on the backside of a skin, a bolt hole inspection, or a thickness measurement of metal cladding. The knowledge and skills required to perform these tests are perishable. The purpose of training and qualification goes beyond the
ability to turn on a machine and follow a detailed procedure. To be effective the technician needs to be able to determine when something is not working properly (not always obvious), determine what to do about it and properly evaluate the resulting indications. It is therefore very important that technicians receive refresher training in those aspects of the method that they may not be using on a regular basis but might be required to use on short notice with no preparation. In addition, complacency or over- confidence tends to increase with time when no defects are found. Alternatively a technician may become less confident and over- sensitive in his inspections resulting in over rejections. The training, qualification and proficiency of NDT personnel in
the aviation industry are critical. The accept/reject decisions are often not simply black and white. Some knowledge of aircraft design; metal and composite materials; manufacturing, repair, and their associated flaws; in-service issues such as corrosion, fatigue, and various types of structural damage; and hands-on practical experience on actual retired aircraft are essential. For example, a well-known service company that specializes in
pipeline and power generation, was asked to X-ray some aircraft parts. They did a fine job of producing the X-rays and made excellent images, but admitted that they could not or would not evaluate them because they did not understand the parts, or the nature of the aircraft corrosion that they were looking for. The types of flaws that in-service technicians are looking for vary widely. Some of the unique aviation circumstances that are likely to be encountered are shown in Figure 1: compressor stalls that put pressure on many components, extreme crosswind landings that could cause wingtip or fuselage damage, bird or lightning strikes that can cause significant damage, landing with the nose gear turned 90 degrees, engines inhaling ground equipment, runway excursions and more. All of these situations require the technician to have an idea of the construction and the stresses encountered in the incident in order to assess the likely location of invisible damage.
Figure 1. Wide varieties of structural damage. Aviation Maintenance |
avmain-mag.com | October / November 2011 31
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