Inspections and testing for cracks

Under part 91, a small aircraft needs an annual inspection within the last 12 calendar months, done by a mechanic with an inspection authorization. If it carries people for hire or is used for flight instruction for hire in an aircraft the instructor provides, it also needs a 100-hour inspection, which an A&P can do. The 100 hours can be exceeded by up to 10 to fly to a place where the inspection can be done, but those extra hours count toward the next 100. An annual counts as a 100-hour; a 100-hour doesn't count as an annual. Larger operators use a progressive or continuous airworthiness program instead.

Finding cracks: nondestructive testing (NDT)

• Visual: the most used. A good light, a mirror, a magnifier, and a borescope to look inside engines and structure.
• Dye penetrant: a red or fluorescent dye soaks into cracks open to the surface, the excess is cleaned off and a developer draws it out. Works on any non-porous material, but only finds surface-breaking flaws.
• Magnetic particle: the part is magnetized and iron particles gather at cracks on or just under the surface. Only for ferromagnetic metals like steel, and the part is demagnetized after.
• Eddy current: a probe induces currents in any conductive metal; cracks, corrosion and heat damage change them. Finds surface and near-surface flaws, even through paint.
• Ultrasonic: high-frequency sound reflects off flaws inside a part; also measures thickness and finds disbonds.
• Radiography (X-ray): shows inside structure, but needs trained people and radiation safety.
• Tap test: tapping composite or bonded panels with a coin or small hammer; a dull sound marks a delamination or disbond.