How to Complete Eddy Current Testing of Aircraft

Eddy current testing for aircraft is a tried and true method of ensuring the efficacy of metal components. While it doesn’t work for everything—like composite material—it’s ideal for evaluating crucial components such as multilayer aluminum bodies, fasteners, and bolt holes. It’s also possibly the best way to review material for surface and subsurface cracking, which is common due to the harsh conditions airplanes face. 

The average speed of an airplane, 575 mph, ensures passengers can travel quickly, but it also puts metal components under a lot of stress. Add in the pressure that occurs in changing altitude, and it’s no wonder that aircraft safety checks have such stringent standards. The smallest incongruity could splinter into something catastrophic under those conditions, making eddy current testing of aircraft an absolute necessity. Luckily, today’s devices are designed to complete this quickly and efficiently to minimize downtime while protecting the safety of passengers. 

Common Aircraft Testing Techniques

The aerospace industry is a fast-moving one. That’s why it’s surprising that many popular nondestructive testing (NDT) methods for aircraft are so time-consuming. Two of the most popular include dye penetrative testing (PT) and magnetic particle testing (MT). 

	Magnetic Particle Testing	Dye Penetrant Testing
Process	Tiny particles are applied to the surface, and then an electromagnetic current magnetizes these particles and pulls them into incongruities. Typically, this is done in two separate magnetizations, as flaws in line with the current can be difficult to view.	This process leverages capillary actions, in that liquid will be drawn into flaws or incongruities when applied to the surface of a part. The dye used can be visible to the naked eye, or it can be UV based.
Pros	Useable on complex shapes Does not require the removal of most finishes and paint Allows the completion of a large number of parts Can detect subsurface incongruities  Cost-effective and easy to learn	Works on conductive and nonconductive materials Easy to process large numbers of parts Can provide visual evidence of cracks for estimatesLow cost and easy to learn
Cons	Orientation can cause missed discontinuities Requires lots of equipment and multiple steps Only works with ferromagnetic materials Some coatings will require removal before testing Cleanup is time-consuming Small subsurface imperfections can be difficult to detect Limited ability to record inspection and prove traceability.	Coatings and paints require removal from the surface before an inspection& Does not detect subsurface incongruities Does not work on porous materials Ineffective in extreme temperaturesRequires extensive cleanup Limited traceability and trackability of results Can be highly subjective based on the individual doing the test

While both PT and MT provide viable paths to airplane inspection, there are a few drawbacks. The biggest comes from the limitations concerning subsurface detection capabilities. Eddy current testing for aircraft provides a more in-depth understanding of those issues that aren’t immediately visible. 

Eddy Current Testing as an Alternative

Eddy current testing (ECT) leverages a wound coil—or one printed on a circuit board—to push an alternating current into conductive materials and create a field. That field will flow around flaws found within a material and make them visible to the operator. 

ECT offers a lot of benefits. Chief among them is that the surface of the material requires no preparation and that subsurface flaws are as easily detectable as surface ones. This process also provides a computerized record of the inspection, which is vital in the heavily regulated airline industry. Finally, eddy current devices can operate in a wide range of temperatures, making them suitable for field use. 

That’s not to say eddy current is perfect. The accuracy of results is highly dependent on user training and experience. If the coil is not correctly engaged, they may see false readings that lead to unnecessary repairs. Aside from that, when only one coil is in play, it can be extremely time-consuming because the user must scan small areas. 

This is where eddy current array (ECA) comes in. A grid of multiple coils can compare signals firing in axial or transverse orientations. The process expands the coverage to the point where the user gets a 3D model of the part. Also, coil redundancy ensures the results are repeatable for inspectors, making ECA a significant time-saver in the airline industry. 

The Process of Eddy Current Testing An Aircraft

The actual process of eddy current testing an aircraft may seem pretty simple to most non-specialists. However, it’s important to remember that it will vary significantly based on the type of instrument used. Despite these discrepancies, some similar threads will apply to most eddy current inspections. 

1. An experienced operator will be assigned to the task. This individual must meet standards for formal training, field experience, eye exams, and tests. Often, the NDT equipment operator will work specifically in the aerospace industry, though some more extensive operations will offer more than one specialty.
2. The inspector will provide an approved instrument that is in line with current government and aircraft manufacturer standards. As technology expands, these standards can grow, so it’s essential to invest in the right technology. 
3. The equipment will be calibrated before the inspection using a test plate or via a digital model. Here, the rejectable level of reading will be established, as well as the base standard. 
4. The probe will be applied to the part requiring inspection as the operator evaluates the reading for incongruities. The reading is compared to the acceptable standard. 
5. The aircraft receives clearance or is grounded based on the results. In either case, records of those results will be maintained in line with standards. 

While this seems like a straightforward process, it’s one that will need to occur repeatedly to various parts. As a result, most of those in the industry either keep a certified NDT inspector on staff or work with an experienced third party. 

Above all, eddy current testing of aircraft is not a layman’s job. It’s one that requires experience and understanding of the unique technology involved. Through this, those in the industry can make their technology safer while minimizing downtime. 

To learn more about eddy current testing of aircraft and how it can improve your inspections, check out our aircraft NDT inspection webinar.

Zetec provides the technology and training you need for highly effective eddy current testing of aircraft. Contact us to learn more or see our aerospace NDT industry page.