“When it comes to metal fatigue testing, there are a number of different ways to test the materials,” says Beau Klingbeil, general manager of Eclipse Aerospace Inc.’s Chicago service center, which provides complete servicing of the manufacturer’s twin-engine, single pilot jet line of aircraft. “Eddy current is the least expensive, least invasive way of doing those tests, so we typically end up using it on a weekly basis.”
While meter-type instruments are traditionally limited to aluminum airframe inspection with a set frequency and probe type, more sophisticated impedance plane units that can be set to a variety of frequencies allow for testing in a much wider range of applications. However, these versatile tools can be quite complicated to use – almost too complicated – even for those with some training.
Fortunately, for busy A&P mechanics – aircraft maintenance technicians who hold an airframe and/or powerplant (A&P) certificate issued by the Federal Aviation Administration (FAA) – that do not specialize in NDT testing, yet are responsible for a breadth of repairs, streamlined testing using sophisticated impedance plane devices is now possible. These devices can provide very accurate test results and dramatically simplify operation while utilizing a wide range of probe types.
Simplifying lift-off and operation
All-digital eddy current instruments, utilizing incremental push-button adjustment instead of rotational knobs, are common in aviation repair. However, operating the devices can be complex to the point that mechanics unfamiliar with their peculiarities are often forced to pull out the manual to set up and use them.
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 accuracy of the measured material. During lift-off with such instruments, mechanics typically end up pressing buttons and waiting for the computer processor to rotate through the set-up process, which can be agonizingly slow.
“With most digital eddy current units, you push buttons to incrementally set a variety of different parameters,” says Klingbeil, an A&P mechanic who is Level 2 certified in eddy current testing. “You have to constantly go back and forth to adjust things like the single amplitude, phase angle, and vertical/horizontal position.”
Klingbeil turned to an impedance plane eddy current instrument by Centurion NDT, a Streamwood, Ill.-based manufacturer of portable eddy current and ultrasonic equipment. “I can step up for an eddy current test in minutes,” Klingbeil says of the NDT-1100, which is portable, easy to use, and performs a variety of tests.
The instrument, which is about the size of an Apple iPad with an LCD screen, will locate surface and near-surface defects and conductivity changes in magnetic and non-magnetic materials, and can cover about 85 percent of the applications for eddy current testing that might be performed. This includes crack detection; sorting classes of materials according to hardness alloy, carbon content, tensile strength, and grain structure; and measuring coating thickness, sheet thickness, and the relative conductivity of critical materials.
The instrument’s automatic balance/null feature is designed to reduce set-up 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, company officials say.
The unit is very easy to use at lift-off and during operation, which tends to eliminate common errors of adjustment that can otherwise occur, Klingbeil explains. “You just enter the frequency with a push button, and then turn knobs to fine-tune everything else. Because of how easy it is to use, I don’t deal with a lot of false indications. It eliminates most of those.
“When you’re adjusting things like the single amplitude, the phase angle and even the vertical and horizontal position, you don’t use a push button that functions very incrementally,” Klingbeil adds. “Instead, you can fine-tune it exceptionally well and accurately with knobs.”
Mechanics often must deal with a full range of testing from the airframe to the wheels, so it is important to select an eddy current instrument that accommodates a large array of probe and coil types.
In the case of Eclipse Aerospace’s Chicago service center, Klingbeil stocks about a half dozen probes from Centurion NDT, which makes a wide variety of probes and coils. Digital frequency selection, probe drive, and impedance adjustments enable the operator to optimize system performance for the probes or coils selected.
The 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 types of wheels. “The pencil probe testers have different angles because some areas we have to NDT are hard to reach,” says Klingbeil. “One pencil probe tester is straight, one is angled 90 degrees to improve reach.”
Among the most specialized probes are those used to detect heat damage along the wheel bead seat, the critical area where the rubber edge of a tire contacts the wheel. In this regard, Centurion NDT offers a library of wheel bead seat probes, which accommodate the variety of wheels manufactured for the industry.
To improve accuracy and ease eddy current testing of the wheel bead seat, the company 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 diameter size of the wheel half. With greater consistency, even the smallest cracks are more clearly visible on the screen, without the more erratic results common to manual methods.
Overall, Klingbeil recommends such simplified, versatile eddy current test units to other A&P mechanics at fixed base operators and major haulers alike.
“I’m an airframe and power plant mechanic, so NDT is not my primary job; it’s something I’ve certified along the way,” says Klingbeil. “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.”
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