On April 28, 1988, the skin of an aging Aloha Airlines 737 peeled off at 24,000 feet. A crew member was lost in the resulting explosive decompression, and the passenger cabin was opened to the frigid, oxygen-poor sky. Only sheer luck, the intrinsic strength of the 737 airframe, and the skill of the pilots managed to get this aircraft back on the ground safely.
So what happened? According to the NTSB, “the probable cause of this accident was the failure of the Aloha Airlines maintenance program to detect the presence of significant disbonding and fatigue damage which ultimately led to failure of the lap joint … and the separation of the fuselage upper lobe.” These were problems that could have been detected using nondestructive eddy current testing. As a result of the Aloha Flight 243 near-disaster, detecting such faults in this manner has become standard aircraft maintenance procedure.
But what is eddy current testing, and how can this nondestructive testing (NDT) process find potentially disastrous flaws safely and reliably? That’s what this article is all about.
Eddy Current Testing 101
According to the Merriam-Webster dictionary, an ‘eddy’ is “current of water or air running contrary to the main current ... [especially] a circular current.” Eddy currents also occur in the realm of electricity, through the process known as electromagnetic induction. (History note: Electromagnetic induction was discovered by Michael Farraday in 1831. Now you know.)
When an alternating current is applied to a conductor such as copper wire, a magnetic field develops in and around the conductor. Because it flows like a eddy current in water or air, we call this induced magnetic field an ‘eddy current’.
The size of this eddy current expands as the AC rise to maximum, and recedes as the AC is reduced to zero. Put another conductor close to the first conductor, and new eddy currents form. The best part: All of these currents can be measured, quantified and used to establish standard readings for intact materials such as aluminum and steel.
This is where eddy current testing gets interesting. If a piece of steel has a crack in it, the eddy current’s magnetic reading changes; the larger the crack, the larger the detectable change. The same is true of surface blemishes, areas affected by corrosion (because the corrosion changes the elemental nature of the material being tested), and problems at meeting points between disparate metals. This is why eddy current testing could have detected the delamination, corrosion, and cracking that fatigued the airframe of Aloha’s troubled 737, before it failed in flight.
“Eddy current testing can not only detect cracks, but also indicate the relative size and depth of them,” says Dave Jankowski, Eddy Current Business Leader at GE Inspection Technologies. “With the right equipment, a skilled inspector can locate, size, and image cracks and corrosion in aerospace structures.” An advanced form of eddy current testing, known as Pulsed Eddy Current (PEC) testing, can even detect corrosion and cracks in various skins of a multi-layered aluminum fuselage, by sweeping through a range of frequencies.
Typically, eddy current testing employs a handheld detection probe — one that is actually an encased circular electromagnet with AC flowing through it — held close to the area of interest. Sometimes a second ‘search’ coil is used to detect fluctuations in the induced magnetic current. Other times, the changes in the primary probe’s current are measured.
In either instance, the changes are evaluated by an eddy current measurement/interpretative device. These devices are typically handheld units about the size of paperback novels, with keyboards that allow a number of tests to be selected. Computerized measurement devices are becoming common, with the recorded data being fed into analytical software programs.
“In the aircraft industry, there are two areas where eddy current testing is commonly used; the airframe and the engines,” says Dr. Norio Nakagawa, adjunct professor of aerospace engineering at Iowa State University and a guiding light for the NDT Resource Center educational website (www.ndt-ed.org). “The testing works best in near-surface applications. It is simple and easy to use, which makes eddy current testing a valuable tool for aircraft maintenance technicians.”
Eddy current testing has many uses in the maintenance hangar. On airframes, for instance, “it is primarily deployed for detecting cracks and faults in riveted areas,” says Dr. Nakagawa. “Since these rivets literally hold the airframe together, the integrity of these joints and their holes is paramount to the aircraft’s overall integrity.”
Areas that take a lot of stress, such as undercarriages and engine casings, are also prime targets for eddy current testing. “The amount of stress undercarriages experience during repeated takeoffs and landings makes them vulnerable to cracks and other forms of fatigue,” says Jankowski. “Meanwhile, if the engines are kept running too long after landing, materials in those areas can be inadvertently backtempered; making them brittle and more prone to failure. The symptoms of these problems can all be detected using eddy current testing.”
Because it is built upon electromagnetic induction, eddy current testing is only useful for detecting flaws on or near the surface of the conductive material being checked. The PEC technique can achieve deeper penetrations, but will eventually encounter a limit in the penetration depth it can measure.
As well, only a skilled technician can interpret the wealth of information offered by an eddy current test. A novice can see that there’s a problem being detected, but not much else. This is why effective eddy current testing requires the tester to be properly trained. The NDT Resource Center web site offers a number of training recommendations at www.ndt-ed.org. The site also has a detailed rundown on the science of eddy current testing for interested readers.
One last hint: Eddy current testing equipment needs to be well-built and properly calibrated to ensure accurate, meaningful readings. Careful and consistent probe positioning during testing is also a must.
The bottom line
Eddy current testing is a profoundly powerful, accurate, yet easy-to-use NDT approach to finding cracks, flaws, and other forms of surface and near-surface damage in airframes and engines. It is an essential tool for any aircraft technician, whether working on Cessnas or 777s.