The Ultimate Guide to Aircraft NDT Inspection
Introduction
Aircraft NDT inspection is how we find cracks, corrosion, and disbonds in flying structure without taking the part apart or cutting into it. Every commercial airframe and engine in service today depends on it. Fatigue cracks start small, often below the surface or under a fastener head where no one can see them, and the job of nondestructive testing is to catch them long before they become a problem.
This guide walks through how NDT is actually applied to aircraft, organized the way a repair station thinks about it: by airframe zone, by method, and by the rules that govern who can sign for the work. If you want the broader background on the technology itself, start with our Ultimate Guide to Nondestructive Testing. This page is the aviation counterpart, and it links out to the specific task articles where we get into individual inspections in detail. For bonded structure specifically, see our complete guide to composite and honeycomb inspection in aircraft.
NDT Methods Used on Aircraft
No single method finds everything. A good aviation NDT program matches the method to the defect, the material, and the access. Here is how the common methods break down on an airframe.
Eddy Current (ET) and Eddy Current Array (ECA)
Eddy current is the workhorse for aluminum airframe inspection. It finds surface and near-surface cracks around fastener holes, in skin splices, and along chem-mill steps, and it works through paint and thin coatings. Array probes (ECA) cover a wider footprint in one pass and produce a C-scan image, which speeds up large-area scans and improves repeatability. Our guide to eddy current testing covers the principles in depth, and the 737 crown skin chem-mill ECA inspection is a good real-world example of where array probes earn their keep.
Ultrasonic Testing (UT and PAUT)
Ultrasonic testing sends sound into the part and reads the echoes. It measures remaining wall thickness on skins, finds subsurface cracks in thick structure, and inspects joints that eddy current cannot reach. Phased array (PAUT) steers the beam electronically so a single probe can cover a range of angles, which is how we inspect lap seams, buttstrap runouts, and structural fittings without dozens of separate setups. See our ultrasonic testing guide for the full method breakdown.
Radiography (RT)
Radiography uses X-rays or gamma to image the inside of a part on film or a digital detector. On aircraft it shows trapped water in honeycomb, internal corrosion, and cracks in thick or multilayer joints that surface methods miss. It is the method of choice when you need to see what is happening under a doubler or inside a closed structure. Our radiographic testing guide covers film, computed, and digital radiography along with the radiation safety side.
Infrared Thermography (IRT)
Thermography is one of Baron’s specialties and the fastest growing method for composite and bonded structure. It heats the surface slightly and watches how heat flows away. Disbonds, delaminations, water in honeycomb core, and crushed core all change the heat flow and show up as a thermal pattern. It covers large composite panels quickly and needs no couplant. We cover it fully in the infrared thermography guide, and you can see it applied on the Airbus rudder honeycomb core and on Boeing engine fan cowls.
Liquid Penetrant (PT/FPI) and Magnetic Particle (MT)
Penetrant and magnetic particle are the classic surface methods. Fluorescent penetrant (FPI) finds surface-breaking cracks in non-porous parts and is the standard for engine components like fan blade leading edges. Magnetic particle works only on ferromagnetic parts such as steel landing gear and engine shafts, where it finds surface and slightly subsurface cracks. Both are covered in our penetrant and magnetic particle guide, and the FPI of fan blade leading edges shows FPI in a real engine shop setting.
Visual and Borescope (VT)
Visual is the oldest method and still the first one applied. Remote visual with a borescope lets an inspector look inside an engine, a wing box, or any closed cavity without disassembly. It is fast, low cost, and often the trigger for a more detailed inspection with one of the methods above.
NDT by Airframe Zone
On a heavy check, the inspection plan is organized by zone. Each area of the aircraft has its own typical damage and its own preferred methods.
Fuselage
The pressurized fuselage cycles every flight, so fatigue cracking around fastener holes, lap splices, and skin doublers is the main concern. Eddy current and ECA dominate here, with PAUT and radiography brought in for multilayer joints. The 737NG lap splice inspection is a textbook fuselage example driven by an airworthiness directive.
Wings
Wing lower skins, spar caps, and the joints under pylons carry high tension loads and see eddy current, ultrasonic thickness, and radiography. Buttstrap runouts and the structure under reinforcing plates are common inspection points where PAUT and X-ray are used together.
Empennage and Flight Control Surfaces
Rudders, elevators, and stabilizers are increasingly composite and honeycomb. Disbond and water ingress are the usual findings, which puts thermography and bond testing front and center. See the rudder disbond thermography and Airbus elevator panel articles for specifics.
Landing Gear
Gear is high-strength steel, so magnetic particle is the primary method, backed by eddy current on bushings and bores and ultrasonic on thick sections. Overhaul shops run these inspections at every gear cycle.
Engine and Nacelle
Rotating engine parts get FPI, high-frequency eddy current on blade roots and pin holes, and ultrasonic on disks. Nacelle and cowl composite panels get thermography. Borescope inspection ties the on-wing and shop-visit programs together.
Standards and Certifications
Aircraft NDT is one of the most heavily regulated forms of inspection there is, and for good reason. The work has to be traceable to a procedure, the procedure has to be approved, and the inspector has to be qualified.
FAA Part 145 and 14 CFR 43
A repair station performing NDT on certificated aircraft operates under 14 CFR Part 145 with maintenance performed per Part 43. Baron NDT holds FAA Repair Station certificate CRS# 5NDR545D, which means our procedures, equipment, and records are subject to FAA oversight.
NAS 410 and SNT-TC-1A
Personnel qualification in aerospace follows NAS 410 (or the European EN 4179 equivalent), which sets training hours, experience, and eye exam requirements for Level I, II, and III inspectors. The general industry recommended practice SNT-TC-1A covers similar ground on the industrial side. Either way, a written practice defines exactly how Baron’s people are certified.
OEM NDT Manuals and ASTM Standards
The actual inspection technique usually comes from the manufacturer’s NDT manual: Boeing, Airbus, Bombardier, and Embraer each publish detailed NDT procedures keyed to the structure. Method standards such as ASTM E1417 for penetrant, E1444 for magnetic particle, E1742 for radiography, and the relevant eddy current and ultrasonic practices fill in the technical baseline.
Airworthiness Directives
Many inspections exist because the FAA issued an airworthiness directive after a fleet-wide problem was found. AD 2013-08-15 drove the 737 crown skin chem-mill inspections, and AD 2023-13-05 drove the 737NG lap splice work. An AD is mandatory, with a stated method, threshold, and repeat interval.
Advantages and Limitations
The big advantage of NDT is obvious: it finds damage without destroying or even removing the part, so an aircraft can be inspected and returned to service quickly. It catches problems while they are still small, supports damage-tolerance programs, and produces records that prove the structure was checked.
The limits are real too. Every method has a smallest detectable flaw size, and that depends on the equipment, the technique, and the inspector. Access is a constant challenge, since a lot of structure is buried under skins, sealant, or other parts. Results carry some operator dependence, which is exactly why personnel qualification and procedure control matter so much. No method finds every defect, so good programs layer methods rather than relying on one.
Best Practices
A few things separate a reliable aircraft NDT program from a box-checking one. Always work to the current revision of the OEM NDT manual or AD, because techniques and intervals change. Calibrate on reference standards that match the part, and verify calibration through the job, not just at the start. Match the method to the defect and the material instead of forcing a favorite technique onto every part. Document findings clearly, with location, indication size, and the disposition, so the next inspector and the engineering authority can act on it. And keep inspectors current: NAS 410 vision exams and recertification exist because skill and eyesight both drift over time.
Frequently Asked Questions
What is aircraft NDT inspection?
It is the use of methods like eddy current, ultrasonic, radiography, thermography, penetrant, and magnetic particle to find cracks, corrosion, and disbonds in aircraft structure and engines without damaging the part. It keeps flying structure safe between and during heavy maintenance.
Which NDT method is best for finding cracks in an aluminum airframe?
Eddy current and eddy current array are usually first for surface and near-surface cracks in aluminum, especially around fasteners and in skin splices. Ultrasonic or radiography is added when the crack may be deeper or hidden under other layers.
Who is allowed to perform NDT on certificated aircraft?
Inspectors qualified to NAS 410 (or EN 4179) and working under an FAA Part 145 repair station’s written practice and approved procedures. Level III personnel approve procedures and oversee the program.
How does NDT relate to airworthiness directives?
Many ADs require a specific NDT inspection at a set threshold and repeat interval to address a known fatigue or corrosion issue. Performing the inspection per the AD and recording it is how an operator stays compliant.
Can composite parts be inspected with NDT?
Yes. Thermography, ultrasonic, bond testing, and radiography all inspect composite and honeycomb structure for disbonds, delaminations, and trapped water. Thermography is especially effective on large bonded panels and is one of Baron’s core capabilities.
Talk to Baron NDT
Baron NDT is an FAA Part 145 repair station (CRS# 5NDR545D) and a Service-Disabled Veteran-Owned Small Business performing aircraft NDT inspection across all of these methods and airframe zones. We hold Boeing Conformity Review approval and run a NAS 410 qualified team out of our Jacksonville, FL aviation facility. If you have an AD coming due, a composite repair to verify, or an engine component that needs FPI, call us at 904-304-2907 and we will get it scheduled.