Look for the signs
Sometimes there are signs that lead to the discovery of defects. An example would be loose bolts, structural hardware, and rivets. Loose, dished, missing, and working rivets sometimes have a dark deposit around them — often referred to as "smoking rivets." This discoloration or streaking might be the kind of clue that a trained and experienced person would use to point to larger structural defects and other problems. Discolorations of coatings or nicotine stains are often associated with skin cracks. Contaminants or residues from lavatory water, hydraulic fluid or poor drainage can be signs of problems.
Visual inspection is the way that corrosion is often detected. Training on the causes and types of corrosion must include what the different types of corrosion look like. Sometimes using a punch or a scribe in the suspect area will lead to further investigation with removal of protective coatings. Everytime I have removed a chunk of the structure with a scribe, thus indicating intergranular or exfoliation corrosion, we did a lot of grinding and rework. Furthermore, the condition of surface coatings and cladding will affect future corrosion protection. Corrosion is found on aircraft structure, engines and systems. Likewise, mold and algae, and other biological contamination should be investigated because the resulting acidity can cause corrosion. Any debris should be examined for powdery aluminum oxide or iron oxide.
Let's expand our earlier reference to visual inspection procedures. These procedures are often for a specified aircraft or component. The source information may be from Federal Aviation Administration (FAA) approved documents like Airworthiness Directives, accepted documents like Advisory Circulars, manufacturer's maintenance manuals and service bulletins, structural inspection programs like SSID's, and general NDT manuals. Whatever the technique, the inspection procedure needs to contain certain logical steps.
A standard format might have these four steps:
1. The basis for the inspection including any relevant background
2. Preparation, calibration and cleaning requirements
3. Implementation and performing the inspection including description or illustrations of possible defects
4. Evaluation of observations with reject/accept criteria and reliability through simple, reporting documentation. Identifying the procedure with the level of inspection is very helpful: walk around, general visual, detailed visual or special detailed visual that is supplemented by NDI equipment. Disposition of the findings may also be impacted by whether they are flight or safety critical.
Reporting documentation supports the requirements for record keeping in FAR Part 43. Defects documented by written reports, squawk sheets, photographs or videos may become part of the Maintenance Record entry. The disposition of the findings or defects may have the final corrective action, but often there will be follow up evaluation. A new inspection requirement, which must be tracked, may now exist. Records need to show the location of the finding as well as its size, shape, and disposition such as rework. Sketches or engineering drawings of repair schemes may assist in future evaluations. Photographs, either stills or digital format, need to be clearly identified and dated. They also need show the location on the aircraft according to the manufacturer's standard system.
There is a lot to consider when thinking about visual inspections. In some cases, we noted that the objective of the visual inspection is aided by the use of NDI techniques. In all cases, you and your eyes make the difference as to whether the defect is found. By finding the defects before they become safety or flight critical, we support the continued airworthiness of every aircraft in the fleet. Keep 'em Flying.
AMTEC USA, 4774 Murrieta St., Bldg. 1, Chino, CA 91710, (909) 464-0605; Borescope sales and repairs for all makes and models.
Centurion NDT, 707 Remington Rd., Schaumburg, IL 60173, (847) 884-4949; Offers the Centurion NDT Model ED-1100 Impedance Plane Eddy Current instrument that is economical and easy to operate. Unit features a large, 6-in. LCD display and proven circuitry. Weighing 7 lbs., the instrument comes encased in a rugged, aluminum housing. Visit our web site at www.centurionndt.com.
Cooperheat-MQS Inc., 5858 Westheimer, #625, Houston, TX 77057, (713) 735-6900; Heat treating and non-destructive testing for aerospace industries. AS9000 certified. Over 900 certified technicians throughout the U.S. and Canada. Web site is www.cooperheat-mqs.com.
Machida offers flexible borescopes, video systems, light sources, and accessories. For technical support contact
"The Big Five" By Joseph Stump Nondestructive inspection (NDI) in the field of aviation came into its own in the 1940s. While still in its infancy, it represented a major innovation in...