Damage to an aircraft is not always obvious to the naked eye: Miniscule cracks, warping, and separation within laminates and parts are all potential safety hazards.
To hunt down unseen damage and prevent part failure, artisans of the non-destructive inspection (NDI) program aboard Fleet Readiness Center Southwest (FRCSW) employ an eclectic array of tooling and technology designed to pinpoint defects without harming the aircraft or its parts.
The NDI program operates in facilities throughout the command and is used to investigate the cause of defects found during routine maintenance or visual inspections, or on components that have failed during use or because of age.
NDI inspections are applicable to all airframes serviced by FRCSW. In the 5,000 square-foot NDI shop in Building 472, about 10 NDI technicians support the E2/C2, V-22 Osprey and F/A-18 airframes, and the LM2500 engine program.
"Other than the V-22, we're very minimal on the vertical lift because they have their own NDI technicians there," noted NDI supervisor Ray Arellano.
Inspection methods and repairs are determined by the composition of the material being examined.
NDI technicians in Building 472 use five primary methods to evaluate aircraft components: magnetic particle inspections, fluorescent dye penetrant, eddy current, ultrasonic testing, and temper edge/metal testing.
Magnetic particle inspections are used to detect damage near the surface or subsurface of components made of ferromagnetic material, like iron, and non-ferromagnetic material, like aluminum.
Ferromagnetic parts are magnetized and bathed with small ferromagnetic particles coated in fluorescent dye. When exposed to a black light, any damage appears dark green.
Non-ferromagnetic parts are coated in dyed florescent oil. When the oil is removed and a blotting agent applied, damages appear when exposed under a black light.
Applicable to both ferro and non-ferromagnetic materials is the eddy current inspection. The procedure uses a probe that induces electrical eddy (swirling) currents into the part. A magnetic field is produced by a coil in the probe. When the probe is connected to a conductor of electricity, like aluminum, the magnetic field creates the eddy currents that decrease in intensity as depth in the material increases. Any distorting to the currents indicates a defect and is displayed on a monitor.
Eddy current inspections are used to look for cracks on surfaces and other areas where the probe can make contact. It is also typically used in drilling areas, like the inside and circumference of rivet and fastener holes.
To detect internal damage to larger areas of aircraft like wings, doors and helicopter blades, ultrasonic and x-ray inspections are used. Ultrasonic inspections are used on components where access is limited to one side and the suspect area is inaccessible. Sound is transmitted into the component at varying degrees.
"The temper edge testing is used on varying degrees of tempered steel," he said. "It's not really an NDI process, we're looking for burns. Temper edge etches the part by removing a minimal portion of the part's surface. Whenever a machine grinds on steel you want to look for burns because now it's been hardened in that area. Depending on how bad it is, that material can be removed."
Temper metals testing is done by dipping the part in hot water, causing the metal to expand, then dipping in acid and a neutralizer. The process detects any burns that may have been caused by grinding or machining.
After defects are identified and repairs made, parts are often returned to the NDI shop for verification testing.
"It depends on the part, but typically they come back to be re-evaluated once the repair is done. If we find corrosion, for example, it is addressed and repaired and we want to make sure that all of the corrosion is gone, so it will come back to us for verification," Arellano said.
Components that have cleared final NDI testing are either placed in a ready-for issue (RFI) status or returned to the original source.
The Building 472 NDI shop inspects approximately 200 components weekly.