Nondestructive Inspection Specialists Ensure Aircraft and Mission Integrity
Senior Airman Shelby Thurman
908th Airlift Wing Public Affairs
Inside the 908th Maintenance Squadron is a little shop that is full of big energy… electromagnetic energy, that is.
The fabrication career field in the Air Force is full of many different specialties, including nondestructive inspection. Other specialties included are machinist metals processing (welding), fabric airframe repair, and corrosion control. Before NDI was its own specialty, each of the aforementioned specialties performed NDI themselves. Once the Air Force realized that there was an increased need for special qualifications and training in NDI, the job became its own unique specialty.
Since some aircraft pieces have imperfections that are smaller than what the human eye can perceive, NDI technicians use five different techniques to inspect the components of the 908th Airlift Wing’s C-130H Hercules.
"We use X-ray, ultrasound, eddy current, magnetic particles, and fluorescent penetrants to inspect aircraft components to ensure component integrity," said Master Sgt. Seth Vandiver, an NDI technician with the 908th MXS.
The first technique the NDI technicians use is radiographic testing, also known as X-ray inspection. This technique is typically used to detect cracks, corrosion, check for internal damage to components, and to see if there are any foreign objects embedded in components. The X-ray tube converts electrical energy into electromagnetic radiation. The film underneath the component being tested is a sheet of clear cellulose or triacetate which has been treated on both sides with a blend of gelatin and silver halide compounds.
"When we use our control unit, it generates X-rays to a piece of film so we can get an image of any piece we put there," said Master Sgt. Jerome Williams, an NDI technician with the 908th MXS and the non-commissioned officer in charge. "The perk of the new system is that we can use the same film two times which saves costs without losing quality."
These film images are then scanned into a computer where technicians can zoom to inspect the piece’s integrity even closer.
Another technique that NDI technicians employ is ultrasonic testing. When most people think of ultrasound tests they probably think of hospitals or veterinary clinics. The principle is the same in that ultrasonic tests use high frequency sound waves to detect subsurface discontinuities. This technique is predominantly used to test the thickness of an aircraft’s skin and the airframe’s structural integrity. Cracks, laminations, shrinkage cavities, forging bursts, porosity, bonding faults and other abnormalities are the common items being searched for.
The third method NDI technicians use is magnetic particle testing. With this technique, the specialists apply a mixture of a base oil and fluorescent iron oxide particles onto the piece being inspected. They then magnetize the piece with low voltage high amperage equipment. Having the material being completely coated with this mixture makes it obvious for flaws to be detected when zapped with currents. This is because if there is a flaw, then a magnetic leakage field forms around the crack or imperfection. A black light is held over the area for technicians to easily spot the flaw. This type of testing is usually reserved for ferromagnetic materials like iron, nickel and cobalt alloys.
While fracture critical items such as crankshafts, connecting rods, engine mounts, and landing gears receive magnetic particle testing, other solid and nonporous materials receive fluorescent penetrant testing.
These fluorescent penetrants are low viscosity liquids that are specifically engineered to creep into the most minute flaws and hairline fractures. The fluid is composed of a fluorescent compound that shines brightly when it is exposed to ultraviolet light. This technique is seen as being the most cost-effective in terms of equipment, materials, energy exerted, and time spent overall.
Even though there are many positive points for using fluorescent penetrant tests, these penetrants can only see flaws on the surface level of materials.
For inspecting the structural integrity of an aircraft component instead of just its outer layer, NDI technicians jump to their go-to method: Eddy current. The machine they use sends electrical currents throughout the part they are inspecting. Whenever there is a disruption in the flow of energy the electrical current sends a reading to the monitor. These disruptions are proof of a crack in a component that might not have been visible to the human eye.
"We are always looking for the smallest things like cracks, sealants, or paints," said Senior Airman Chandra O’Bryant, an NDI technician with the 908th MXS.
O’Bryant laughed while adding that, "Sometimes we feel like crime scene investigators."
The technicians focus on any part of the aircraft ranging from engines to windows or landing gears. Some parts that need inspecting are either too large or too delicate for transporting to their shop, so this requires the technicians to take their gear out to the flight line. But for the most part, pieces of the aircraft come to them.
Working in NDI requires these Airmen to always pay special attention to detail. Having a sharp eye is what makes it possible for imperfections to be noticed and then repaired.
"It’s important that every process we do ensures the integrity of the aircraft parts and the aircraft themselves in order to keep our airplanes flying so that we can continue the mission," said Williams.
Photo By Senior Airman Shelby Thurman | Senior Airman Treashear Britt, a nondestructive inspection technician with the 908th Maintenance Squadron, plugs in a chord to the x-ray imaging system Jan. 8, 2022, at Maxwell Air Force Base, Alabama. The system converts electrical energy into electromagnetic radiation which is then sent towards the object being inspected. (U.S. Air Force photo by Senior Airman Shelby Thurman)