The Ultimate Guide to Eddy Current Testing (ET and Eddy Current Array)

Eddy current testing is one of the fastest ways to find a crack you cannot see. No couplant, no film, no chemicals. You energize a coil, hold it near a conductive part, and read how the part talks back through the impedance of that coil. When something interrupts the induced currents flowing in the metal, the instrument shows it. For aircraft aluminum, that speed and sensitivity is exactly why eddy current sits at the center of so many fleet inspections. For a real Boeing example, see the eddy current array special detailed inspection of Boeing longitudinal skin splices.

This guide walks through how the method actually works, where conventional eddy current ends and eddy current array begins, the probes and frequencies that matter, and the standards that govern the work. It is written from the bench, not the brochure. If you want the broader picture of how this method fits next to ultrasonics, radiography, and penetrant, start with our ultimate guide to nondestructive testing and come back here for the eddy current detail.

Introduction: what eddy current testing is

Eddy current testing, often shortened to ET, is an electromagnetic NDT method for conductive materials. A coil carrying alternating current creates a changing magnetic field. Bring that field close to a conductor and it induces small circulating currents, the eddy currents, in the surface and near-surface layers of the part. We put this to work on aircraft every week, for example our eddy current array inspection of 737 crown skin chem-mill steps, part of the wider aircraft NDT inspection scope. Those currents generate their own opposing field, which changes the impedance of the original coil. The instrument measures that change.

A crack, a corrosion pit, a conductivity shift, or a change in part thickness all disturb the eddy current flow. The disturbance shows up as a signal the inspector can read on an impedance plane display. Because the whole interaction happens through a magnetic field, the probe never has to touch bare metal and there is no need for a coupling medium. You can inspect through thin paint and primer, which is a real advantage on a finished airframe.

How eddy currents work

The physics in plain terms

Three things drive the result: the test frequency, the conductivity of the material, and the magnetic permeability of the material. Aluminum and titanium are non-ferromagnetic, so permeability is constant and easy to deal with. Carbon steel is ferromagnetic, which complicates things and is one reason eddy current sees far more use on aircraft aluminum than on industrial carbon steel piping.

Skin effect and depth of penetration

Eddy currents are densest at the surface and fall off with depth. This is the skin effect. The standard depth of penetration is the depth where current density drops to about 37 percent of its surface value. Higher frequency concentrates the currents near the surface and gives sharp sensitivity to surface cracks. Lower frequency drives the field deeper so you can reach subsurface cracks or second-layer corrosion, at the cost of some surface resolution. Picking the frequency is half the inspection.

Surface versus subsurface inspection

For a surface crack in a fastener hole or a fillet radius, you want high frequency, often hundreds of kHz up into the low MHz range. For a crack in a second or third layer of a lap joint, or for hidden corrosion between skins, you drop into the low kHz range. Multi-frequency and pulsed eddy current extend this further, letting one scan separate near-surface signals from deeper ones.

Conventional eddy current versus eddy current array

Conventional single-coil ET

Conventional eddy current uses one coil, or a small pair of coils, and covers area by manual scanning. It is light, portable, and proven. A surface probe rocked around a fastener head, a bolt-hole probe spun inside a reamed hole, a pencil probe run along a radius. For decades this has been the workhorse for fastener-hole crack detection and conductivity checks on aircraft structure. Our overview of how eddy current finds cracks covers the conventional side in more detail.

Eddy current array (ECA)

Eddy current array puts many small coils in a single probe and fires them in a controlled sequence so adjacent coils do not interfere. One pass of an array covers a wide footprint at once and builds a C-scan style image of the surface. Two things change with ECA. First, coverage speed goes up dramatically because you are no longer rastering a single coil back and forth. Second, you get a positional map, which makes flaws easier to interpret and easier to document for the records.

On large skin areas this matters a lot. A good example is the eddy current array inspection of 737 crown skin chem-mill steps, where the array images the chem-milled pockets and steps quickly and consistently across a big curved panel. ECA also shows up alongside phased array ultrasonics on lap-splice work such as the Boeing 737NG lap splice inspection for AD 2023-13-05, where HFEC handles the surface layer and PAUT handles the buried cracks.

Probes, frequency, and calibration

Probe types

Surface probes for general crack detection. Bolt-hole probes that rotate inside a fastener hole. Pencil probes for tight radii and weld toes. Sliding-probe units sized to scan along rivet rows. Each is tuned to a frequency range and a coil geometry that suits the target. Absolute coils read against a fixed reference, differential coils compare two adjacent points and shine at finding small abrupt flaws while ignoring slow material changes like liftoff or gradual thickness drift.

Reference standards and calibration

Eddy current is a comparative method. You set the instrument up on a reference standard with known artificial flaws, EDM notches or drilled holes of specified depth, before you ever touch the part. The standard matches the alloy, thickness, and geometry of the work as closely as possible. You verify liftoff response, set the phase angle so a crack signal points the way you expect, and confirm the smallest target flaw gives a clear, repeatable indication. Skip the standard and the numbers mean nothing.

Applications: where eddy current earns its keep

Aircraft aluminum airframe

This is the home turf. Fastener-hole cracks in wing and fuselage structure, surface cracks at fillet radii and lugs, second-layer cracking in lap joints, and conductivity testing to check heat treat condition or to map heat damage after a lightning strike or overheat event. Eddy current is also the standard tool for measuring nonconductive coating thickness on aluminum.

Engine and rotating hardware

High frequency eddy current inspects fan blade dovetail slots, blade root features, and pin holes where fluorescent penetrant is impractical or where a quick recurring check is needed between heavier shop visits. The sensitivity to tight fatigue cracks in a small radius is hard to beat.

Tubing and heat exchangers

Outside aviation, eddy current and its cousins remote field and near field testing are the primary methods for in-service inspection of non-ferromagnetic heat exchanger and condenser tubing. A bobbin probe run through the tube ID finds wall loss, pitting, and cracking from the inside.

Standards and certifications

Eddy current work is governed by both the method documents and the personnel-qualification documents. The ones you will see most often:

• ASTM E309 for eddy current examination of steel tubular products, and ASTM E1004 for determining electrical conductivity by the eddy current method.
• ASME Boiler and Pressure Vessel Code, Section V, Article 8 for eddy current examination of tubular products in the industrial world.
• NAS 410, the aerospace standard for NDT personnel qualification and certification, plus SNT-TC-1A as the recommended practice many written practices are built on.
• OEM NDT manuals from Boeing, Airbus, Bombardier, and Embraer, which give the specific probe, frequency, standard, and accept-reject criteria for each task. On a repair station these documents are the law for the job.
• 14 CFR Part 145, the rule set for the repair station itself, and 14 CFR Part 43 for maintenance performed on the aircraft.

Many eddy current inspections exist because an Airworthiness Directive requires them. The 737 crown skin chem-mill checks trace to AD 2013-08-15, and the lap-splice work mentioned earlier traces to AD 2023-13-05. The AD names the inspection, the OEM service bulletin names the method and the standard, and the inspector executes it under the Part 145 quality system.

Advantages and limitations

Advantages

Fast and clean. No couplant, no consumables, minimal surface prep, and you can inspect through thin paint. Highly sensitive to small surface and near-surface cracks. Gives instant results on a portable instrument. Works for crack detection, conductivity, coating thickness, and material sorting all from the same family of equipment. Array probes add wide coverage and a recorded image for the file.

Limitations

Conductive materials only, so no plastics or composites. Limited depth of penetration, so it is a surface and near-surface method, not a tool for finding flaws deep in a thick forging. Ferromagnetic materials are harder to inspect and usually need saturation or a different method. Signals are sensitive to liftoff, edge effects, and geometry, so interpretation takes a trained eye. And because it is comparative, the result is only as good as the reference standard and the setup behind it. When you need volumetric coverage of a thick weld, ultrasonics is the better answer, which is why we keep a separate ultimate guide to ultrasonic testing.

Best practices

• Match the reference standard to the part: same alloy, thickness, curvature, and fastener type. A generic block gives generic confidence.
• Choose frequency for the target depth. High frequency for surface cracks, low frequency for second-layer and subsurface flaws, multi-frequency when you need to separate the two.
• Set and verify phase and liftoff before scanning, and recheck calibration on a schedule during long inspections.
• Control surface condition. Excess paint build-up, loose debris, or a rough doubler can mask or mimic a flaw. Note it before you scan.
• Use array probes for large repetitive areas and recorded imaging, and conventional probes for tight radii, single fastener holes, and access-limited spots.
• Follow the OEM NDT manual exactly. The probe, frequency, and accept-reject criteria in that manual are not suggestions.
• Document the setup, the standard used, and the indications found so the inspection is repeatable and defensible.

Frequently asked questions

Can eddy current testing find cracks under paint?

Yes, within reason. Eddy current reads through thin nonconductive coatings, so light paint and primer are usually not a problem. Thick build-up increases liftoff and reduces sensitivity, so heavy or peeling coatings should be noted and may need to be addressed before inspection.

What is the difference between ET and eddy current array?

Conventional ET uses a single coil that you scan by hand to cover area. Eddy current array uses many coils in one probe fired in sequence, covering a wide path in a single pass and producing a positional image. ECA is faster on large areas and easier to document. Conventional probes still win for tight radii and individual fastener holes.

Does eddy current work on steel?

It can, but it is harder. Steel is ferromagnetic, which complicates the signal and usually requires magnetic saturation or a specialized probe. Eddy current is most effective on non-ferromagnetic conductors like aluminum and titanium, which is why it dominates aircraft aluminum inspection rather than carbon steel piping.

How deep can eddy current testing inspect?

It is a surface and near-surface method. Penetration depends on frequency, conductivity, and permeability, but practical depth is typically a few millimeters in aluminum. For deeper or volumetric coverage, ultrasonic testing or radiographic testing is the right tool.

What certification does an eddy current inspector need?

In aerospace, personnel are qualified and certified to NAS 410, with written practices often modeled on SNT-TC-1A. Inspectors are certified by method and level, and Level III personnel approve procedures and accept-reject criteria. The work itself is performed under the repair station’s Part 145 quality system.

Why is a reference standard so important?

Eddy current is a comparative method. The instrument is set up against known artificial flaws in a standard that matches the part, and every later signal is judged against that baseline. Without the right standard, the signal amplitudes and phase angles have no meaning.

Conclusion

Eddy current testing is the method you reach for when you need fast, clean, sensitive crack detection on conductive material, especially aircraft aluminum. Conventional probes handle the tight spots and single holes, eddy current array covers large skin areas with a recorded image, and the right frequency and reference standard make the difference between a defensible result and a guess.

Baron NDT performs conventional eddy current and eddy current array inspections on aircraft structure under our FAA Part 145 repair station certificate, with NAS 410 qualified personnel and procedures approved by a Level III. Whether you are working an Airworthiness Directive, a recurring structural check, or a one-off finding, we can run the inspection to the OEM NDT manual and give you records you can stand behind. Call Baron NDT at 904-304-2907 or reach out through baronndt.com to schedule eddy current inspection at our Jacksonville aviation facility.