The Non-Destructive Inspection (NDI) technician is the personal physician for the C-17 Globemaster III. Much like a regular doctor, the 437th Maintenance Squadron’s NDI shop at Joint Base Charleston, South Carolina, looks over the C-17 to catch problems before the entire aircraft, as well as its crew, can be put into jeopardy. Instead of tumors and disease, NDI technicians look for cracks and other defects in each part of the airplane they inspect. JB Charleston celebrated 3 million flying hours for the C-17 fleet earlier this year. The milestone is the equivalent of one aircraft being airborne for 342 years. So the mileage on the C-17 makes aircraft part inspection especially crucial. “I am most proud of how many lives we have saved and mishaps we have been able to prevent by finding cracked, damaged and faulty parts,” said Staff Sgt. John M. Price, the NDI assistant NCO in charge. “Removing them from service eliminated any potential risks they could have caused if they continued to be used in flying operations.”
Many bases provide NDI support to the C-17, but JB Charleston is the only one used for testing. The shop’s five active-duty Airmen, two Air Force Reserve technicians and two civilian technicians inspect metal objects ranging from nuts and bolts to large hooks and sheets of metal. Parts vary from those weighing hundreds of pounds and designed to hold the C-17’s main landing gear to a 2-inch bolt that weighs only ounces. JB Charleston’s NDI was named the best laboratory in the Air Force in 2006 against almost 200 other labs. Its Airmen have also received numerous individual awards. Although NDI technicians feel a sense of accomplishment when they find a crack in an airplane part, the key is the inspection process, said Tech. Sgt. Ryan Michalec, the shop’s NCO in charge. “Doing the job is actually fairly simple. It’s knowing the science behind it,” he said. “It’s understanding how to interpret what you’re looking at, not only physically, with the parts, penetrant and magnetic particle testing, but also with the machines that are telling you what’s going on.” Once the NDI technicians find a crack, they refer the part to aircraft structural maintenance for repair. The part will then return to NDI for a post inspection after it’s repaired. While inspecting JB Charleston’s 50 C-17s, the shop uses six inspection methods: fluorescent penetrant, eddy current, magnetic particle, bond testing, ultrasonic and X-ray testing methods. Liquid penetrant testing is one of the oldest and simplest NDI methods, dating back to the 19th century, where it was used on railroad parts with kerosene and an oil mixture, Michalec said. Today, a florescent penetrant is used to detect any surface-connected discontinuities, such as cracks from fatigue, quenching and grinding, as well as fractures, porosity, and flaws in joints. A penetrant is applied to the test part, rinsed with water, submerged in an emulsifying liquid, rinsed with water again, then followed by a developer. The developer helps to draw the penetrant out of the flaw where an invisible indication becomes visible. Inspection is performed under ultraviolet light.
“The reason we use fluorescent light is because the visual acuity of someone with 20/20 vision is about one-3000th of an inch, meaning that is what you can actually see,” Michalec said. “But we’re looking for stuff you can’t see. We’re looking for stuff that, if you’re looking at a part, you have no idea there’s a crack on there unless you specifically knew where and what to look for. “The good thing about this method is it’s sensitive, and we can find very tiny cracks with it, which is great because if you’re in the aircraft world, you know that little, tiny cracks can turn into really big cracks with the right amount of stress on that particular part.” Eddy currents are circular electrical currents induced in a conductive metal by reacting to an alternating magnetic field. The testing uses electromagnetic induction to detect flaws in conductive materials. It can detect particularly small cracks on or near the surface. “The technology on that particular inspection has increased a lot over the last 30 years,” Michalec said. “The capability has gone from basically a needle on a meter to a machine that actually reads conductivity like a tiny metal detector. “You’re going to get these little circular electrical currents flowing inside of that metal called eddy currents. Those circular electrical currents are going to create another magnetic field that’s pushing against the magnetic field coming out of the probe. Electricity and magnetism don’t like to go through air. When that secondary magnetic field hits that crack, it’s going to disappear, and the machine is made to read what extent the eddy currents dissipate, so we can estimate what kind of depth we have in the crack.”
Because the C-17 is made mostly out of bonded structures, NDI uses ultrasonic testing quite often. It’s the same technology used to detect a fetus in the womb, but instead of a baby, the NDI technician is looking for a crack. “It’s a different technique, but the same theory applies,” Michalec said. “We’re placing sound into a material, looking for a reflection from the back side of it, and if we see anything in between there, we can tell you there’s a crack and the depth of the crack.” The same theory also applies to bond testing, Michalec said. The idea is to put sound into the part being inspected and look for changes in the sound waves on the equipment’s screen. One common method of bond testing is the tap test. The technician taps the surface of the test structure and evaluates the sound the knock produced. The tap test method has demonstrated the ability to detect cracks, corrosion, impact damage and disbanding of bonded structures. “It’s like finding a stud in the wall,” Michalec said.
Radiography involves the photographic record produced by the passage of radiation, such as X-rays or gamma rays through an object onto film. When X-ray film is exposed to radiation, an invisible change called a latent image is produced on the film itself, once the film is developed. Along with the variety of testing methods, one of the advantages of the NDI career field is its versatility, Michalec said. He’s done inspections on virtually every Air Force airplane during his career, in addition to Marine Corps helicopters, the Navy’s EA-6B Prowler and oil analysis testing on the Army’s special operations helicopters while he was deployed to Afghanistan and Iraq. “I don’t think there’s but one or two military aircraft that I haven’t had my hands on in my 13-year career,” Michalec said. “I enjoy it, because this is one career field where we can go and work on anything. If it rolls, rotates or flies, we can work on it. That makes it a unique kind of career field because if you are a C-17 crew chief, you’re a C-17 crew chief. If you’re an F-16 (Fighting Falcon) crew chief, you’re an F-16 crew chief. You are going to stay with that platform. “But for us, we can go to different bases and work on different platforms. Whether they are helicopters, fighters, cargo, bombers or attack aircraft, NDI can do a job on them, and the equipment would also be the same. We use the same equipment across the Air Force, with the same standards.” Another aspect of the job that remains the same, from tech school through a decade or so in the career field, is the satisfaction that comes from finding a crack. When Michalec was a tech school instructor, he encouraged his students by telling them, “Just wait until you find your first crack.” They don’t always find defects in the C-17 parts they inspect, which is obviously a good thing for the Air Force and the Airmen who fly the aircraft. But there is an adrenaline rush when they do.