Nondestructive Testing: Keeping planes flying longer

Keeping planes flying longer By Tim Kunkel Three decades ago, manufacturers used expiration dates to determine when a part needed to be replaced. If a spar had more than 25,000 operating hours, it was probably discarded - even if there were...


Keeping planes flying longer

By Tim Kunkel

Three decades ago, manufacturers used expiration dates to determine when a part needed to be replaced. If a spar had more than 25,000 operating hours, it was probably discarded - even if there were no defects in the part. This had to be done to ensure safety. The growth of non-destructive testing (NDT) techniques, such as eddy current, liquid penetrant, and magnetic particle inspection, has given the industry another option - one that has saved aircraft owners considerable amounts of money, while maintaining the highest possible safety level.

In testing a part, an NDT inspector has many tools at his or her disposal. The most obvious is a simple visual inspection of a part. Obvious cracks, corrosion, and distortions don't require more advanced testing. Those parts are replaced immediately. Flaws deep in the metal, fatigue cracks, or very light surface corrosion, however, often require more advanced techniques to identify and measure. Today, there are five basic NDT methods used to identify these flaws - liquid penetrant, magnetic particle, eddy current, ultrasonic, and radiographic. Among these, the most commonly used in testing aviation parts are eddy current, liquid penetrant, and magnetic particle.

Eddy current
Eddy current can be the most effective at identifying and measuring the severity of flaws at or near the surface of a metal part. A small electromagnetic pulse (EMP) is injected into a part. Defects are measured when the path of the electricity meets a defect and causes a needle to jump or a graph to spike on a monitor.

However, to effectively test any component, the NDT inspector may use a variety of test methods - even if no defects are found using eddy current. NDT inspectors determine which tests to use based on a variety of factors, including the material being tested and the accessibility of the part.

Liquid penetrant testing
Liquid penetrant is an effective way to test ferrous and nonferrous materials for cracks, laps, cold sheets, porosity, corrosion, and other defects open to the surface. It is important that the part be thoroughly cleaned before the penetrant is applied. Cleaning methods include vapor degreasing, solvent wipe, detergent, or alkaline cleaners. Paints, varnishes, scale, carbon, and other contaminants not removed by these cleaning methods should be removed with chemical cleaners.

Tim Kunkel applies a magnetic particle bath to the wheel bolts of a Cessna Citation V (left) and inspects them under a blacklight for any defects (right). Tim Kunkel applies a magnetic particle bath to the wheel bolts of a Cessna Citation V and inspects them under a blacklight for any defects.

In conducting the test, the inspector applies a fluorescent liquid to a surface using a combination of spray and brush. The liquid is allowed to "dwell" for a set time. Depending on the substance used, this time can vary. Warm water is used to remove excess penetrant, and the part is dried with circulating warm air dryers. A developer is then added and allowed to dwell.

Once the part has been prepared using a liquid penetrant and the appropriate developer, it is examined in a darkened area using a ultraviolet (black) light capable of producing a minimum intensity of 1,200 microwatts per centimeter squared and a wave length between 3,200 and 3,800 angstrom units.

Under the ultraviolet light, surface defects are exaggerated and made visible. An experienced NDT inspector or technician can then determine whether any damage has occurred and compare it to the parameters set forth by the manufacturer to determine if a part needs to be discarded or replaced.

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