Aeronautical Maintenance

The standard intervals are as follows:

Transit Checks

These are performed before each flight, including stopovers. They consist of a quick inspection where general aspects of the aircraft are checked, such as the condition of the tires, oil level, possible existence of any structural damage, etc.

Daily Checks

They consist of an oil level check. However, if the oil level needs to be checked before the first takeoff, the aircraft must taxi for at least two minutes to warm up the oil. Although this procedure is not common, it may be necessary in certain cases.

48-Hour Checks

This type of inspection replaces the daily inspection for many aircraft models, depending on each airline’s specifications. They may include more detailed checks than daily inspections, and some examples of inspections carried out are wheel and brake checks, fluid replacements (engine oil, hydraulic fluid, etc.), inspection and replacement of auxiliary power unit oil, and visual inspection of the fuselage, wings, interior, and cockpit.

Time Limit Checks

Some inspections have maintenance measures assigned based on the number of hours the system in question has been operating. This assignment is established for engine inspections, aircraft controls, and other systems that operate continuously during flight and/or taxiing.

Operating Limit Cycle Checks

For other aircraft systems, maintenance tasks are performed according to the monitoring of a specific program that depends on the number of operating cycles. For example, the landing gear is only used for takeoffs and landings, the number of which varies depending on flight planning. The structure, engine components, turbine blades, and other components are subject to cyclic stresses, so they will have numerous maintenance tasks.

PS Checks

Some airlines like American Airlines perform checks every 2 or 3 days, known as “Periodic Service” (“Periodic Service” in English), and consist of a visual inspection of the aircraft. These types of checks are generally performed at night or during the day, during periods of aircraft inactivity, and usually involve an estimated effort of two hours of work on average.

Letter Checks

These checks are identified by the letters A, B, C, and D. This classification, which we will see in depth below, provides a maintenance program that allows checks to be performed at the optimal time for the system or equipment in question. This makes the program more flexible to the operator’s needs. However, some operators choose to continue organizing maintenance in blocks, by hourly intervals or cycles.

A Check

They are performed every 400-600 hours or every 200-300 cycles (takeoff and landing are considered a single cycle for the aircraft), depending on the aircraft’s time. They require between 50 and 70 man-hours and are normally performed in ground hangars, with a minimum duration of 10 hours.

The regularity of these checks depends on the type of aircraft, as well as the total cycles or number of flight hours since the previous check. The airline can delay this type of event if certain predetermined conditions are met.

B Check

They are performed approximately every 6-8 months. They require between 160 and 180 hours of work depending on the type of aircraft and last between 1 and 3 days. These inspections are carried out in airport hangars.

C Check

They are carried out every 20-24 months when the aircraft reaches a certain number of flight hours, although the regularity of this type of check can also be established by the manufacturer. They are much more thorough than B checks, as they require the inspection of a greater number of parts.

These checks involve taking the aircraft out of service, so it cannot leave the inspection site until the check is completed. This check also requires more space than A and B checks, so it is performed in a base maintenance hangar. The time required to complete C checks is 1 to 2 weeks, assuming an estimated effort of 6000 man-hours.

D Check

These are the most comprehensive and demanding checks for the aircraft. This is a type of check that is performed approximately every 6 years and consists of an inspection in which practically all aircraft undergo an inspection and repair procedure. In this case, even the paint must be completely removed in order to perform a more thorough inspection than that carried out on the fuselage in previous cases.

This type of inspection typically involves an effort of approximately 50,000 man-hours and is carried out over about two months, depending on the aircraft and the number of technicians assigned to perform the work.

D checks also require more space than previous checks and, therefore, must be carried out at a suitable maintenance base. The requirements and the extensive effort required for this type of check result in a total cost of approximately one million dollars per check. For this reason, most airlines (especially those with large fleets) are forced to plan D checks years in advance to minimize the impact on their operations.

It’s worth noting that older aircraft are often retired from fleets due to the high cost of a D check compared to the cost of acquiring a new aircraft. On average, throughout its service life, a commercial aircraft undergoes three D checks before being retired.

Due to the significant amount of time it takes for an aircraft to undergo a D check, airlines often take advantage of this opportunity to carry out various types of cabin improvements such as seat renovations, upgrades to in-flight entertainment systems, etc.