Non-destructive testing (NDT) is an important aspect of aircraft maintenance, as it allows for the inspection of aircraft components and structures without causing damage. There are several NDT methods that are commonly used in the aviation industry, including:

1. Visual inspection: Visual inspection involves examining the surface of an aircraft component or structure using the naked eye or specialized instruments. This is often the first step in NDT and can identify obvious defects or imperfections.

2. Radiographic testing: Radiographic testing uses X-rays or gamma rays to create an image of the internal structure of an aircraft component or structure. This can be used to detect cracks, corrosion, or other internal defects that may not be visible to the naked eye.

3. Ultrasonic testing: Ultrasonic testing uses high-frequency sound waves to inspect the material or component. This can be used to detect surface and subsurface defects, such as cracks, voids, and inclusions.

4. Eddy current testing: Eddy current testing uses electromagnetic fields to detect imperfections in conductive materials. This can be used to inspect the surface of aircraft components for cracks or other defects.

5. Magnetic particle testing: Magnetic particle testing uses a magnetic field to detect surface and slightly subsurface imperfections in ferromagnetic materials. This can be used to inspect aircraft components made of ferrous materials, such as steel or iron.

NDT inspections of aircraft components and structures are typically required at regular intervals in order to ensure the safety and reliability of the aircraft. The specific NDT methods used and the frequency of inspections will depend on the type of aircraft, the component or structure being inspected, and the regulatory requirements for that particular aircraft.