Non-Destructive Testing (NDT)

Non-Destructive Testing encompasses every inspection methodology that evaluates the integrity of a material, component, or structure without altering its ability to return to service. In aviation, NDT is required throughout an aircraft's operational life by Airworthiness Directives, Supplemental Structural Inspection Documents (SSIDs), Structural Repair Manuals (SRMs), and operator maintenance programs. NDT is the enforcement mechanism for structural airworthiness: it is how an operator demonstrates that fatigue cracks, corrosion, and manufacturing defects detected by design analysis have not yet initiated or propagated to serviceable limits in actual metal.

The seven primary NDT methods used in aviation are distinct in their physics, their application domains, and their personnel qualification requirements.

Visual inspection (VT) is the most common method and the baseline for all other NDT. It includes direct unaided inspection, enhanced visual inspection with optical aids (magnifier, mirror, borescope), and remote visual inspection (RVI) via videoscope. FAA Advisory Circular AC 43-204 covers visual inspection of aircraft. Borescope inspection is a sub-method of RVI and the dominant technique for turbine engine internal inspection. Visual inspection finds surface-visible corrosion, mechanical damage, fluid leaks, and fastener distress; it cannot detect tight fatigue cracks or subsurface flaws.

Dye penetrant inspection (PT) detects surface-breaking discontinuities by applying a low-viscosity penetrant (colored or fluorescent) to the cleaned surface, allowing capillary action to draw penetrant into any surface crack, then removing excess penetrant and applying a developer that draws the entrapped penetrant back to the surface as a visible indication. Fluorescent penetrant under UV illumination is the higher-sensitivity version. PT is used on non-ferromagnetic parts — aluminum landing gear components, titanium fittings, compressor blades — where magnetic particle inspection is not applicable. ASTM E1417 provides the standard practice for liquid penetrant testing.

Magnetic particle inspection (MT) is applicable only to ferromagnetic materials (steel, some nickel alloys). A magnetic field is induced in the part, and fine ferromagnetic particles (dry powder or wet suspension) are applied to the surface; particles congregate at discontinuities where flux leakage exits the part. MT is used extensively on steel landing gear components, engine steel shafts, and certain steel structural attachments. ASTM E1444 is the standard practice. Magnetization must be removed (demagnetization) after inspection.

Eddy current inspection (ET) exploits electromagnetic induction: an alternating-current coil is placed near the conductive part surface, inducing eddy currents; subsurface or surface discontinuities alter the eddy current pattern in ways detectable as impedance changes in the coil. Eddy current is particularly effective for fastener-hole inspection — detecting cracks around fastener holes in aluminum wing and fuselage skins without removing fasteners — and for corrosion thickness measurement. Boeing SB 737-53-1180 and similar Airworthiness Directives routinely invoke eddy current as the required inspection method for specific structural locations. ASTM E2349 covers eddy current examination.

Ultrasonic inspection (UT) uses high-frequency sound waves (typically 1–25 MHz) introduced into the part to detect internal discontinuities via reflected pulse-echo signals, transmitted wave attenuation, or time-of-flight diffraction. UT is used for corrosion mapping, lamination detection in composites, and inspection of heavy section forgings. ASTM E2375 covers ultrasonic examination of forgings.

Radiographic inspection (RT) uses penetrating radiation — X-ray or gamma-ray — transmitted through the part to expose a film or digital detector, revealing internal density variations caused by cracks, porosity, or material inclusions. RT requires radiation safety controls under 10 CFR Part 20 in the US and equivalent national regulations in other jurisdictions. Common applications include casting inspections and inspection of complex assemblies where other methods cannot achieve required access. ASTM E1742 covers radiographic examination.

Infrared thermography (IRT) detects defects by observing differential heat dissipation patterns on or near the part surface using an infrared camera. In aviation, IRT is used for composite delamination detection and for bond-line integrity verification in honeycomb sandwich structures. The method is inherently two-dimensional and requires thermal excitation of the part. ASTM E2582 covers IRT for composite structures.

Personnel certification: US industry standard is SNT-TC-1A published by the American Society for Nondestructive Testing (ASNT), defining Level I (performs inspections under supervision), Level II (performs and evaluates), and Level III (develops procedures, approves programs). Many Part 145 Repair Stations require Level II for structural NDT inspections and Level III for NDT program management. Under EASA Part-145, AMC 145.A.35(a)(3) establishes that NDT tasks are to be performed by personnel certified in accordance with EN 4179 or NAS 410 (the aerospace industry equivalents of SNT-TC-1A). AC 43-3 covers the regulatory framework for NDT in aviation maintenance; §43.13(a) requires that maintenance be performed using methods, techniques, and practices acceptable to the Administrator as described in the manufacturer's current instructions or as otherwise acceptable to the FAA.

Structural NDT is a recurring cost center for aviation operators, particularly for older airframes undergoing Supplemental Structural Inspection Document (SSID) programs. Boeing's SSID for the 737 Classic, defined in Boeing Document D6-38278, and the equivalent for the 737 NG defined in D6-82489, prescribe hundreds of NDT tasks at specific aircraft age and cycle thresholds — tasks that require qualified Level II or III personnel, specialized equipment, and significant access preparation (panel removal, insulation blanket removal, fastener removal). The cost of a major SSID inspection event at high aircraft cycles can exceed the aircraft's market value, at which point operators retire rather than inspect.

For smaller training operators, NDT relevance is highest around landing gear overhauls (magnetic particle and dye penetrant on steel components are universal gear overhaul requirements), annual inspection corrosion assessment (visual and sometimes eddy current), and post-incident structural evaluation (any hard landing requiring eddy current or dye penetrant on affected structure per the SRM procedure). Failure to document NDT findings correctly — or to use a qualified facility when the task requires it — is an airworthiness violation that can create personal liability for the certifying mechanic and certificate-action exposure for the operator.

Aviatize's maintenance control module tracks NDT tasks as distinct maintenance items within the approved maintenance program, each carrying the correct threshold and repeat interval, required method (PT, MT, ET, UT, etc.), and qualification requirement. When an upcoming NDT task appears on the maintenance schedule, the work order generated includes the method requirement and any documented equipment or personnel qualification specifications, ensuring that maintenance coordinators procure qualified inspection services rather than delegating to general-purpose mechanics without the required authorization.

The digital data and records module stores the NDT inspection results — qualified inspector's name, certification level, method used, equipment identification, findings, and disposition — as a structured record attached to each maintenance task completion. For operators subject to SSID programs or ADs requiring periodic NDT, the compliance and auditing module tracks each AD task completion with the required documentation, providing auditors and the authority with a clean compliance trail. This closes the common gap between work performed and work documented that generates findings during authority ramp checks and base audits.