Section 7.18

Disassembly, Inspection, Repair & Assembly

Defect recognition and visual inspection; non-destructive testing methods and their applications; repair schemes and limits; SB and AD compliance.

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Main ideas

Covers defect recognition, cleaning, non-destructive testing, repair schemes and limits, and compliance with service bulletins and airworthiness directives.
Common defects: cracks, corrosion, wear, distortion, fretting, pitting, dents, scores and heat/overload damage. Visual inspection, aided by magnification and borescopes, is the first line.
NDT — non-destructive testing — finds defects without damaging the part. DYE PENETRANT reveals surface-breaking flaws in any non-porous material (limit: surface-breaking only).
MAGNETIC PARTICLE finds surface and slightly sub-surface cracks in FERROMAGNETIC materials only — it cannot be used on aluminium or austenitic stainless steel.
EDDY CURRENT finds surface and near-surface cracks in conductive materials and works through thin coatings (limit: skilled interpretation, limited depth). ULTRASONIC finds internal and volumetric defects and measures thickness (limit: needs a couplant, access and skill).
RADIOGRAPHY (X-ray) images internal and volumetric defects but needs radiation-safety controls and is poor at fine cracks lying across the beam. A borescope gives visual access to internal areas.
Repairs are carried out only to an approved scheme (SRM, AMM or an approved repair design) and within stated limits; SBs and ADs are complied with as mandated, and the work is recorded.
⚠ Exam trap: magnetic particle will NOT work on aluminium — it is non-ferromagnetic; use penetrant or eddy current instead.

Key formulas

Solved examples

A surface crack is suspected in an aluminium fitting. Which NDT methods are suitable, and which is not?
Dye penetrant or eddy current are suitable for a surface crack in aluminium. Magnetic particle is not — aluminium is non-ferromagnetic, so no flux leakage forms and the method cannot work.
Which NDT method would you choose to detect an internal void in a thick component, and what does it require?
Ultrasonic testing — it sends high-frequency sound into the material and times the reflection from an internal defect. It requires a couplant between probe and part, adequate access, and a skilled operator to interpret the trace; radiography is the alternative for volumetric internal defects.

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