There are several concerns about visual inspection. The person performing the visual inspection needs the appropriate training so that they have the knowledge and experience to detect the defect. To be good at visual inspection, a person must have a working knowledge of the aircraft including systems, structure, and components.
Visual inspection requires defects to be located, identified, and evaluated. These defects may have had their origin in the manufacturing process, but the important thing is to find them as they developed while in service.
Any situation that impedes free access to the inspection area could impact reliable results. The person performing the inspection has to see the inspection area. Unusual body positions may be difficult and tiring. Mirrors with attached lights sometimes make difficult access easier. Keep in mind that flashlights may need to be approved by Underwriters Laboratory (UL) as suitable for use in a hazardous atmosphere. MIL-F-3747E, flashlights with standard incandescent lamps are good explosion-proof inspection lights. Ladders and platforms often permit closer access but at times create their own limitations when trying to get to hard-to-access area with equipment in hand.
Let there be light
Lighting is of particular importance to get good results for visual inspection. The light is needed on the inspection area. The light needs to be uniform so that there are no light and dark areas. Shadows and reflective glare need to be minimized. This sometimes is a challenge in areas where there is limited access. Remember that if we can't see the defect for lack of adequate light, we can't detect it. With good lighting, we can determine if preparation of the area was adequate. Further cleaning may be required to remove dirt. Cleaning may remove evidence of the crack, but it can't remove the crack. Always observe safety precautions when inspection areas have been recently cleaned in case there is any residual cleaner left in the area. This is particularly important in confined, poorly ventilated areas. Refer to the cleaner's Material Safety Data Sheet (MSDS) for more information.
The trained visual inspector needs an inspection procedure that identifies specific locations, details of the inspection, and if possible, examples of known defects for comparisons. If applicable, visual inspections have to have procedures and techniques that are reliably repeatable with the same results from different inspectors. Visual inspection aids may be configured or modified for different applications whether airframe, engines, systems, accessories, or components. Specific applications may require the visual results be compared with pictures of known sample defects or guidelines clearly showing techniques to determine the extent of damage. Reference to acceptable limits needs to be readily available to the inspector. Sometimes inspection procedures must have detailed guidelines of how to measure what is seen with inspection aids like borescopes.
Seeing is believing
Visual inspection not only requires "looking" but also "seeing." Better seeing is based on knowledge about the item being inspected. Consistent results are the product of constant inspection techniques. Rather than scan a wide area, an effective technique is to look at specific targets. These targets focus the eyes and attention to possible defects. This is called step scanning. The idea is to move the point of observation in small increments. Sometimes, looking off to the side of the specific area will permit the inspector to identify the defect. Look from left to right or top to bottom and then reverse the scanning pattern. The goal here is to develop a pattern that time after time will find the defect.
The standard tools of inspection, for finding cracks, are the mirror, magnifying device, flashlight and note pad. Cracks are evidence of structural failure and fatigue. If cracks are inaccessible, then sometimes, the only way to see them is by a long handled mirror with a flashlight attachment. Once the crack is identified, it will be necessary to determine the extent of the crack. Sometimes surface cracks lead to the discovery of more extensive cracks in supporting structure for riveted assemblies. Other NDI techniques may verify the extent of a crack. In these cases, dye penetrant, eddy current and ultrasonic aids will confirm the visual "find." Magnetic particle inspection techniques will verify the extent of cracks on ferrous components.
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...