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Maintenance

A deeper look

Regular inspections of wheels are important, but the diversity of tyres is challenging for maintenance personnel
 
Aircraft wheels take a lot of punishment, from environmental factors like rain, snow and ice, temperature changes, and, of course, the high speed impact of a landing. The most critical area is the wheel bead seat, where the rubber edge of a tyre contacts the metal of the wheel, as this is susceptible to fatigue cracking and heat damage.
 
The preferred approach to test wheels made of aluminium or magnesium alloys for cracks or other damage is to utilise eddy current devices along with wheel-specific probes. Eddy current testing is a non-destructive test (NDT) method that uses electromagnetic induction or electricity and magnetism to create a magnetic field in the item inspected, and 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 probes that fit the precise size and measurements of the wheel bead seat. 
 
However, 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, almost no two aircraft wheels are alike, and a single large wheel manufacturer might have hundreds of different wheels varying in size, diameter and circumference. This presents a challenge for many MRO operations because a wide range of wheel-specific probes are required. On top of this, manufacturers are continuously updating or modifying aircraft. The Boeing 737 is up to revision eight, the 737 MAX, he notes, while long range versions and increased MTOW models may have modified landing gear and different wheels and/or tyres. Because of this, 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. 
 
In addition to having the appropriate probe on hand for the aircraft wheel, it is also essential, of course, to have a reliable, easy-to-use, eddy current device. As an example of this, the company’s eddy current device, the ED-1100, has been incorporated by UTC Aerospace Systems as an instrument to use in wheel maintenance inspection. The portable unit, with an LCD screen and 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 for eddy current testing that might be performed. 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.
 
Unlike traditional all-digital units, the instrument has an automatic balance/null feature that substantially reduces 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.
 
As an example of wheel testing, a UTAS NDT manual describes an eddy current test with a contour bead seat probe. The procedure inspects for cracks in aluminium or magnesium alloy wheel halves, wheel bases or side rims using 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 centre 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. Because of this, to improve accuracy and ease testing of the wheel bead seat, some companies like Centurion NDT also offer a motorised wheel turntable accessory. 
 
Instead of manually running the probe around the wheel, which can lead to less reliable test results, the motorised 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 are on the line during every take off and landing, says Strass, 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. 

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