What NDT methods are used for aircraft structural inspection and when is each appropriate?

NDT method selection depends on defect type and material: Visual inspection (surface damage, corrosion, obvious defects); Dye penetrant (surface cracks in non-porous materials); Magnetic particle (surface/near-surface cracks in ferromagnetic materials); Eddy current (surface cracks, subsurface flaws in conductive materials, used for fastener hole inspection); Ultrasonic (internal flaws, delamination in composites, thickness measurement); Radiography (internal defects, FOD, complex geometries); Thermography (disbonds, water ingress in composites). Selection considers: defect type expected, material, accessibility, required sensitivity, and inspector training. Probability of detection (POD) data validates inspection reliability for damage tolerance.