Updated: May 6, 2021
Aircraft maintenance is vitally important when it comes to safety in the skies. Every registered aircraft, from small back yard kit planes to the jumbo A-380, must undergo regularly scheduled maintenance to be remain airworthy. The most commonly scheduled are known as A, B, C, and D Checks. Each check is progressively more complex - requiring more time, more mechanical and inspection skills, more specialized equipment, and more technical support, etc.
Among all of the aircraft models and types flying, each version has different specific requirements for each level of inspection. Checks are scheduled based on a combination of flight time and the specific number of flight cycles (take-offs and landings). Flight time and cycles vary significantly depending on the routes being flown. For instance, inter-island flights in Japan have short flight times but many cycles compared to long haul flights from San Francisco to Hong Kong.
The logic for scheduled checks and maintenance originated in 1968 when the MSG-1 guidelines were introduced for the Boeing 747. You can learn more about current scheduled maintenance planning in MSG-3.
737NG - A Check Overview
The 737 is the most flown commercial jetliner in the world with many different versions in operation. The basic 737 has gone through multiple rejuvenations. The newest one, the 737 MAX, began service in 2017. The 737NG (Next Generation) is currently the most flown version with several models in operation, including the −600/-700/-800/-900.
The A Check is one step up in complexity compared to the preflight inspection or walk-around. Intentionally designed into the 737NG is improved reliability with an important feature being reduced maintenance costs. The A Check occurs at 500 flight hours (FH). Individual airlines do have considerable flexibility to group specific tasks into different check levels based on different maintenance operation constraints such as aircraft availability, manpower, and other resources. As a result, many, if not most, airlines have combined many B Check tasks into either A or C Checks.
Typically, A Checks are scheduled to occur any time after 450FH but before 500 (some airlines allow up to 600FH). Because A Checks are performed every 500FH, they also happen several times before the heavier C and D Checks. A typical flight year for a 737NG is 3,000 hours meaning A Checks occur about 6 times per year.
Some Airworthiness Directives (ADs), FAA Service Bulletins (SBs), Modifications (MODS), and Engineering Orders (EOs) that do not have to be incorporated immediately will be included during the scheduled checks along with the normally scheduled maintenance. However, only a limited number of these changes can be done during an A Check because these checks are typically done at a designated airport ground station rather than a full-service maintenance facility.
Because of the frequency of A Checks, not all tasks required before a C Check need to be accomplished during every A check. Hence, multiple A Checks are subdivided into A-1, A-2, A-3, etc. Some tasks, such as changing certain fluids, are done at each check but others required less frequently are scheduled for future A Checks.
General A Check Procedures
A typical A Check on B737 requires between 6 and 24 hours to complete with the average taking close to 10 hours. The man-hour requirement is between 150 and 180 hours when no extensive ADs, SBs, MODS, or EOs are involved.
The check begins similar to a Daily Check with a visual external inspection looking for damage, corrosion, and missing or distorted/deformed parts. Additional access panels are opened for a more extensive inspection. Although these checks are not performed at full-service facilities, some special tooling and test equipment is required. The minimum A
Aircraft Maintenance A Check:
· Proper emergency light functionality.
· Activating the Built-in Test Equipment (BITE) of the Flap/Slat Electronics Unit.
· Pressure check of the crew oxygen system.
· Lubrication of the nose gear actuator.
· Pressure checking the parking brake accumulator.
· Filter replacement, fluid checks, and fluid replacement.
We researched a few companies that provide the airlines replacement parts for an aircraft A check.
Here are a few:
Not related to the operation and structure of the aircraft, many airlines take advantage of this down time to perform a deeper cleaning of the aircraft interior than can be accomplished between turn around's. Often carpets are cleaned and seat covers are cleaned or replaced.
The 737NG requires much less maintenance compared to older aircraft. Less maintenance along with other costly and important factors such as lower fuel burn and better reliability are significant factors making it the most flown aircraft in the sky.