Metallic airframe structures endure punishing cycles of stress, vibration, temperature change, and corrosion throughout an aircraft's service life. Hidden damage — a fatigue crack buried beneath a rivet head, a disbond invisible to the naked eye, or subsurface corrosion inside a thick aluminum spar — can grow silently until it threatens structural integrity. Non-destructive testing (NDT), sometimes called non-destructive inspection (NDI) or non-destructive evaluation (NDE), is the family of methods that allows an Aviation Maintenance Technician (AMT) to detect exactly these kinds of hidden flaws without cutting, breaking, or otherwise permanently altering the part under examination. Mastery of NDT is not optional knowledge for airframe work — it is a core airworthiness skill, and it appears prominently on the FAA Airframe Knowledge Test.
This article covers the five primary NDT methods used on metallic airframe structures: visual inspection, dye penetrant, magnetic particle, eddy current, and ultrasonic testing. Each method has a specific physical principle, a specific range of detectable defects, defined materials it works on, and procedural steps the AMT must follow. Understanding all five — their strengths, limits, and the conditions under which each is appropriate — is the foundation of sound structural inspection practice.
Visual Inspection
Visual inspection is both the simplest and the most frequently performed NDT method. It requires no special equipment beyond good lighting, a magnifying glass, a mirror, and a borescope for confined spaces. The AMT examines surfaces directly for cracks, corrosion, dents, scratches, distortion, and missing fasteners. Despite its apparent simplicity, visual inspection is only effective for surface defects that are open to the surface and large enough to see. Subsurface cracks, tight fatigue cracks, and defects beneath paint or sealant will typically be missed. Because of these limitations, visual inspection is always the starting point of any inspection, but it is routinely supplemented by one or more of the methods described below.
Dye Penetrant Inspection
Dye penetrant inspection (also called liquid penetrant inspection) reveals surface-breaking cracks in any non-porous material — including aluminum alloys, titanium, stainless steel, magnesium alloys, and even non-metallic composites. It does not work on porous materials because the background bleed-out masks indications. The underlying principle is capillary action: a brightly colored or fluorescent dye penetrates into open cracks by capillary force, and a developer then draws that dye back out and spreads it visibly on the surface.
The standard procedural steps are: (1) Clean the surface thoroughly to remove all contamination. (2) Apply penetrant and allow dwell time — typically 5 to 30 minutes depending on the penetrant type and the suspected flaw size; a tight fatigue crack needs more dwell time than a coarse defect. (3) Remove excess penetrant from the surface without washing it out of the crack. (4) Apply developer, which acts as a blotter. (5) Inspect under white light (visible dye) or UV/black light (fluorescent dye). (6) Post-clean the part. Fluorescent penetrant systems are more sensitive than visible-dye systems because the bright glow against a dark background enhances small indications. The critical limitation: dye penetrant only finds defects open to the inspected surface. Subsurface voids or cracks that don't break the surface are completely invisible to this method.
Magnetic Particle Inspection
Magnetic particle inspection (MPI) is restricted to ferromagnetic materials — primarily iron and steel alloys. Aluminum, titanium, magnesium, and copper alloys are non-magnetic and cannot be inspected with this method. The principle is simple: when a ferromagnetic part is magnetized, any crack or void disrupts the magnetic flux lines and causes them to leak out at the defect, creating a small localized magnetic field called a flux leakage field. Fine iron particles — applied either dry or suspended in a liquid — are attracted to these leakage fields and accumulate as a visible indication over the flaw.
Two types of magnetization are used. Circular magnetization (current passed directly through the part, or through a central conductor) detects longitudinal cracks running parallel to the current path. Longitudinal magnetization (using a coil or yoke wrapped around or placed against the part) detects transverse cracks perpendicular to the induced field. Because a crack running parallel to the magnetic field lines produces little to no flux leakage, the part must often be inspected in two directions, roughly 90 degrees apart, to ensure complete coverage. MPI can detect both surface and near-surface subsurface defects, which is an advantage over dye penetrant. After inspection, the part must be thoroughly demagnetized to prevent residual magnetism from interfering with instruments, attracting ferrous debris to bearings, or affecting compass systems.
Eddy Current Inspection
Eddy current inspection is one of the most versatile NDT methods for aviation metallic structures because it works on any electrically conductive material and can detect both surface and subsurface flaws without direct contact or surface preparation as extensive as other methods. It can even inspect through paint and thin coatings. The principle: an alternating current through a probe coil induces a swirling electrical current — an eddy current — in the conductive material beneath it. Any crack, corrosion thinning, or conductivity change disrupts these eddy currents. The probe detects the resulting change in the coil's impedance and displays it on a meter, oscilloscope, or digital instrument.
Eddy current is the preferred method for detecting fatigue cracks around fastener holes, a common failure initiation site in aluminum airframe skins and spars. Special rotary scanner probes can be inserted directly into a fastener hole to inspect the surrounding structure. Eddy current is also used extensively to measure coating thickness and to detect corrosion under the skin without removing paint. The depth of penetration is controlled by frequency: lower frequencies penetrate deeper (useful for subsurface inspection) while higher frequencies are more sensitive to near-surface defects. One key limitation is that the method requires careful calibration against reference standards with known defect sizes, and interpretation of the display requires trained, practiced technicians.
Ultrasonic Inspection
Ultrasonic testing (UT) uses high-frequency sound waves — typically in the range of 0.5 MHz to 25 MHz — to probe the interior of materials. A transducer (probe) converts electrical energy into sound waves that travel through the metal. When a sound wave encounters an interface — such as a crack, void, delamination, or the far wall of the part — it reflects back to the transducer, which converts the echo into an electrical signal displayed on an instrument. By measuring the time of flight of the echo, the instrument precisely calculates the depth of the flaw within the material.
Two common UT techniques apply to airframe work. Pulse-echo uses a single transducer to send and receive; it is excellent for measuring material thickness and detecting internal cracks. Through-transmission uses separate sending and receiving transducers on opposite sides of the part; a reduction in received signal indicates an intervening flaw. Ultrasonic inspection requires a couplant — a gel, oil, or water medium — between the probe and the part surface to efficiently transmit sound (air gaps block the waves). UT can inspect thick materials and complex geometries that other methods cannot reach, and it provides precise depth information, but it also requires significant operator training and careful probe manipulation to ensure complete coverage.
Why These Methods Matter
Fatigue cracking is one of the leading structural threats to metal airframes. Because cracks often initiate at microscopic stress concentration points — a scratched fastener hole, a corrosion pit, a machining mark — and grow invisibly for many flight cycles before becoming detectable visually, scheduled NDT inspections are written directly into maintenance programs and airworthiness directives. Choosing the wrong method for a given material or defect type means a dangerous flaw could be completely missed, even after a diligent inspection. An AMT who understands which method to apply — and why — is a genuine safety asset.
Key Numbers and Rules
- Dye penetrant: Works on any non-porous material; finds surface-breaking defects only; dwell time typically 5–30 minutes; fluorescent systems more sensitive than visible-dye systems.
- Magnetic particle: Ferromagnetic materials only (steel/iron); detects surface and near-surface flaws; requires two magnetization directions roughly 90° apart; demagnetization required after inspection.
- Eddy current: Any electrically conductive material; detects surface and subsurface flaws; works through paint; lower frequency = greater penetration depth; requires calibration standards.
- Ultrasonic: Works on most solid materials including thick sections; measures flaw depth precisely; requires couplant; two main modes are pulse-echo and through-transmission.
- Visual: All materials; surface defects only; enhanced by magnification, lighting, borescopes; always the first step but rarely sufficient alone for structural inspections.
Common Test Traps
- Applying magnetic particle to aluminum: Aluminum is non-ferromagnetic. MPI simply does not work on it. The FAA knowledge test frequently offers MPI as a distractor for aluminum structure questions — always choose eddy current or dye penetrant for aluminum.
- Assuming dye penetrant finds subsurface flaws: It does not. Liquid penetrant inspection requires the defect to be open to the surface. If the question describes a subsurface crack in aluminum, the correct answer is eddy current or ultrasonic — not penetrant.
- Forgetting demagnetization after MPI: Leaving a steel part magnetized is not just an oversight — it can cause compass errors and attract debris to precision surfaces. Demagnetization is a required step, not optional cleanup.
- Confusing frequency and penetration depth in eddy current: Higher frequency = shallower penetration, more surface sensitivity. Lower frequency = deeper penetration. The relationship is inverse, and the test exploits students who assume higher frequency means deeper inspection.
- Skipping couplant in ultrasonic testing: Without couplant, most of the ultrasonic energy reflects at the air-metal interface and never enters the part. The inspection will produce invalid results. This procedural requirement is commonly tested.