Every aircraft is a collection of carefully engineered parts that must perform reliably under extreme stress, vibration, temperature swings, and fatigue cycles. Over time, even the best materials can develop cracks, voids, corrosion, or disbonds that are invisible to the naked eye. Non-destructive testing (NDT) — also called non-destructive inspection (NDI) or non-destructive evaluation (NDE) — gives aviation maintenance technicians (AMTs) a toolkit of scientific methods to find those hidden defects without damaging the part or removing it from service. Understanding NDT is essential for passing the FAA AMT General knowledge test and, more importantly, for keeping aircraft safe.
The FAA addresses NDT principles in the Aviation Maintenance Technician Handbook — General (FAA-H-8083-30), which describes the most common inspection methods, their physical principles, and their proper applications. Each method exploits a different physical phenomenon — light, sound, magnetism, electricity, or radiation — and each has strengths and limitations that make it better suited to certain materials or defect types. A competent AMT must know which method to apply, how to prepare the surface, and how to interpret results correctly.
Visual Inspection
Visual inspection (VI) is the foundation of all maintenance checks and is often performed before any other NDT method. The technician examines surfaces directly with the naked eye or with optical aids such as magnifying glasses, borescopes, and fiber-optic light guides. A borescope is especially valuable for inspecting the interior of engine cylinders, turbine stages, and enclosed airframe cavities without disassembly.
Effective visual inspection requires good lighting, a clean surface, and a systematic scan pattern to avoid missing areas. Although it seems simple, visual inspection is the most widely used NDT method in aviation maintenance and catches a large percentage of discrepancies — corrosion pitting, obvious cracks, impact damage, and missing fasteners. Its limitation is that it only reveals surface conditions; subsurface defects require additional methods.
Dye Penetrant Inspection
Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI), detects surface-breaking discontinuities in non-porous materials. The process relies on capillary action: a brightly colored or fluorescent liquid penetrant is applied to the cleaned surface and drawn into any crack or void. After a specified dwell time, excess penetrant is removed from the surface, and a developer is applied that draws the trapped penetrant back out, creating a visible indication against a contrasting background.
There are two main types: visible dye (red dye examined under white light) and fluorescent (examined under ultraviolet/black light in a darkened area). Fluorescent penetrant is more sensitive and preferred for critical components. DPI is generally applicable to non-porous materials such as metals and ceramics, but its use on plastics and composites is limited — some of these materials are porous or absorbent, or may be chemically attacked by penetrant fluids, so compatibility must be verified before use. It cannot detect subsurface defects and requires scrupulous surface preparation; oil, paint, or plating can seal cracks and produce false-negative results. The basic steps are: clean, penetrant application, dwell, remove excess, develop, inspect, and post-clean.
Magnetic Particle Inspection
Magnetic particle inspection (MPI) finds surface and near-surface defects in ferromagnetic materials — iron, nickel, cobalt, and most steels. When a magnetic field is induced in a part, any discontinuity interrupts the field and creates a leakage field at the surface. Fine ferromagnetic particles (dry powder or wet suspension) applied to the surface are attracted to these leakage fields, forming a visible indication at the defect location.
Magnetization can be achieved by passing current directly through the part (contact method), using a yoke (an external electromagnet), or by threading a conductor through a hollow part. Fluorescent wet particles examined under UV light offer the highest sensitivity. After inspection, the part must be demagnetized — residual magnetism can attract ferrous debris and interfere with onboard compass systems. MPI is fast and reliable for steel landing gear, engine crankshafts, and attachment fittings, but it is completely ineffective on aluminum, magnesium, titanium, and composite structures.
Eddy Current Inspection
Eddy current inspection exploits electromagnetic induction and is one of the most versatile NDT methods for aviation. An alternating current passes through a probe coil, generating an alternating magnetic field that induces circulating electrical currents — called eddy currents — in any nearby conductive material. Defects, material variations, or changes in thickness alter the flow of eddy currents, which in turn changes the impedance of the probe coil. The instrument detects and displays this impedance change.
Eddy current is widely used for inspecting aluminum airframe structures, detecting fatigue cracks around fastener holes, measuring coating thickness, and checking heat-treat condition. It can detect surface and near-surface defects without contact (the probe can work through paint) and requires minimal surface preparation. The penetration depth depends on the test frequency — lower frequencies penetrate deeper but with reduced sensitivity, while higher frequencies are more sensitive near the surface. Eddy current equipment requires skilled operators to interpret the impedance-plane displays and distinguish real defects from geometry changes or lift-off variations.
Ultrasonic Inspection
Ultrasonic testing (UT) uses high-frequency sound waves — commonly in the range of about 1 to 10 MHz for aviation applications, though the broader usable range for UT can extend from roughly 0.5 to 25 MHz depending on the application — that are well above the range of human hearing. A transducer converts electrical energy into sound waves and couples them into the material through a couplant (gel, oil, or water). The sound travels through the material until it strikes a boundary — either the far wall of the part or an internal defect — and reflects back to the transducer, which converts the returning echo into an electrical signal displayed on a screen.
The time it takes for the echo to return indicates the depth of the reflector, and the amplitude of the echo indicates its size relative to calibration standards. UT can detect subsurface cracks, voids, delaminations in composites, and inclusions deep within thick sections — capabilities that surface-only methods cannot provide. Two common UT techniques are pulse-echo (single transducer sends and receives) and through-transmission (separate send and receive transducers on opposite sides, useful for composites). Proper coupling, calibration with reference standards, and interpreter skill are critical for accurate results.
Radiographic Inspection
Radiographic inspection (X-ray and gamma ray) passes ionizing radiation through a part and records the transmitted intensity on film or a digital detector. Denser or thicker areas absorb more radiation and appear lighter on the radiograph; voids, cracks, and porosity allow more radiation through and appear as darker areas. Radiography excels at revealing internal defects in castings, welds, and assemblies where other methods cannot reach, and it can inspect hidden structural areas without disassembly.
X-rays are produced by an electrical X-ray tube and are preferred when fine detail and variable energy are needed. Gamma rays are produced by radioactive isotopes (such as iridium-192) and are useful in field conditions where electrical power is unavailable. Radiography requires strict radiation safety protocols, controlled access zones, and proper shielding. Personnel must be trained and monitored for radiation exposure per regulatory requirements. A key limitation is that cracks oriented parallel to the beam may not be visible, so proper beam angulation is essential.
Why NDT Matters for Airworthiness
Aircraft structures experience millions of stress cycles over their service lives. A fatigue crack that begins as a microscopic surface flaw can propagate rapidly to catastrophic failure if undetected. NDT is mandated by manufacturer maintenance manuals, Airworthiness Directives (ADs), and FAA-approved inspection programs because it allows defects to be caught at the subcritical stage — when they can be repaired or the part replaced before any safety consequence occurs. NDT is not optional; it is the backbone of damage-tolerant and safe-life design philosophy.
Key Numbers and Rules
- DPI dwell time — varies by penetrant type and material; always follow the penetrant manufacturer's instructions and the applicable maintenance manual (typically 5–30 minutes for most metals).
- MPI demagnetization — required after every MPI; the acceptable residual field level is not a single fixed FAA number and depends on the applicable specification and maintenance manual (commonly on the order of a few gauss); always verify against the manual.
- Eddy current frequency — lower frequency = deeper penetration; higher frequency = greater surface sensitivity.
- UT frequency range — commonly about 1 to 25 MHz for aviation applications; higher frequencies yield finer resolution but less penetration depth.
- Ferromagnetic limitation — MPI works only on ferromagnetic metals; it will not work on aluminum, titanium, magnesium, or composites.
- Surface preparation — DPI requires a completely clean, bare surface; coatings must be removed or the test is invalid.
- Radiographic safety — ionizing radiation hazard; controlled access area required during exposures; personnel dosimetry required.
Common Test Traps
- MPI on aluminum: A classic FAA knowledge test trap. MPI only works on ferromagnetic materials. Aluminum, magnesium, titanium, and composites require eddy current, DPI, or UT instead.
- DPI for subsurface defects: DPI reveals only surface-breaking discontinuities. Choosing DPI to find an internal void or inclusion is incorrect; UT or radiography is needed for subsurface detection.
- Forgetting demagnetization: After MPI, the part retains a magnetic field. Failing to demagnetize can cause compass errors and attract metallic debris — always demagnetize after MPI.
- Eddy current on non-conductors: Eddy current requires an electrically conductive material. It cannot inspect non-conductive composites (e.g., fiberglass), ceramics, or plastics.
- Radiographic crack orientation: A crack parallel to the X-ray beam may be nearly invisible on the film. The beam must be properly angled to the suspected defect orientation to reveal it; this is why multiple exposures or angles may be required.