Nondestructive Inspection and Visual Methods

10-17 • Flying the aircraft to a base where repairs, alterations, or maintenance are to be performed or to a point of storage • Delivering or exporting the aircraft • Production flight testing new production aircraft • Evacuating aircraft from areas of impending danger • Conducting customer demonstration flights in new production aircraft that have satisfactorily completed production flight tests Additional information about special flight permits may be found in 14 CFR part 21. Application forms for special flight permits may be requested from the nearest FAA FSDO.

Nondestructive Inspection/Testing

The preceding information in this chapter provided general details regarding aircraft inspection. The remainder of this chapter deals with several methods often used on specific components or areas on an aircraft when carrying out the more specific inspections. They are referred to as nondestructive inspection (NDI) or nondestructive testing (NDT). The objective of NDI and NDT is to determine the airworthiness of a component, without damaging it, that would render it unairworthy. Some of these methods are simple, requiring little additional expertise, while others are highly sophisticated and require that the technician be highly trained and specially certified.

Training, Qualification, & Certification The product manufacturer or the FAA generally specifies the particular NDI method and procedure to be used in inspection. These NDI requirements are specified in the manufacturer’s inspection, maintenance, or overhaul manual, FAA ADs, supplemental structural inspection documents (SSID), or SBs. The success of any NDI method and procedure depends upon the knowledge, skill, and experience of the NDI personnel involved. The person(s) responsible for detecting and interpreting indications, such as eddy current, x-ray, or ultrasonic NDI, must be qualified and certified to specific FAA or other acceptable government or industry standards, such as MIL-STD-410, Nondestructive Testing Personnel Qualification and Certification or ATA iSpec 105, Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods. The person must be familiar with the test method, know the potential types of discontinuities peculiar to the material, and be familiar with their effect on the structural integrity of the part. Additional information on NDI may be found by referring to Chapter 5 of FAA AC 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair.

Advantages & Disadvantages of NDI Methods disadvantages of common NDI methods. This table could be used as a guide for evaluating the most appropriate NDI method when the manufacturer or the FAA has not specified a particular NDI method to be used. General Techniques Before conducting NDI, it is necessary to follow preparatory steps in accordance with procedures specific to that type of inspection. Generally, the parts or areas must be thoroughly cleaned. Some parts must be removed from the aircraft or engine. Others might need to have any paint or protective coating stripped. A complete knowledge of the equipment and procedures is essential and, if required, calibration and inspection of the equipment must be current.

Visual Inspection

Visual inspection can be enhanced by looking at the suspect area with a bright light, a magnifying glass, and a mirror. Some defects might be so obvious that further inspection methods are not required. The lack of visible defects does not necessarily mean further inspection is unnecessary. Some defects may lie beneath the surface or may be so small that the human eye, even with the assistance of a magnifying glass, cannot detect them. 10-18 Surface Cracks When searching for surface cracks with a flashlight, direct the light beam at a 5 to 45 degree angle to the inspection surface towards the face. [Figure 10-7] Do not direct the light beam at such an angle that the reflected light beam shines directly into the eyes. Keep the eyes above the reflected light beam during the inspection. Determine the extent of any cracks found by directing the light beam at right angles to the crack and tracing its length. Use a 10-power magnifying glass to confirm the existence of a suspected crack. If this is not adequate, use other NDI techniques, such as penetrant, magnetic particle, or eddy current to verify cracks.

Borescope Inspection by use of a borescope is essentially a visual inspection. A borescope is a device that enables the inspector to see inside areas that could not otherwise be inspected without disassembly. Borescopes are used in aircraft and engine maintenance programs to reduce or eliminate the need for costly teardowns. Aircraft turbine engines have access ports that are specifically designed for borescopes. Borescopes are also used extensively in a variety of aviation maintenance programs to determine the airworthiness of difficult to reach components. Borescopes typically are used to inspect interiors of hydraulic cylinders and valves for pitting, scoring, porosity, and tool marks; search for cracked cylinders in aircraft reciprocating engines; inspect turbojet engine turbine blades and combustion cans; verify the proper placement and fit of seals, bonds, gaskets, and subassemblies in difficult to reach areas; and assess foreign object damage (FOD) in aircraft, airframe, and powerplants. Borescopes may also be used to locate and retrieve foreign objects in engines and airframes.

Borescopes are available in two basic configurations. The simpler of the two is a rigid type, small diameter telescope with a tiny mirror at the end that enables the user to see around corners. The other type uses fiber optics that enable greater flexibility. [Figure 10-8] Many borescopes provide images that can be displayed on a computer or video monitor for better interpretation of what is being viewed and to record images for future reference. Most borescopes also include a light to illuminate the area being viewed. Liquid Penetrant Inspection Penetrant inspection is a nondestructive test for defects open to the surface in parts made of any nonporous material. It is used with equal success on such metals as aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium. It may also be used on ceramics, plastics, molded rubber, and glass.

Penetrant inspection detects defects, such as surface cracks or porosity. These defects may be caused by fatigue cracks, shrinkage cracks, shrinkage porosity, cold shuts, grinding and heat-treat cracks, seams, forging laps, and bursts. Penetrant inspection also indicates a lack of bond between joined metals. The main disadvantage of penetrant inspection is that the defect must be open to the surface in order to let the penetrant get into the defect. For this reason, if the part in question is made of material that is magnetic, the use of magnetic particle inspection is generally recommended. Penetrant inspection uses a penetrating liquid that enters a surface opening and remains there, making it clearly visible to the inspector. It calls for visual examination of the part after it has been processed, increasing the visibility of the defect so that it can be detected. Visibility of the penetrating material is increased by the addition of one or two types of dye: visible or fluorescent.

The visible penetrant kit consists of dye penetrant, dye remover emulsifier, and developer. The fluorescent penetrant inspection kit contains a black light assembly, as well as spray cans of penetrant, cleaner, and developer. The light assembly consists of a power transformer, a flexible power cable, and a hand-held lamp. Due to its size, the lamp may be used in almost any position or location. The steps for performing a penetrant inspection are: 1. Clean the metal surface thoroughly. 2. Apply penetrant. 3. Remove penetrant with remover emulsifier or cleaner. 4. Dry the part. 5. Apply the developer.

6. Inspect and interpret results. Interpretation of Results The success and reliability of a penetrant inspection depends upon the thoroughness that the part was prepared with. Several basic principles applying to penetrant inspection are: 1. The penetrant must enter the defect in order to form an indication. It is important to allow sufficient time so the penetrant can fill the defect. The defect must be clean and free of contaminating materials so that the penetrant is free to enter. 2. If all penetrant is washed out of a defect, an indication cannot be formed. During the washing or rinsing operation, prior to development, it is possible that the penetrant is removed from within the defect, as well as from the surface.

3. Clean cracks are usually easy to detect. Surface openings that are uncontaminated, regardless of how 10-19 Visual Penetrant Dye Eddy Current Ultrasonic Magnetic Particle X-Ray Radiography Isotope Radiography Method Advantages • Inexpensive • Highly portable • Immediate results • Minimum training • Minimum part preparation • Portable • Inexpensive • Sensitive to very small discontinuities • 30 minutes or less to accomplish • Minimum skill required • Can be portable • Inexpensive • Sensitive to small discontinuities • Immediate results • Moderate skill required • Detects surface and subsurface discontinuities • Relatively fast • Portable • Detects surface and subsurface discontinuities • Moderate speed • Immediate results • Sensitive to small discontinuities • Thickness sensitive • Can detect many variables • Portable • Inexpensive • Sensitive to very small discontinuities • Immediate results • Little part preparation • Wide range of materials and thickness can be inspected • Detects surface and internal flaws • Can inspect hidden areas • Permanent test record obtained • Minimum part preparation • Portable • Less inexpensive than x-ray • Detects surface and internal flaws • Can inspect hidden areas • Permanent test record obtained • Minimum part preparation Disadvantages • Surface discontinuities only • Generally only large discontinuities • Misinterpretation of scratches • Locate surface defects only • Rough or porous surfaces interfere with test • Part preparation required (removal of finishes and sealant, etc.) • High degree of cleanliness required • Direct visual detection on results required • Surface must be accessible • Rough surfaces interfere with test • Part preparation required (removal of finishes and sealant, etc.) •Semi-directional requiring general orientation of field to discontinuity • Ferro-magnetic materials only • Part must be demagnetized after test • Surface must be accessible to probe • Rough surfaces interfere with test • Electrically conductive materials • Skill and training required • Time consuming for large areas • Surface must be accessible to probe • Rough surfaces interfere with test • Highly sensitive to sound beam discontinuity orientation • High degree of skill and experience required for exposure and interpretation • Depth of discontinuity not indicated • Safety hazard • Very expensive (slow process) • Highly directional, sensitive to flaw orientation • High degree of skill and experience required for exposure and interpretation • Depth of discontinuity not indicated • Safety hazard • Must conform to federal and state regulations for handling and use • Highly directional, sensitive to flaw orientation • High degree of skill and experience required for exposure and interpretation • Depth of discontinuity not indicated fine, are seldom difficult to detect with the penetrant inspection.

4. The smaller the defect, the longer the penetrating time. Fine crack-like apertures require a longer penetrating time than defects such as pores. 5. When the part to be inspected is made of a material susceptible to magnetism, it should be inspected by a magnetic particle inspection method if the equipment is available. 6. V isible penetrant-type developer, when applied to the surface of a part, dries to a smooth, white coating. As the developer dries, bright red indications appear where there are surface defects. If no red indications appear, there are no surface defects. 7. When conducting the fluorescent penetrant-type inspection, the defects show up (under black light) as a brilliant yellow-green color and the sound areas appear deep blue-violet.

8. It is possible to examine an indication of a defect and to determine its cause as well as its extent. Such an appraisal can be made if something is known about the manufacturing processes that the part has been subjected to. The size of the indication, or accumulation of penetrant, shows the extent of the defect and the brilliance is a measure of its depth. Deep cracks hold more penetrant and are 10-20 Fay 45° KEEP EYE ABOVE REFLECTED LIGHT BEAM Crack open to surface Incandescent light beam Line of sightReflected light beam Rigid Flexible broader and more brilliant. Very fine openings can hold only small amounts of penetrants and appear as fine lines.

[Figure 10-9] False Indications With the penetrant inspection, there are no false indications in the sense that they occur in the magnetic particle inspection. There are, however, two conditions that may create accumulations of penetrant that are sometimes confused with true surface cracks and discontinuities. The first condition involves indications caused by poor washing. If all the surface penetrant is not removed in the washing or rinsing operation following the penetrant dwell time, the unremoved penetrant is visible. Evidences of incomplete washing are usually easy to identify since the penetrant is in broad areas rather than in the sharp patterns found with true indications. When accumulations of unwashed penetrant are found on a part, the part must be completely reprocessed. Degreasing is recommended for removal of all traces of the penetrant.

False indications may also be created where parts press fit to each other. If a wheel is press fit onto a shaft, penetrant shows an indication at the fit line. This is perfectly normal since the two parts are not meant to be welded together. Indications of this type are easy to identify since they are regular in form and shape.

Eddy Current Inspection

Electromagnetic analysis is a term describing the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is the eddy current. Eddy currents are composed of free electrons under the influence of an induced electromagnetic field that are made to “drift” through metal. Eddy current is used to detect surface cracks, pits, subsurface cracks, corrosion on inner surfaces, and to determine alloy and heat-treat condition. Eddy current is used in aircraft maintenance to inspect jet engine turbine shafts and vanes, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat, or frame damage. Eddy current may also be used in repair of aluminum aircraft damaged by fire or excessive heat.

Different meter readings are seen when the same metal is in different hardness states. Readings in the affected area are compared with identical materials in known unaffected areas for comparison. A difference in readings indicates a difference in the hardness state of the affected area. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions. Figure 10-10 shows a technician performing an eddy current inspection on a fan blade. Basic Principles When an alternating current (AC) is passed through a coil, it develops a magnetic field around the coil, which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current. If this coil is placed in such a way that the magnetic field passes through an electrically conducting specimen, eddy currents are induced into the specimen. The eddy currents create their own field that varies the original field’s opposition to the flow of original current.

The specimen’s susceptibility to eddy currents determines the current flow through the coil. The magnitude and phase of this counter field is dependent primarily upon the resistance and permeability of the specimen under consideration and enables us to make a qualitative determination of various physical properties of the test material. The interaction of the eddy current field with the original field results is a power change that can be measured by utilizing electronic circuitry similar to a Wheatstone bridge.