the trained inspector and are used to detect defects which may affect airworthiness. Loose rivets tend to emit "smoke" ', discoloration, and streaking (localized).
d. Corrosion. Corrosion is primarily detected by visual inspection. Since its detection is critical, inspection personnel should be familiar with the appearance of the common types of corrosion and have training and experience in corrosion detection on aircraft structural and engine materials. Corrosion Control for Aircraft (AC 43-4A) contains technical information on corrosion identification, photos illustrating examples of typical corrosion, and guidance on treatment of corrosion on aircraft structural, engine, and related materials. It may be necessary to remove corrosion to assess the condition of the underlying material. It is important not to destroy the surface finish, or renove protective coatings which would be difficult to replace, such as the "cladding" found on the skin surface of major transports. In general aviation, an inspector will sometimes probe a part with a center punch in an area of suspected corrosion; if more than a minor indentation is observed, corrosion is present.
e. Inspection After Maintenance. After a component has been serviced, repaired, or otherwise maintained, it should be inspected for abnormalities. A similar inspection of the surroundings should be carried out when a component or accessory is reinstalled in the aircraft. Look for loose material, missing coatings, other damage, etc.
10. AIRFRAME VISUAL INSPECTION. It is necessary not only to know how to examine, but where to examine, when inspecting details for defects with which one is familiar from past inspections. The flashlight and the mirror are standard tools which should accompany the inspector on all inspections. The inspector should become skillful in using these tools. Also, magnifiers should be readily at hand for some types of inspections. Other aids which may be available for specialized requirements are described in Chapters 3 and 4. The flashlight/mirror combination can be used to illuminate an area as well as view it and is particularly valuable in inaccessible spots. Magnifiers are invaluable for defining the extent and character of cracks and problems in riveted assemblies.
Visual Detection of Surface Cracks. When searching for surface cracks with a flashlight, the light should be directed at a 5- to 45-degree angle (see Figure 1-18). The light beam should not be directed at such an angle that the reflected light beam shines directly into the eyes. The eyes should be focused above the beam during the inspection. The extent of any cracks can be determined by directing the light beam at right angles to the crack and tracing its length. A 10-power magnifying glass should be used to confirm the existence of a suspected crack. If this chap 1 Par 9 Page 21
is not adequate, other NDI techniques-such as penetrant, magnetic particle, or eddy current-may be used to verify cracks.
EYE ABOVE
REFLECTED
LIGHT BEAM
INCIDENT
LIGHT BEAM 5° TO 45 LINE OF SIGHT REFLECTED LIGHT BEAM
FAYING CRACK OPEN
SURFACES TO SURFACE FIGURE 1-18. USING A FLASHLIGHT TO INSPECT FOR SURFACE CRACKS
b. Visual Detection of Hidden Defects. The mirror can be used with great effect in locations that cannot be viewed directly. The mirror is frequently attached to a long arm so that it can be directed around corners. It is frequently necessary to shine a light onto the mirror and indirectly illuminate the area requiring or undergoing inspection.
c. Hardware and Fasteners. Rivets, bolts, and other miscellaneous structural hardware should be inspected for looseness, integrity, proper size and fit, and corrosion. Dished, cracked, or missing rivet heads and loose rivets should be identified and recorded, using the arsenal of visual inspection techniques available. The inspector should ensure that sufficient light is available to be able to really see every fastener. Once again, a
d. Control Systems. Cables, control rods, rod ends, fairleads, pulleys, and all other similar items should be examined for integrity, structural soundness, and corrosion.
e. Visual Inspection for Corrosion. Inspection of an aircraft for corrosion follows a systematic pattern.
(1) Clues. The aircraft is initially observed for clues as to the care with which it has been maintained. Have obvious Chap 1 Page 22 Par 10
corrosion control measures been taken? Have drains been cleaned, paint replaced, and debris removed?
Locations. Inspect likely corrosion sites. These include galleys and food service areas, lavatories, bilges, tank drains, and fastenings. when debris is found, it should be examined for iron oxide and the characteristically white powdery aluminum hyaride. Biological contamination (mold, algae), which may feel greasy or siippery, frequentiy causes corrosion since it alters the acidity of any moisture it contains. Caulking and sealing compounds should be tested for sound adhesion since corrosion can intrude under such materials. Nutplates should be investigated for underlying corrosion. Tap tests should be performed frequently and any dull sounding areas investigated carefully.
The use of a center punch or awl to indent a surface should be used with care, since awl or center punch pricks can serve as sites for the initiation of fatigue cracks. The omission of fuel additives by some fuel vendors can accelerate the deterioration of tankage on small aircraft. In such cases, it is necessary to drain tanks and inspect them using lighted borescopes or other aids. Flight and control surfaces are difficult to inspect since access is difficult. Extensive use of aids is recommended for such locations.
Sites. Careful detailed examination of corrosion sites is then accomplished to define the extent of corrosion.
This can lead to removal of skin panels or other measures to further define the extent of damage.
f. Disbonds. Many aircraft have extensive regions of adhesive bonding of the structure alone or in combination with rivets. It is frequently necessary to detect disbonds and adhesive failure in structure. It should be remembered that, in adhesively bonded structures, evidence of corrosion frequently signals the loss of bond integrity. A good example of this condition is the pillowing which appears behind rivets. If the structure is bonded as well as riveted, one can be sure that where pillowing exists the bond is destroyed. Unfortunately, the reverse is not also true. It is quite possible for an uncorroded joint to be disbonded. Frequently, the only means for detecting such a disbond is to mechanically distort the structure and determine whether any adhesion exists in the area of interest.
Specifically, the waffle doublers, which act as tear preventers in pressurized aircraft, should be tested where possible.
g. Painted Surfaces. Painted surfaces should be inspected loose or blistered paint and for evidence of corrosion. Refer to AC 43-4A for examples of corrosion.
h. Windshields and Ports. Transparent surfaces-such as windshields, windows, and ports-should be inspected for cracks, crazing, and seeds using a Larascope or similar device. This de- Chap 1 Par 10 Page 23
vice uses a plastic prism to permit viewing the interior of transparent plastic or glass window material. The prism is optically coupled to the window being inspected by use of an oil couplant, enabling the operator to see inside the glass. The prism can be manipulated to observe if any cracks are emanating from drill holes or from edges of the transparent material.
i. Other Surface Discontinuities. Other surface discontinuities-such as discoloration from overheating; buckled, bulging, or dented skin; cracked, chafed, split, or dented tubing; chafed electrical wiring; delaminations of composites; and damaged protective finishes-may be found by careful visual inspection.
11. ENGINE VISUAL INSPECTION. Some superficial clues to engine condition are leaks and evidence of smoke. Details observed in engine inspection which differ from airframe inspection generally relate to surface finish and the means for inferring underlying conditions.
a. Forgings, Weldments, Castings, and Machined Parts. The surface texture of the components differs from that of the rolled and formed surfaces most common in airframes. The components are therefore inspected using different criteria. So many engine defects are related to surface features of the material that a glossary of terms is used in most engine manuals to describe visible surface characteristics in terms of the process which caused them and its effect on satisfactory performance of the part. As a result, much of the current guidance relating to engine visual inspection is related to manufacturing processes.
This document covers only those visual inspection issues relating to operational and maintenance inspection. The Engine Visual Inspection Glossary in this document, however, describes defects which may be found by maintenance inspectors in the field, but are more often found in the manufacturing process rather than "in-service" operations or maintenance.
b. Engine Environment. Another difference related to engine inspection is associated with the environment in which engines are expected to operate. Many engine parts are expected to survive in intense heat. Inspections within the protective enclosures surrounding these areas often require complex visual aids such as light pipes, borescopes, and special inspection fixtur- INSPECTION FOR SPECIFIC TYPES OF CORROSION. This section provides general information for visual inspection of aircraft structure and components for specific types of corrosion. Visual inspection is the primary method for inspecting aircraft structure and components for corrosion. Other NDI techniques such as eddy current, ultrasonic, radiographic, magnetic particle, and penetrant inspection are also used to supplement visual inspection. These other NDI techniques are generally used when costly Chap 1 Page 24 Par 10
disassembly would be required to gain access to hidden structure or components to accomplish visual inspections. They are also used to measure or estimate material loss due to corrosion and to verify the effectiveness of corrosion removal. Inspection of aircraft for corrosion is a continuing requirement and should be done on a daily basis.
Corrosion. Corrosion is the electrochemical deterioration of a metal because of its reaction with the surrounding environment. While new and better materials are continuously being developed, this progress is offset, in part, by a more aggressive operational environment. Corrosion is a complex phenomenon which can take many different forms and the resistance of aircraft materials to corrosion can drastically change with only a small environmental change.
b. Catastrophic Corrosion Events. Corrosion is most often thought of as a slow process of material deterioration, taking place over a significant period of time (examples being general corrosion, pitting, exfoliation, etc.). Other forms of corrosion degradation can occur very quickly, in days or even hours, wit!
catastrophic results. These torms such as stress corrosion cracking, environmental embrittlement, and corrosion fatigue) depend on both the chemical and mechanical aspects of the environment and can cause catastrophic structural failure without warn-
metals; however, the use of corrosion resistant metals and the application of protective coatings minimize aircraft corrosion.
Many other factors contribute to the amount and degree of aircraft corrosion such as the operational environment, materials used in the original design, and the amount of operational preservation provided during maintenance and repair.
d. Aircraft Environment. Aircraft exposed to salt air, heavy industrial pollution, and/or over water operations will experience more corrosion problems than aircrait operated in a ary environment. Corrosion is caused by the presence of salts in moist air or by some other abatement to corrosion present in the chemical content of the water or elements in the metal.
e. Manufacturers' Handbooks. Manufacturers' handbooks should be used as a general guide when an area is to be inspected for corrosion. Some manufacturers have developed videos explaining the process of corrosion and separate corrosion control manuals that are the basis for a complete aircraft corrosion inspection and program. On aircraft for which the manufacturer has not published a recommended corrosion inspection schedule and treatment program, the recommendations of Advisory Circular (AC) 43-4A, Corrosion Control for Aircraft, should be followed.
Chap 1 Par 12 Page 25
f. Uniform Etch Corrosion. Uniform etch corrosion results from a relatively uniform chemical attack on a metal surface (see Figure 1-19). On a polished surface, this type of corrosion is first seen as a general dulling of the surface, and if the attack is allowed to continue, the surface becomes rough and possibly frosted in appearance.
FIGURE 1-19. UNIFORM ETCH CORROSION (Courtesy of Thomas Flournoy) Chap 1 Page 26 Par 12
g. Pitting Corrosion. The most common form of corrosion on aluminum and magnesium alloys is pitting (see Figure 1-ZV) • it is first noticeable as a white or gray powdery deposit, which, when removed reveals uneven corrosive attack which is revealed as pits or holes in the surface.
FIGURE 1-20. PITTING CORROSION Galvanic Corrosion. Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte (see Figure 1-21). The electromotive force (EMF) ries lists metals in the order of their ability to corrode as follows.
1. Magnesium Alloys 12. Copper
2. Zinc (plate) 13. Monel 3 Beryllium 14. Stainless Steels Cadmium (plate) 15. Titanium Alloys
5. Aluminum Alloys* 16. Silver
6. Tin (plate) 17. Palladium 7 . Copper (plate) 18. Gold 8 • Nickel (plate) 19. Rhodium
9. Chromium (plate) 20. Platinum
10. Brass (yellow or naval) 21. Carbon/Graphite
11. Bronze 2024 (copper alloye) used mostly in fuselages and the 7000 lzinc alloy) series used mostly in flying and control surfaces have about the same corrosion potential. The exceptions are rolled, extruded, and forged aluminum (anisotropic grain structure) which have been heat treated of cold worked. These alloys are susceptible to exfoliation corrosion, Chap 1 Par 12 Page 27
FIGURE 1-21. GALVANIC CORROSION OF MAGNESIUM ADJACENT TO STEEL FASTENER (Courtesy of Thomas Flournoy) Chap 1 Page 28 Par 12
8/14/97 AC 43-204 The rate and degree of corrosion depends on the relative surface areas of the two metals and their location on the electromotive force series. Adjacent metals in the EMF series have low corrosion susceptibility whereas widely separated metals will corrode rapidly. For example, an aluminum fastener in contact with a relatively inert Monel (nickel, copper, steel) structure may corrode severely, while a Monel bracket secured to a large aluminum member would result in a relatively superficial attack on the aluminum sheet.
i. Concentration Cell or Crevice Corrosion. Concentration cell corrosion is corrosion of metals in a metal-to-metal joint;
corrosion at the edge of a joint, even through joined metals are identical; or corrosion of a spot on the metal surface covered by a foreign material (see Figure 1-22 for the three types of crevice corrosion).
LOW METAL ION CONCENTRATION HIGH OXYGEN CONCENTRATION
- HIGH METAL ION CONCENTRATION LOW OXYGEN CONCENTRATION
METAL ION CONCENTRATION CELL OXYGEN CONCENTRATION CELL
RIVETED LAP JOINT
• FOREIGN MATERIAL CREATES
PASSIVE FILM LOW OXYGEN REGION WHICH
PROTECTS EXPOSED PREVENTS THE RE-FORMATION
SURFACE OF PASSIVE FILM ACTIVE METAL
ACTIVE - PASSIVE CELL FIGURE 1-22. CONCENTRATION CELL CORROSION J. Intergranular Corrosion. Intergranular corrosion is an attack along the grain boundaries of a material. Rapid selective corrosion at the grain boundary can occur with subsequent delamination (see Figure 1-23). High-strength aluminum alloys such as 2014 and 7075 are more susceptible to intergranular corrosion if they have been improperly heat-treated and are then exposed to a corrosive environment.
Chap 1 Par 12 Page 29
FIGURE 1-23. INTERGRANULAR CRACKING AND CORROSION ON
A WINGSPAR CHORD Exfoliation Corrosion. Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted up by the force of expanding corrosion products occurring at the grain boundaries just below the sur- The lifting up or swelling is visible evidence of exfoliation corrosion (see Figure 1-24). Exfoliation is most prone to occur in rolled and wrought products such as extrusions, thick sheet, thin plate, and certain die-forged shapes. This is in contrast with cast products that tend to have a more homogeneous grain structure.
Chap 1 Page 30 Par 12
FIGURE 1-24. SEVERE EXFOLIATION CORROSION (Courtesy of Thomas Flournoy)
13.-199. RESERVED .
Chap 1 Par 12 Pages 31 through 42