In aircraft maintenance and repair, eddy current devices have long been used to non-destructively test wheels, struts, propellers, airframes, hubs, engine components, and other parts for flaws such as cracks, fatigue, or corrosion in aluminum and steel alloys.
Traditionally, manufacturers used multiple eddy current test instruments to address the full scope of work – inspecting surfaces, welds, tubing, and bolt holes.
“When it comes to metal fatigue testing, there are several different ways to test the materials,” says Beau Klingbeil, general manager of Eclipse Aerospace Inc.’s Chicago service center, which services the manufacturer’s twin-engine, single-pilot jet line. “Eddy current is the least expensive, least invasive way of doing those tests, so we typically end up using it on a weekly basis.”
Meter-type instruments are often limited to aluminum airframe inspection with a set frequency and probe type, but more sophisticated impedance-plane units can be set to various frequencies for more applications. However, these tools can be complicated to use, even for those with some training.
Fortunately, for busy airframe and powerplant (A&P) mechanics who do not specialize in non-destructive testing (NDT) yet are responsible for broad repairs, streamlined testing using sophisticated impedance-plane devices is possible. These devices provide accurate test results and dramatically simplify operation while using various probe types.
Simplifying lift-off, operation
All-digital eddy current instruments that use incremental push-button adjustment, instead of rotating knobs, are common in aviation repair. However, operating the devices can be so complex that mechanics unfamiliar with their peculiarities must refer to the manual for setup and use.
This is particularly true when it comes to lift-off, which is required at the start of eddy current testing to offset any surface abnormalities or hand movement that could distort measurement accuracy. During lift-off with such instruments, mechanics typically end up pressing buttons and waiting for the computer processor to rotate through the slow setup process.
“With most digital eddy current units, you push buttons to incrementally set a variety of different parameters,” says Klingbeil, an A&P mechanic and Level 2-certified in eddy current testing. “You have to constantly go back and forth to adjust single amplitude, phase angle, and vertical/horizontal position.”
Klingbeil found a better alternative by turning to an impedance-plane eddy current instrument by Centurion NDT, a Streamwood, Illinois-based manufacturer of portable eddy current and ultrasonic equipment.
“I can step up for an eddy current test in minutes and be ready to go with the NDT-1100,” Klingbeil says. “The portable unit is very easy to use and performs a variety of tests.”
The instrument, about the size of an iPad, will locate surface and near-surface defects and conductivity changes in magnetic and non-magnetic materials, and can cover about 85% of the applications that eddy current testing might perform – crack detection; sorting materials according to hardness alloy, carbon content, tensile strength, and grain structure; and measuring coating and sheet thickness, and the relative conductivity of critical materials.
Unlike traditional all-digital units, the instrument has an automatic balance/null feature that reduces setup time for manual operation. By moving the probe and turning a phase control knob on the front panel, lift-off can be completed in seconds.
According to Klingbeil, the unit eliminates common adjustment errors that otherwise occur at lift-off and during operation.
“Unlike traditional all-digital units, the instrument has an automatic balance/null feature that reduces setup time for manual operation.”
“You just enter the frequency with a push button, and then turn knobs to fine tune everything else,” he says. “I don’t deal with a lot of false indications. It eliminates most of those.”
Regarding ease of use, he adds, “When you’re adjusting things such as the single amplitude, the phase angle, and the vertical and horizontal position, you don’t use a push button that functions incrementally. Instead, you can fine tune it exceptionally well and accurately with knobs.”
Mechanics must test everything from the airframe to the wheels, so it is important to select an eddy current instrument that accommodates an array of probe and coil types.
Klingbeil stocks six probes from Centurion NDT. Digital frequency selection, probe drive, and impedance adjustments allow the operator to optimize system performance for whichever probes or coils are selected. Eclipse’s Chicago service center uses two pencil-probe testers with different frequencies, two probes for wheel-specific testing, and two bolt-hole probes to test different wheel types.
“The pencil probe testers have different angles because some areas we have to NDT are hard to reach,” Klingbeil says. “One pencil probe tester is straight, one is angled 90° to improve reach.”
Among the most specialized probes are those that detect heat damage along the wheel bead seat, the critical area where the rubber edge of a tire contacts the wheel. To accommodate various wheels, Centurion NDT offers several wheel bead-seat probes.
To improve accuracy and ease eddy current testing of the wheel bead seat, the company also offers a wheel turntable accessory. Instead of manually running the probe around the wheel, which can lead to less reliable test results, the motorized turntable provides a constant rotational speed, specific to the wheel diameter. Greater consistency than can be achieved manually makes even the smallest cracks more visible on the screen.
“I’m an airframe and power plant mechanic, so NDT is not my primary job; it’s something I’ve certified along the way,” Klingbeil says. “If you’re a novice learning how to do NDT, it’s a good unit because of the simplicity and accuracy. You could use it at a service center platform or in the field for mobile response. This will do everything that other units will but is a lot faster and more portable.”