Aircraft maintenance is the broad set of tasks that keep planes safe to fly: inspection, repair, overhaul, part replacement, and preventive servicing. It covers everything from a quick walkaround before takeoff to a months-long teardown where technicians strip an airliner to its bare frame. The FAA treats inspection as an inseparable part of maintenance, not a separate activity, and every commercial aircraft operates under a structured program that schedules work across three major areas: routine scheduled maintenance, unscheduled repairs when something breaks, and specific requirements for major components like engines and landing gear.
The Letter Check System
Airlines organize scheduled maintenance into progressively deeper inspections, commonly called A, B, C, and D checks. Each level builds on the one before, with longer intervals, more downtime, and a wider scope of work.
A checks happen roughly every 400 to 600 flight hours, or every 200 to 300 flights. Technicians do a general visual inspection of the aircraft interior and exterior, looking for damage, deformation, corrosion, or missing parts. They also run engine function checks, test emergency lights, lubricate nose gear components, and verify parking brake pressure. An A check can often be completed overnight at a gate.
B checks occur about every six to eight months. These are more targeted, covering tasks like checking the alignment of landing gear components or inspecting wheel well hydraulic tubing for corrosion and fluid leaks. Many airlines have folded B check tasks into their A and C check schedules, so not every operator uses this category as a standalone event.
C checks are deep inspections that cover a majority of the aircraft’s parts. Technicians examine load-bearing structures on the fuselage and wings for corrosion and damage, test electrical bus systems, and perform thorough lubrication of all fittings and cables. A C check pulls the aircraft out of service for one to two weeks and typically requires hangar space.
D checks, sometimes called “heavy maintenance visits,” happen every 6 to 10 years depending on the aircraft type. This is the most intensive event in a plane’s life. The aircraft is essentially taken apart so technicians can inspect every structural element for hidden damage and corrosion. Paint is stripped, interior fittings are removed, and wiring and plumbing are examined. A D check can take weeks to complete and requires thousands of labor hours. Some older aircraft are retired rather than undergo the expense of a D check.
Scheduled vs. Unscheduled Maintenance
Scheduled (preventive) maintenance is preplanned. It follows a calendar or flight-hour interval, and its whole purpose is to catch wear and deterioration before they cause a failure. The letter checks above are all forms of scheduled maintenance. So are routine oil sampling, filter changes, and tire replacements done at predictable intervals.
Unscheduled (corrective) maintenance is the opposite: something breaks or degrades unexpectedly, and resources are redirected to fix it. A bird strike, a hydraulic leak discovered during a walkaround, or an in-flight warning light that flags a faulty sensor are all examples. Corrective maintenance is reactive by nature, and airlines work hard to minimize it because unplanned repairs ground aircraft and disrupt schedules.
Health Monitoring and Predictive Maintenance
Modern aircraft are packed with onboard sensors that continuously track system performance and structural condition. The data they generate feeds into what the FAA calls Integrated Aircraft Health Management, a process that combines real-time monitoring with ground-based analysis to spot problems before they become failures.
This approach goes by several names depending on what’s being watched: engine condition monitoring for powerplants, structural health monitoring for airframe components, and health and usage monitoring systems for helicopters. In each case, the principle is the same. Sensors detect degradation trends, and when a reading crosses a preset threshold, an alert triggers a maintenance action. Operators often set their own alert levels more conservatively than the manufacturer’s defaults. For example, an airline might choose to replace a landing gear brake assembly at 5 percent wear remaining instead of waiting until just 2 percent is left, reducing the risk of costlier damage.
The practical result is that maintenance shifts from a fixed schedule toward decisions based on actual component condition. A part that’s wearing faster than expected gets replaced sooner; one that’s still healthy stays in service longer.
The Minimum Equipment List
Not every piece of equipment on an aircraft needs to be working for a flight to depart legally. Each aircraft type has a master minimum equipment list, developed by the manufacturer and approved by regulators, that identifies which items can be temporarily inoperative at the start of a flight. Airlines then create their own version, approved by their national aviation authority, tailored to their operations.
If a cabin reading light fails or a backup instrument goes offline, the MEL may allow the plane to continue flying under specific conditions or limitations while the repair is deferred to a maintenance base. But the system has clear boundaries. Multiple unrelated failures are treated with extra caution, and the final decision to accept an aircraft with any inoperative equipment rests with the captain. An aircraft cannot fly outside the conditions set by its MEL without explicit regulatory permission.
Who Performs the Work
In the United States, aircraft maintenance technicians must hold an FAA mechanic certificate with either an airframe rating, a powerplant rating, or both. There are two paths to eligibility. The first is graduating from an FAA-certified aviation maintenance technician school, which typically takes 18 to 24 months. The second is accumulating on-the-job experience, either in the military or civilian workforce, for 18 months (for a single rating) or 30 months (for both). Either way, candidates must pass written knowledge tests plus oral and practical exams before receiving their certificate.
Airlines can perform maintenance in-house or contract it out to FAA Part 145 repair stations. These certified facilities must meet strict requirements: an FAA-approved repair station manual, a quality control manual, appropriate equipment and housing, qualified personnel, and an approved training program. The FAA inspects all of these elements before granting certification.
Why It Matters for Safety
Maintenance-related errors have been linked to roughly 15 percent of major aircraft accidents. An FAA review of nearly 21,000 general aviation accident reports over a ten-year period found that about 7 percent cited a maintenance-related cause or contributing factor. That places maintenance alongside pilot error and weather as one of the most significant categories in accident investigation.
The layered system of scheduled checks, real-time monitoring, regulatory oversight, and technician certification exists specifically to keep that number as low as possible. Each layer catches what the others might miss: a sensor flags an engine trend that a visual inspection wouldn’t reveal, while a hands-on C check finds structural corrosion that no sensor is positioned to detect. The redundancy is deliberate.