Par 5-5 Page 5-5
(2) Heat treating cracks are caused by stresses setup by unequal heating or cooling of portions of a part during heat treating operations. Generally, they occur where a part has a sudden change of section that could cause an uneven cooling rate, or at fillets and notches that act as stress concentration points.
(3) Grinding cracks are thermal type cracks similar to heat treating cracks and can occur when hardened surfaces are ground. The overheating created by the grinding can be caused by the wheel becoming glazed so that it rubs instead of cutting the surface; by using too little coolant; by making too heavy a cut; or by feeding the material too rapidly. Generally, the cracks are at right angles to the direction of grinding and in severe cases a complete network of cracks can appear. Grinding cracks are usually shallow and very sharp at their roots, which makes them potential sources of fatigue failure.
(4) Etching cracks can occur when hardened surfaces containing internal residual stresses are etched in acid.
(5) Plating cracks can occur when hardened surfaces are electroplated. Generally, they are found in areas where high residual stresses remain from some previous operation involving the part.
e. In-Service Flaws. These flaws are formed after all fabrication has been completed and the aircraft, engine, or related component has gone into service. These flaws are attributable to aging effects caused by either time, flight cycles, service operating conditions, or combinations of these effects. The following are brief descriptions of some in-service flaws.
(1) Stress corrosion cracks can develop on the surface of parts that are under tension stress in service and are also exposed to a corrosive environment, such as the inside of wing skins, sump areas, and areas between two metal parts of faying surfaces.
(2) Overstress cracks can occur when a part is stressed beyond the level for which it was designed. Such overstressing can occur as the result of a hard landing, turbulence, accident, or related damage due to some unusual or emergency condition not anticipated by the designer, or because of the failure of some related structural member.
(3) Fatigue cracks can occur in parts that have been subjected to repeated or changing loads while in service, such as riveted lap joints in aircraft fuselages. The crack usually starts at a highly stressed area and propagates through the section until failure occurs. A fatigue crack will start more readily where the design or surface condition provides a point of stress concentration.
Common stress concentration points are fillets; sharp radii; or poor surface finish, seams, or grinding cracks.
(4) Unbonds, or disbonds, are flaws where adhesive attaches to only one surface in an adhesive-bonded assembly. They can be the result of crushed, broken, or corroded cores in adhesive-bonded structures. Areas of unbonds have no strength and place additional stress on the surrounding areas making failure more likely.
Par 5-5 Page 5-6
(5) Delamination is the term used to define the separation of composite material layers within a monolithic structure. Ultrasonic is the primary method used for the detection of delamination in composite structures.
5-6. SELECTING THE NDI METHOD. The NDI method and procedure to be used for any specific part or component will generally be specified in the aircraft or component manufacturer’s maintenance or overhaul manuals, SSIDs, SBs, or in Ads.
NOTE: Some Ads refer to SBs which may, in turn, refer to manufacturer’s overhaul or maintenance manuals.
a. Appropriate Method. The appropriate NDI method may consist of several separate inspections. An initial inspection may indicate the presence of a possible flaw, but other inspections may be required to confirm the original indication. Making the correct NDI method selection requires an understanding of the basic principles, limitations, and advantages and disadvantages of the available NDI methods and an understanding of their comparative effectiveness and cost.
b. Other Factors. Other factors affecting the inspection are:
• The critical nature of the component;
• The material, size, shape, and weight of the part;
• The type of defect sought;
• Maximum acceptable defect limits in size and distribution;
• Possible locations and orientations of defects;
• Part accessibility or portability; and
• The number of parts to be inspected.
c. Degree of Inspection. The degree of inspection sensitivity required is an important factor in selecting the NDI method. Critical parts that cannot withstand small defects and could cause catastrophic failure require the use of the more sensitive NDI methods. Less critical parts and general hardware generally require less-sensitive NDI methods.
d. Material Safety Data Sheets (MSDS). The various materials used in NDI may contain chemicals, if improperly used, can be hazardous to the health and safety of operators and the safety of the environment, aircraft, and engines. Information on safe handling of materials is provided in MSDS. MSDS, conforming to Title 29 of the Code of Federal Regulations (29 CFR) part 1910, § 1200, or its equivalent, must be provided by the material supplier to any user and must be prepared according to Federal Standard (FED-STD)-313.
e. Advantages and Disadvantages. Table 5-1 provides a list of the advantages and disadvantages of common NDI methods. Table 5-1, in conjunction with other information in the AC, may 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.
Par 5-6 Page 5-7 TABLE 5-1. ADVANTAGES AND DISADVANTAGES OF NDI METHODS
METHOD ADVANTAGES DISADVANTAGES VISUAL Inexpensive Highly portable Immediate results Minimum training Minimum part preparation Surface discontinuities only Generally only large discontinuities Misinterpretation of scratches DYE PENETRANT Portable Inexpensive Sensitive to very small discontinuities 30 min. or less to accomplish Minimum skill required 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 of results required MAGNETIC PARTICLE Can be portable Inexpensive Sensitive to small discontinuities Immediate results Moderate skill required Detects surface and subsurface discontinuities Relatively fast 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 Ferromagnetic materials only Part must be demagnetized after test.
EDDY CURRENT Portable Detects surface and subsurface discontinuities Moderate speed Immediate results Sensitive to small discontinuities Thickness sensitive Can detect many variables Surface must be accessible to probe Rough surfaces interfere with test Electrically conductive materials Skill and training required Time consuming for large areas ULTRASONIC Portable Inexpensive Sensitive to very small discontinuities Immediate results Little part preparation Wide range of materials and thickness can be inspected Surface must be accessible to probe Rough surfaces interfere with test Highly sensitive to sound beam discontinuity orientation High degree of skill required to set up and interpret Couplant usually required
Par 5-6 Page 5-8 X-RAY RADIOGRAPHY Detects surface and internal flaws Can inspect hidden areas Permanent test record obtained Minimum part preparation 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 ISOTOPE RADIOGRAPHY Portable Less expensive than X-ray Detects surface and internal flaws Can inspect hidden areas Permanent test record obtained Minimum part preparation 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
5-7. TYPES OF INSPECTIONS. NDT methods are techniques used both in the production and in-service environments without damage or destruction of the item under investigation.
Examples of NDI methods are as follows:
• Visual inspection,
• Magnetic particle,
• Penetrants,
• Eddy current,
• Radiography,
• Ultrasonic,
• Acoustic emission,
• Thermography,
• Holography,
• Shearography, and
• Tap testing.
5-8.–5-14. [RESERVED.]
Par 5-15 Page 5-9 SECTION 2. VISUAL INSPECTION
5-15. GENERAL. Visual inspection is the oldest and most common form of NDI for aircraft.
Approximately 80 percent of all NDI procedures are accomplished by the direct visual methods.
This inspection procedure may be greatly enhanced by the use of appropriate combinations of magnifying instruments, borescopes, light sources, video scanners, and other devices discussed in this AC. Visual inspection provides a means of detecting and examining a wide variety of component and material surface discontinuities, such as cracks, corrosion, contamination, surface finish, weld joints, solder connections, and adhesive disbonds. Visual inspection is widely used for detecting and examining aircraft surface cracks, which are particularly important because of their relationship to structural failures. Visual inspection is frequently used to provide verification when defects are found initially using other NDI techniques. The use of optical aids for visual inspection is beneficial and recommended. Optical aids magnify defects that cannot be seen by the unaided eye and also permit visual inspection in inaccessible areas.
5-16. SIMPLE VISUAL INSPECTION AIDS. It should be emphasized that the eye mirror-flashlight is a critical visual inspection process. Aircraft structure and components that must be routinely inspected are frequently located beneath skin, cables, tubing, control rods, pumps, actuators, etc. Visual inspection aids such as a powerful flashlight, a mirror with a ball joint, and a 2 to 10 power magnifying glass are essential in the inspection process.
a. Flashlights.
(1) Flashlights used for aircraft inspection should be suitable for industrial use and, where applicable, safety approved by the Underwriters Laboratory or equivalent agency as suitable for use in hazardous atmospheres such as aircraft fuel tanks. Military Specification MIL-F-3747E, Flashlights: Plastic Case, Tubular (Regular, Explosion-Proof Heat Resistant, Traffic Directing, and Inspection-Light), provides requirements for flashlights suitable for use in aircraft inspection. However, at the present time, the flashlights covered by this specification use standard incandescent lamps and there are no standardized performance tests for flashlights with the brighter bulbs: Krypton, Halogen, and Xenon. Each flashlight manufacturer currently develops its tests and provides information on its products in its advertising literature. Therefore, when selecting a flashlight for use in visual inspection, it is sometimes difficult to directly compare products.
(2) The following characteristics should be considered when selecting a flashlight:
foot-candle rating; explosive atmosphere rating; beam spread (adjustable, spot, or flood);
efficiency (battery usage rate); brightness after extended use; and rechargeable or standard batteries. (If rechargeable, how many hours of continuous use and how long is required for recharging?) If possible, it would be best to take it apart and inspect for quality of construction and to actually use the flashlight like it would be used in the field. Inspection flashlights are available in several different bulb brightness levels:
• Standard incandescent (for long battery life).
• Krypton (for 70 percent more light than standard bulbs).
• Halogen (for up to 100 percent more light than standard bulbs).
• Xenon (for over 100 percent more light than standard bulbs).
Par 5-16 Page 5-10
b. Inspection Mirrors. An inspection mirror is used to view an area that is not in the normal line of sight. The mirror should be of the appropriate size to easily view the component, with the reflecting surface free of dirt, cracks, worn coating, etc., and a swivel joint tight enough to maintain its setting.
c. Simple Magnifiers. A single converging lens, the simplest form of a microscope, is often referred to as a simple magnifier. Magnification of a single lens is determined by the equation M = 10/f. In this equation, “M” is the magnification, “f” is the focal length of the lens in inches, and “10” is a constant that represents the average minimum distance at which objects can be distinctly seen by the unaided eye. Using the equation, a lens with a focal length of 5 inches has a magnification of 2, or is said to be a two-power lens.
5-17. BORESCOPES. These instruments are long, tubular, precision optical instruments with built-in illumination, designed to allow remote visual inspection of internal surfaces or otherwise inaccessible areas. The tube, which can be rigid or flexible with a wide variety of lengths and diameters, provides the necessary optical connection between the viewing end and an objective lens at the distant, or distal tip of the borescope. Rigid and flexible borescopes are available in different designs for a variety of standard applications and manufacturers also provide custom designs for specialized applications. Figure 5-1 shows three typical designs of borescopes.
a. Borescopes Uses. 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; inspect 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.
b. Optical Designs.
(1) Typical designs for the optical connection between the borescope viewing end and the distal tip are:
• A rigid tube with a series of relay lenses;
• A flexible or rigid tube with a bundle of optical fibers; and
• A flexible or rigid tube with wiring that carries the image signal from a Charge Couple Device (CCD) imaging sensor at the distal tip.
(2) These designs can have either fixed or adjustable focusing of the objective lens at the distal tip. The distal tip may also have prisms and mirrors that define the direction and field of view. A fiber optic light guide with white light is generally used in the illumination system, but ultraviolet light can also be used to inspect surfaces treated with liquid fluorescent penetrant or to inspect for contaminants that fluoresce. Some borescopes with long working lengths use light-emitting diodes at the distal tip for illumination.
FIGURE 5-1. TYPICAL BORESCOPE DESIGNS Eyepiece lens Relay Lens Objective lens Incandescent Lamp
(a) A rigid borescope with a lamp at the distal end.
Eyepiece lens Control handles Focusing ring for four-way tip articulation Interchangeable Image guide tips Objective lens Light guide Protective sheath Light source Light guide exit Projection lamp
(b) A flexible fiberscope with a light source.
Field of View Orbital scan Control Focus control Shaft rotation for orbital scan Working length Light guide connector (inside handle) Light guide To light source (integral or separate)
(c) A rigid borescope with a light guide bundle in the shaft.
5-18. VISUAL INSPECTION PROCEDURES. Corrosion can be an extremely critical defect. Therefore, NDI personnel should be familiar with the appearance of common types of corrosion and have training and experience on corrosion detection on aircraft structure and engine materials. (Refer to AC 43-4, Corrosion Control for Aircraft, for additional information on corrosion.)
a. Preliminary Inspection. Perform a preliminary inspection of the overall general area for cleanliness, presence of foreign objects, deformed or missing fasteners, security of parts, Par 5-17 Page 5-11
Par 5-18 Page 5-12 corrosion, and damage. If the configuration or location of the part conceals the area to be inspected, use visual aids such as a mirror or borescope.
b. Corrosion Treatment. Treat any corrosion found during preliminary inspection after completing a visual inspection of any selected part or area.
NOTE: Eddy current, radiography, or ultrasonic inspection can determine the loss of metal to corrosion.
c. Lighting. Provide adequate lighting to illuminate the selected part or area.
d. Personal Comfort. Personal comfort (temperature, wind, rain, etc.) of the inspector can be a factor in visual inspection reliability.
e. Noise. Noise levels while conducting a visual inspection are important. Excessive noise reduces concentration, creates tension, and prevents effective communication. All these factors will increase the likelihood of errors.
f. Inspection Area Access. Ease of access to the inspection area has been found to be of major importance in obtaining reliable visual inspection results. Access consists of the act of getting into an inspection position (primary access) and doing the visual inspection (secondary access). Poor access can affect the inspector’s interpretation of discontinuities, decision making, motivation, and attitude.
g. Precleaning. Clean the areas or surface of the parts to be inspected. Remove any contaminates that might hinder the discovery of existing surface indications. Do not remove the protective finish from the part or area prior to inspection. Removal of the finish may be required at a later time if other NDI techniques are required to verify any visual indications of flaws that are found.
h. Inspection. Carefully inspect the area for discontinuities, using optical aids as required.
An inspector normally should have available suitable measuring devices, a flashlight, and a mirror.
(1) 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. (See Figure 5-2.)
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.
Par 5-18 Page 5-13 FIGURE 5-2. USING A FLASHLIGHT TO INSPECT FOR CRACKS (
2) Other Surface Discontinuities. Inspect for other surface discontinuities, such as:
discoloration from overheating; buckled, bulging, or dented skin; cracked, chafed, split, or dented tubing; chafed electrical wiring; delamination of composites; and damaged protective finishes.
i. Recordkeeping. Document all discrepancies by written report, photograph, and/or video recording for appropriate evaluation. The full value of visual inspection can be realized only if records are kept of the discrepancies found on parts inspected. The size and shape of the discontinuity and its location on the part should be recorded along with other pertinent information, such as rework performed or disposition. The inclusion on a report of some visible record of the discontinuity makes the report more complete.
5-19.–5-24. [RESERVED.]
Par 5-25 Page 5-14 SECTION 3. EDDY CURRENT INSPECTION
5-25. EDDY CURRENT INSPECTION. Eddy current is used to detect surface cracks, pits, subsurface cracks, corrosion on inner surfaces, and to determine alloy and heat-treat condition.
a. Eddy Current Instruments. A wide variety of eddy current test instruments are available. The eddy current test instrument performs three basic functions: generating, receiving, and displaying. The generating portion of the unit provides an alternating current to the test coil.
The receiving section processes the signal from the test coil to the required form and amplitude for display. Instrument outputs or displays consist of a variety of visual, audible, storage, or transfer techniques utilizing meters, video displays, chart recorders, alarms, magnetic tape, computers, and electrical or electronic relays.
b. Principles of Operations. Eddy currents are induced in a test article when an alternating current is applied to a test coil (probe). The alternating current in the coil induces an alternating magnetic field in the article which causes eddy currents to flow in the article. (See Figure 5-3.)
(1) Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the coil accordingly. (See Figure 5-4.) Instruments display the impedance changes either by impedance plane plots or by needle deflection.
(2) Figure 5-5 shows typical impedance plane display and meter display instrument responses for aluminum surface cracks, subsurface cracks, and thickness.
FIGURE 5-3. GENERATING AN EDDY CURRENT
FIGURE 5-4. DETECTING AN EDDY CURRENT Alternating current
- Probe coil Primary magnetic field Conductor - Eddy current Secondary magnetic field FIGURE 5-5. TYPICAL INSTRUMENT DISPLAYS Aluminum surface crack Aluminum subsurface crack Thickness Impedance response Crack response Crack plane thickness a, b, c, \ a instrument response Liftoff Balance point Liftoff Balance point Liftoff Balance point Balance Crack Balance Crack point response point response Meter Balance display point instrument
5-26. EDDY CURRENT COILS AND PROBES. A wide variety of eddy current coils and probes is available. Coils and probes are not always interchangeable between various types of instruments and, for best results, should be matched to a specific instrument and frequency range. Special probe holders can be fabricated to facilitate eddy current inspection of contoured or shaped parts including part edges.
Par 5-25 Page 5-15
Par 5-27 Page 5-16
5-27. FIELD APPLICATION OF EDDY CURRENT INSPECTION. Eddy current techniques are particularly well-suited for detection of service-induced cracks in the field.
Service-induced cracks in aircraft structures are generally caused by fatigue or stress corrosion.
Both types of cracks initiate at the surface of a part. If this surface is accessible, a high-frequency eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity. If the surface is less accessible, such as in a subsurface layer of structure, low-frequency eddy current inspection can usually be performed. Eddy current inspection can usually be performed without removing surface coatings such as primer, paint, and anodic films.
Eddy current inspection has the greatest application for inspecting small localized areas where possible crack initiation is suspected rather than for scanning broad areas for randomly-oriented cracks. However, in some instances it is more economical to scan relatively large areas with eddy current rather than strip surface coatings, inspect by other methods, and then refinish.
5-28. SURFACE INSPECTION.
a. Inspection Technique. Eddy current inspection techniques are used to inspect for surface cracks such as those shown in Figure 5-6.
FIGURE 5-6. TYPICAL SURFACE CRACKS
b. E quipment Requirements. The following are typical eddy current equipment requirements for surface crack inspections.
(1) Instruments must meet the liftoff and sensitivity requirements of the applicable NDI procedures. The frequency requirement is generally 100 hertz (Hz) to 200 kilohertz (kHz).