Another Look at Visual Inspection
By Fred Workley
November 1999
Fred Workley is the president of Workley Aircraft and Maintenance Inc. in Manassas, VA. He is on the technical committees of PAMA and NATA and participates in several Aviation Rulemaking Advisory Committees. He frequently speaks to groups on issues of current interest to the aviation community. He holds an A&P certificate with an inspection authorization, general radio telephone license, a technician plus license, ATP, FE, CFI-I, and advance and instrument ground instructor licenses.
Visual inspection continues to be an important element of maintenance programs. Technology has made great strides in giving maintenance personnel a wide variety of Nondestructive Inspection (NDI) options in dye penetrant, eddy current and ultrasonic. But no matter how advanced and expensive the test equipment, the decision regarding serviceability is often made after you perform some visual task. This is done by the process of observing an object with your eyes that are either unaided or aided. Then you must make an assessment of the aircraft's condition based on all the information that is available to you.
Elements of inspection
There are some basic inspection principles and applications that are used with in-service airplane inspections. We will review these in this article and note how these basic visual inspection practices also support Nondestructive Inspection. Also, visual inspections become elements in inspection programs that are mandated by regulatory requirements, manufacturer's instructions for continued airworthiness and operators' FAA approved maintenance programs. Often maintenance personnel will not have available either FAA or manufacturer's approved processes and procedures for a particular application. In these cases, the person performing the visual inspection has to evaluate each individual inspection outcome and make determinations of airworthiness.
On general aviation aircraft, it is often true that almost all of the inspection is visual. Even transport aircraft requires maybe only 15 to 20 percent to be aided by technology. Visual inspection can often serve as the basis of documentation to adequately support the continued airworthiness of an aircraft. Often the first evidence of most defects are found in visual inspections. Thus, these visual inspections require a level of proficiency by the person performing them. To have anything less would be a concern for the safe operation of the aircraft.
The "eyes" have it
Let's look more closely at visual inspection and then how it supports NDI. Visual inspection is using the eyes either alone or with aids,
such as sensing mechanisms, to determine the condition of the unit so that judgements can be made. Some examples of imaging devices are optical comparators, eddy current imaging, ultrasonic C-scans, and real-time X-rays. There are many advanced applications, which are also used: computed tomography in dye penetrant, automated eddy current scanning, computerized ultrasonic testing, acoustic emission, diagnostic X-ray, magneto-optic eddy current, coherent optics and advanced video imaging.* For our review, we will not consider the analysis of these displays as visual inspection. The visual inspection component of NDI is to validate the procedures and assure the calibration of appropriate standards. Visual inspection ensures that NDI equipment is maintained so that it functions well in the maintenance environment. The bottom line is that visual aids increase the probability of finding a defect or failure so that timely, corrective action can be initiated.
The power of observation
Visual inspection must provide an assessment of the overall condition of a component, system or structure. Defects must be identified early enough so that corrective action is taken before any progression causes a critical condition. Another use of visual inspection is to discover manufacturing or assembly errors. Of importance is to observe any condition of a component or structure that has a defect or malfunction that if not corrected, might lead to failure in flight. In all these situations, visual inspections will initiate the gathering of information upon which judgements are made. Airframe defects are generally of three types: cracks, disbonding and corrosion. On the other hand, visual inspection of aircraft components and systems finds wear, fatigue, interference, corrosion and accidental damage. Visual inspection of engines using borescopes finds nicks, pitting, spalling, burning, erosion, scoring, galling and cracks.
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.
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.
Related Links
November 1999
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.
Everest VIT Inc., 199 Hwy 206, Flanders, NJ 07836, (973) 448-0077 ext. 822; Provides a full line of remote imaging products and services. Our products are designed to help you take a look in areas that are inaccessible, dangerous or too expensive to look at by other means. Visit our web site at www.everestvit.com.
Foerster Instruments Inc., 140 Industry Dr., RIDC Park West, Pittsburgh, PA 15275, (412) 788-8976; Utilizing technology relied upon by the Non-destructive Testing (NDT) industry throughout the world, we employ a variety of electromagnetic test methods: eddy-current; magnetic induction; flux leakage; and magnetic field measurement. Foerster offers comprehensive, customized training programs and customer support that continues long past installation. Our web site is www.foerstergroup.com.
Hellier, 2051 E. Cerritos Ave., Anaheim, CA 92806 (714) 956-2274; Offers non-destructive testing training, qualification examinations and consultation, and is an ASNT and AWS examination center. Hellier also has facilities in Niantic, CT and Houston, TX. Please see our web site at www.hellierndt.com. Krautkramer Branson, 50 Industrial Park Rd., Lewistown, PA17044, (717) 242-0327; Designer, manufacturer, and marketer of high technology ultrasonic and eddy current instruments, transducers, and probes. The Krautkramer instrument product line includes flaw detectors, thickness gauges, hardness testers, and inspection systems. Our web site is www.krautkramer.com.
Lenox Instrument Co. Inc., 265 Andrews Rd., Scottsville Industrial Pk., Trevose, PA 19053, (800) 356-1104; Manufacturer of flexible, rigid, and extendable borescopes and remote video monitoring systems for visual inspection of otherwise inaccessible areas, such as the interiors of airframe fuselages, engines, pipe tubing, valves, and castings. Lenox offers the only videoprobe with remote focus.
Machida Inc., 40 Ramland Rd. South, Orangeburg, NY 10962, (914) 365-0600; Offers flexible borescopes and approved kits designed for the internal visual inspection of various engine models, including turbine and turboprop engines from Pratt & Whitney Canada, Rolls-Royce, Garrett, Allison, Turbomeca, AlliedSignal, and others. Kits contain flexible borescopes with accessories, light sources with adapters and a carrying case.
NDT Engineering Corp., 19620 Russell Rd., Kent, WA 98032, (253) 872-3565; Manufacturer of eddy current probes, ultrasonic transducers and both metallic and non-metallic reference standards. NDT's specializes in custom designs to fit customer application requirements and our product line has over 10,000 different style of probes and transducers.
NDTec Inc., 8385 SW 188 St., Miami, FL 33157, (305) 378-4992 or (888) 638-3248; Offers select training that meets or exceeds the requirements SNT-TC-1A, MIL-STD-410E, NAS 410, and ATA-105. Our instructors are Level III Certified by ASNT. Convenience of arranging training around your schedule — at your facility or ours — with a choice of training in English or Spanish.
Olympus America, IPG, 2 Corporate Center Dr., Melville, NY 11747; Manufacturers of borescopes, fiberscopes, videoscopes, and portable inspection systems for inspecting turbines, airframes, and other critical components. New 3-D stereo measuring system gives highly accurate results without calibrating or aligning. Results are not skewed by surface contours, or by the angle or distance of scope to object. Visit our web site at www.olympusamerica.com.
OPTIM Inc., 64 Technology Park Rd., Sturbridge, MA 01566, (508) 347-5100; Manufacturer of flexible fiberscopes for aircraft maintenance and inspection. Models: FS126 (3.2mm); FS155 (4.0mm); FS200 (5.3mm); FS236 (6.0mm); and IFS-8 (8.0mm).
Panametrics Inc., 221 Crescent St., Waltham, MA 02155, (781) 899-2719; Manufacturer of ultrasonic non-destructive testing equipment for the aircraft maintenance industry. For details on the Model 25DL Precision Thickness Gage and Epoch III and IIIB Flaw Detectors, visit our web site at www.panametrics.com. Our School of Non-destructive Testing conducts classes in ultrasonic testing.
Quality Testing Services Inc., 4003 Fee Fee Rd., Bridgeton, MO 63044, (314) 770-0607 or (888) 770-0607; Offers Level I and Level II courses in liquid penetrant, magnetic particle, ultrasonic, radiographic, and eddy current inspection. Train, qualify, and certify personnel to SNT-TC-1A, MIL-STD 410, and CP-189. QTS is an ASNT authorized exam center and is approved to administer IRRSP and ACCP Level II and ASNT NDT Level III exams. Please visit our web site at http://members.aol.com/qtsstl/.
R-CON NDT, 1620 Stout Rd., Menomonie, WI 54751, (715) 235-7222; Provides sales of all types of NDT equipment and supplies. Also available are NDT training and consulting services — specializing in NDT of composites and the aviation industry. Visit our web site at www.rcon-ndt.com.
Richard Wolf Borescopes, 353 Corporate Woods Pkwy., Vernon Hills, IL 60061, (847) 913-1113; Offers a complete line of rigid borescopes and accessories needed to inspect aircraft.
Sonotech Inc., 774 Marine Dr., Bellingham, WA 98225, (360) 671-9121; ISO 9000 manufacturer of ultrasonic couplants for contact NDE and water additives for immersion and squirter systems. Specializing in operator-safe and environmentally-benign products, including several couplants developed for aircraft industry. Fully traceable products with Certificates of Analysis available.
Staveley NDT Technologies, 421 N. Quay St., Kennewick, WA 99336, (509) 736-2751; Manufactures a full line of high performance ultrasonic, eddy current and bond testing instruments, as well as sensors and X-ray. The 1000 Series of lightweight portables have become the instruments of choice by the U.S. Armed Services and commercial airlines throughout the world.
Titan Tool Supply Co., Inc., P.O. Box 569, Buffalo, NY 14207, (716) 873-9907; High quality borescopes for aviation maintenance and microscopes for inspection of machined parts. Offers a broad range of borescopes for internal viewing and inspection from .028- to 0.800-inch diameters and in lengths from six inches to 30 feet. Visit our web site at www.titantoolsupply.com.
UE Training Systems Inc., 14 Hayes St., Elmsford, NY 10523, (888) 222-7228; Offers certifiable courses in airborne/structure borne ultrasound. A specialized Associate's course for aircraft inspection includes methods for leak detection and mechanical inspection. Our web site is www.uesystems.com.
UXR, 67 W. Easy St., Unit 118, Simi Valley, CA 93065, (805) 527-9219 or (800) 530-3871; Offers new and pre-owned NDT equipment: borescopes, fiberscopes, videoscopes, ultrasonic and eddy current NDT equipment. Visit our web site at www.uxr.com
Walker Scientific Inc., Rockdale St., Worcester, MA 01606, (508) 852-3674; Cost effective alloy sorting is made easy with the ATS-6044T Alloy Thermo-Sorter. For rapid reproducible sorting and classifying of alloys ranging in size from fine wire to steel girders. Alloy Chem-Sort kits are also available when only occasional sorting is required.
Xactex Corp., 3704 Stearman Ave., Pasco, WA 99301, (509) 545-6364; Ultrasonic instruments, transducers and immersion systems for scanning aircraft engines and airframe components. Eddy current probes, both standard and customized for all aerospace applications. NDT application development for all sizes of aircraft.
Zetec Inc., 1370 NW Mall St., Issaquah, WA 98027, (425) 392-5316; Provides complete integrated support for every aspect of eddy current testing used in mission critical applications in industries such as aerospace, nuclear and fossil energy, and petrol/chemical plants. Visit our web site at www.zetec.com.
