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Nondestructive Testing Techniques in Aircraft Maintenance
Release Date:
2022/02/11
Nondestructive testing is a branch of materials science that evaluates material defects (discontinuities), structural defects in workpieces (discontinuities), physical and mechanical properties, and chemical composition without altering or damaging the condition or performance of the materials or workpieces.
Nondestructive testing (NDT), as a critical method for detecting structural damage in aircraft, was adopted relatively late in civil aviation aircraft maintenance. However, the development of NDT has been driven by the formulation and implementation of the “Chinese Civil Aviation Nondestructive Testing Standards,” the introduction of new NDT technologies, and the continuous enhancement of personnel qualifications.
Nondestructive testing has been recognized by airlines for its effectiveness and high reliability.
I. Analysis of the Application Objects of Nondestructive Testing
Nondestructive testing is primarily aimed at detecting damage in aircraft structures, which can be broadly classified into the following five types:
1. Defects generated during the manufacturing process of aircraft structural components;
2. Structural damage caused by excessive loads on the aircraft during takeoff, flight, and landing due to various factors. For example, heavy landings can result in damage to the landing gear and wheel assemblies;
3. Scratches, dents, and other damage caused during routine maintenance;
4. Corrosion damage caused by the service environment—such as the humid air in coastal areas and seafood transported in aircraft cargo holds—is the root cause of corrosion-related failures.
5. Fatigue damage (fatigue cracks) caused by cyclic loading.
If these damages are not properly addressed, they can readily give rise to cracks, such as fatigue cracks, stress-corrosion cracks, and corrosion-fatigue cracks. For instance, stress-concentrated areas in aircraft structures—such as the fillet transitions at the wheel hub–wheel disc interface, the threaded portions of fasteners, and other locations like joints, hole edges, and corners—are particularly prone to fatigue cracking.
The crack initiation and short-crack propagation stages constitute the onset and dominant phase of fatigue; studies have shown that these stages account for as much as 80% of the total fatigue life. Consequently, the crack-initiation life of a structure has become a critical performance indicator of widespread concern.
In particular, in the aerospace field, the complexity of certain structures makes in-service inspection challenging. Furthermore, some structures, owing to their specialized functional requirements, must employ high-strength or ultra-high-strength materials, which typically exhibit poor crack-propagation resistance.
II. Nondestructive Testing Methods and Applications
Some structural damages can be detected through visual inspection or other methods, such as endoscopy. However, when inspecting minute defects or when visual inspection is inadequate, other nondestructive testing methods must be employed, including acoustic emission testing, eddy-current coating thickness measurement, eddy-current conductivity testing, ultrasonic thickness measurement, magnetic particle inspection, and liquid penetrant inspection.
In practical applications, they exhibit distinct differences as well as close interconnections; therefore, a brief introduction is warranted.
1. Eddy current testing is used to inspect surface and near-surface defects in conductive components, such as hub cracks, cracks around fasteners, and corrosion damage to aluminum skin. It is also the most widely used inspection method today.
2. Magnetic particle inspection is used to detect surface and near-surface defects in ferromagnetic components, such as landing gear parts, wheel hub connecting bolts, engine suspension point bolts, and welded joints.
3. Penetrant testing is used to detect surface-opening defects in non-porous materials. Due to its simple equipment and other advantages, penetrant testing is widely applied. In particular, during structural repairs—for example, several years ago when a passenger boarding bridge inadvertently collided with an aircraft cabin door—penetrant testing was employed to accurately determine the length and orientation of the cracks induced by the impact. This not only guided the repair strategy but also ensured the quality of the repair.
4. Ultrasonic testing can be used to inspect internal defects in nearly all aircraft structural components, such as bolts connecting the wing to the fuselage and signs of structural corrosion.
5. Radiographic testing can be used to inspect internal defects in aircraft metallic materials, such as fatigue cracks in fuselage door frames and wing stiffeners. However, due to constraints related to site conditions, radiation protection requirements, and capital investment in equipment, most small domestic airlines do not currently offer this service. That said, radiographic testing is an indispensable inspection method during major overhauls at specialized aircraft maintenance organizations.
6. It is also worth noting the application of acoustic–vibration testing. With the advancement of composite-material technology, composites and honeycomb structures offer high specific strength and specific stiffness, leading to their increasingly widespread use in aircraft. These materials are prone to defects such as delamination, debonding, and cracking; acoustic–vibration testing is primarily employed to detect debonding, voids, and delamination in bonded joints, thereby assessing the integrity of bonded structures made from composites and honeycomb materials. For example, the Canadian-made CF-18 fighter jet requires a full-airframe bond inspection every two years to identify potential debonding and other defects. Given the extensive use of composites in modern aircraft, acoustic–vibration testing holds significant promise for future applications.
III. Application of Nondestructive Testing in Aircraft Maintenance
1. As nondestructive testing (NDT) has become increasingly sophisticated in terms of personnel, equipment, and technological maturity, it can effectively detect structural defects in aircraft, such as fatigue cracks, thereby enabling the implementation of necessary measures to eliminate flight safety hazards. When repairing damaged components, decisions must be made on a case-by-case basis depending on the severity of the damage. To this end, NDT personnel first conduct inspection and assessment of the damaged area, and maintenance personnel then develop a repair plan based on the assessment results to ensure the feasibility and effectiveness of the repair. Following completion of the repair, NDT personnel must again perform nondestructive testing on the repaired area to verify the quality of the repaired component, thus guaranteeing the reliability of aircraft maintenance.
2. Thanks to the rational design of aircraft structures and the continuous improvement and enhancement of nondestructive testing (NDT) technologies, in-service inspections using NDT have become feasible. Specifically, most NDT tasks can now be performed without removing structural components, thereby reducing maintenance time and costs. Moreover, this development provides a solid basis for the innovation and optimization of overall maintenance procedures.
3. Nondestructive testing makes it possible to monitor certain aircraft structural components. In practical inspections, it has been observed that some components exhibit minor defects that do not yet meet the criteria for scrapping; however, given that these components are subjected to significant cyclic loading or high stress levels, it is both feasible and advisable to shorten the inspection intervals for continued service. This approach not only ensures maintenance reliability but also extends the service life of the components.
4. With the application of advanced nondestructive testing technologies, such as real-time acoustic emission monitoring, maintenance practices will undergo a fundamental transformation, shifting from scheduled maintenance to condition-based and reliability-centered maintenance. By continuously monitoring the operational condition of aircraft structures and components, necessary preventive maintenance can be performed on an as-needed basis, which in turn requires appropriate and effective inspection methods. The enhancement of NDT techniques and the ongoing refinement of associated procedures are enabling a transition from current damage localization to comprehensive damage characterization—both qualitative and quantitative—and to reliability assessment, a goal that is entirely achievable. It can thus be said that NDT represents the technological linchpin for revolutionizing aviation maintenance practices.
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