircraft maintenance. Moreover, with our 24/7x365 customer assistance and rapid, same-day delivery aided by our expansive network, you can experience the industry’s best parts procurement services. Contact us today by filling out an Instant RFQ form on our website, and someone from our team will respond within 15 minutes or less.


March 11, 2022
January 3, 2022

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