The preferred approach to test wheels made of aluminum or magnesium alloys for cracks or other damage is to utilize eddy current devices along with wheel-specific probes. The critical area is the wheel bead seat, which is the area most likely to fail because of fatigue or heat damage.
To prevent such failure, maintenance personnel must be able to perform eddy current testing with probes that fit the precise size and measurements of the wheel bead seat.
Eddy current testing – a nondestructive test (NDT) method that uses electromagnetic induction or electricity and magnetism to create a magnetic field in the item inspected – is very effective in detecting a wide range of flaws, such as small cracks, surface defects, and heat damage. These sophisticated NDT devices provide extremely accurate wheel inspection results when paired with the correct probe.
In the aviation industry, however, almost no two wheels are alike, with many varying in size, diameter, and circumference. A single large wheel manufacturer might have up to hundreds of different wheels. This presents a challenge for many MRO operations because when testing the contact of the probe with the surface of the wheel bead seat is critical, so often a wide range of wheel-specific probes are required.
“The circumference and radius of the wheel is usually different for every single make or model of airplane from the Airbus A-300 to the Westland EH-101,” says Ken Strass, owner of Centurion NDT, a manufacturer of portable eddy current and ultrasonic instrumentation that makes a wide variety of probes and coils, in Streamwood, Ill. “This is true of both main and nose gear wheels.”
Compounding the challenge, aircraft manufacturers are continuously updating or reformulating different aircraft.
“The 737 is up to revision eight, the 737 Max,” says Strass. “Aircraft manufacturers may also have long-range versions, and the wheel itself may be slightly different because the tires are different.”
As a result, suppliers such as Centurion NDT must be able to offer an extensive library of aviation wheel bead seat probes to accommodate the variety of wheels manufactured for the industry.
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Inspecting wheel halves and bases
The Goodrich Non-Destructive Testing Manual – from Goodrich Corp., a Fortune 500 company and global supplier of systems and services to the aerospace, defense, and homeland security markets (acquired by United Technologies Corp.) – describes an eddy current test with a contour bead seat probe. The procedure inspects for cracks in aluminum or magnesium alloy wheel halves and wheel bases.
To inspect wheel halves, wheel bases, or side rims with a contour, differential-coil, bead seat probe, the manual states to put the probe in the bead seat of the part and press the balance or null button on the instrument to center the indicator dot on the display.
The manual also states to put the wheel half, wheel base, or side rim on a turntable, hold the probe firmly on the bead seat, and slowly turn the part until the full bead seat is inspected. It instructs to turn the part (or move the probe) at the same speed that was found to give the best signal in the calibration procedure.
In such a procedure, moving the part or probe at a constant speed can be essential to obtaining a good eddy current reading. To improve accuracy and ease testing of the wheel bead seat, some companies like Centurion NDT also offer a motorized 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. This enables the inspector to simply hold the eddy current probe in place as the wheel spins.
The constant rotational speed gives accurate and repeatable results. With greater consistency, even the smallest cracks are more clearly visible on the screen, without the more erratic results common to manual methods.
“Passenger safety and airline reputation is on the line during every takeoff and landing,” Strass says. “So, aircraft repair stations and MROs need to ensure that they have the right equipment for safety inspection available, including eddy current devices, wheel probes, and accessories.”
Eddy current instrumentation
It is essential to have not only the appropriate probe on hand for the aircraft wheel, but also a reliable, easy-to-use, eddy current device. Centurion NDT’s ED-1100 eddy current device, for example, has been incorporated by Goodrich Corp. as an instrument to use in wheel maintenance inspection.
It is used for a variety of applications including detecting flaws in aircraft wheels, struts, propellers, hubs and engine components. It uses absolute, differential and reflection probes to detect flaws or determine properties such as conductivity, hardness, alloy type, and heat treat condition. The portable unit, 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.
Unlike traditional all-digital units, the instrument has an automatic balance/null feature that substantially 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.
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