Good Practices in Fluorescent Penetrant Inspection
By Fred Workley
When I started reading one of the FAA reports published in August 1999, I thought I already knew all about fluorescent penetrant inspection. However, there is new information that I would like to pass on to you.
The Federal Aviation Administration Office of Aviation Medicine issued a report prepared by Colin G. Drury under contract with the FAA titled, Human Factors: Good Practices in Fluorescent Penetrant Inspection. It reviews research that looks at accumulated knowledge of human factors engineering as applied to Nondestructive Inspection (NDI) of critical rotating engine components.
The report points out that good results from fluorescent penetrant inspection (FPI), using liquid penetrant, depends on Procedures, Materials, and Human Factors. Because of the visual nature of liquid penetrant, it is dependent on human performance which includes skill, cognitive ability, and attitude. The report offers 86 best practices that impact probability of detection, as well as reliability, technical, environmental, and organizational issues. Let's look at some of the report's conclusions.
FPI relies on good procedures. Your success depends on the penetrant getting into the crack, which is a capillary opening. It's a good idea to review the new Advisory Circular, AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, dated 9/8/98. Section 5, Penetrant Inspection, Table 5-4 shows a general process.
The first procedure is to clean the component thoroughly with materials that are compatible with the system you are using. After the component is dry, you apply the penetrant. There will be a recommended "dwell" time to permit the penetration into the capillary opening. Next, remove the surface penetrant fluid without removing fluid from the capillary. Then, apply a developer to draw the penetrant from the capillary to the surface. You are looking for the visual contrast to indicate a crack. Now comes the tough part.
Visual inspection will detect the presence of a crack, but you will now have to classify and interpret type, size, and suitability for service of the part. Ask yourself if airworthiness is affected. The magnitude of the crack may or may not be a cause for rejection under the suitable inspection criteria. In order to make these determinations, some procedures may require the use of 5X or 10X magnifying loupe that can be used hands-free. One solution is flip down magnifiers that can be attached to safety glasses. Follow published procedures carefully for best results.
Procedures generally follow these steps: Initiation, Access, Search, Decision, and Response.
Initiation consists of all processes, including cleaning, that lead up to the actual visual inspection. You need to get and understand the applicable work instructions. Check the part to determine that it matches the paperwork. Prepare the inspection tools and adjust the booth lighting so that your eyes adapt to your environment.
Access requires proper positioning of the component as needed throughout the inspection as specified by the inspection system.
The Search begins by visually scanning the component to confirm adequate cleaning. Scanning in sections or areas, back and forth, increases your chance of finding defects, thus increasing the probability of detection.
The Decision function is your chance to compare the crack with known standards. This is when re-testing may be necessary by re-bleeding. Confirmation may be required using a magnifying loupe or a white light.
Your Response may be the confirmation of the crack and marking for later disposition or repair. There are two possibilities: non-conforming action taken on a conforming article, or conforming action taking on a non-conforming article. The type of materials used, along with surface condition and cleaning, impact the results.
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