In the aviation industry, proper aircraft inspection techniques can make the difference between life and death. Most inspections completed on aircraft today are visual inspections. However, not all areas of the aircraft can be accessed for visual inspection and some damage is too small to be detected visually.
Non-Destructive Testing (NDT) was developed as a way to detect and evaluate defects in parts or assemblies, such as corrosion, cracks and other defects quickly and easily without disassembling or destroying the items. NDT is used throughout the design, development, certification, testing, and inspection of aviation components and assemblies to everything from small pieces of hardware to aircraft structures and engines. NDT plays an important role to enhance the quality and safety in the manufacture and maintenance of aircraft and to keep travelling by aircraft one of the safest methods of transportation.
Types of NDT Testing
NDT is generally split between surface techniques and sub-surface techniques. The variety of techniques available in the NDT field requires highly trained specialists who are able to evaluate the components or areas to be tested with the characteristics and advantages of the different techniques to get a comprehensive status of the safety and airworthiness of the aircraft.
Visual inspection is still the most widely used of all nondestructive tests. Light is set to illuminate the item to be inspected and is usually inspected using some type of magnification to enlarge the view of the part and any defects. Imaging-type borescopes can be used in deeper holes of aircraft parts to visually inspect the internal spaces of some inaccessible areas.
Liquid Penetrant Testing
Liquid Penetrant testing is one of the oldest and most common nondestructive methods used in aircraft maintenance. Liquid penetrant testing involves applying a colored liquid to the surface of a nonporous component. The liquid will be drawn into any minute surface defects and will increase the visible contrast between the defect and its background. Fluorescent penetrants can be used with UV light for more critical inspections. The area to be examined needs to be thoroughly cleaned before and after the test
Magnetic Particle Testing
Magnetic Particle testing uses magnetic fields to locate surface and near-surface defects in magnetic materials. Fine magnetic particles are placed on the surface of a magnetized object and are drawn into any defects by the distortion of the magnetic field. Fluorescent magnetic particles can also be used to enhance the inspection. The components tested by this method need to be demagnetized before being returned to service.
Ultrasonic testing is a technique that uses sound frequencies for the detection of internal defects in materials. Sound has a constant velocity in a material so any defect will cause a change in the sound characteristics. Those changes can be measured and graphed and can locate and determine the size of the defect. Ultrasonic techniques can also be used for thickness measurements between surfaces and also for coating thicknesses.
Eddy Current Testing
Eddy Current testing is an important technique that detects surface and subsurface cracks or corrosion on conductive materials. Eddy Currents are electrical currents produced in a component by a reaction with an alternating magnetic field. Defects in the component will change the shape and flow of the eddy currents so that the presence of cracks & other flaws can be detected.