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.
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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
CHAPTER 2. VISUAL INSPECTION PROCEDURES
200. GENERAL Generally, the inspection procedure to be used will be specified by the aircraft or component manufacturers or the FAA in documents such as maintenance or overhaul manuals, AD's, SSID's, or manufacturers' Service Bulletins. If, during maintenance, a condition is found on the aircraft being inspected which may recur on that aircraft or on another of its type, a certified inspector or other appropriate individual should create an inspection procedure covering the condition. Approval of such procedures should be in accordance with appropriate FARS.
201. MATERIAL CONTAINED IN A VISUAL INSPECTION PROCEDURE. Visual inspection procedures should all contain steps in a logical for- A procedure has four basic parts: (1) a basis for the inspection, (2) preparatory arrangements, (3) implementation of the inspection, and (4) evaluation of the results. Table 2-1 is a listing of all the elements to be considered when creating or reviewing an inspection procedure. Each of the items in the Table should be addressed in the procedure if relevant for the specific inspection.
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AC 43 - 204 8/14/97 TABLE 2-1. TOPICS TO BE CONSIDERED IN PREPARING OR
REVIEWING A VISUAL INSPECTION PROCEDURE
BASIS FOR INSPECTION Reason for and purpose of the inspection including any relevant background information.
References to controlling documentation, for example: Advisory Circulars, Airworthiness Directives, Manufacturers and general NDT Manuals, Maintenance and other relevant manuals.
controlling dates: Effective date of the inspection, date before which the inspection should be accomplished, inspection interval, date at which the inspection no longer is necessary.
Effectiveness; aircraft, engine, or part number.
Time and labor required for the inspection.
Conditions under which the inspection is no longer necessary.
PREPARATION Preparation and cleaning of the item being inspected.
Any special requirements of the inspection such as unusual surface preparation (etching, paint removal) or the temperature range over which the inspection is effective.
Identification of any equipment necessary for the inspection together with provisions for periodic calibration thereof.
Identification of the specific materials approved for the inspection. These may be included in a referenced Qualified Products List.
IMPLEMENTATION specific description of the area to be inspected.
Instructions for carrying out the inspection.
Description of the defect to be detected, preferably with an illustration of a sample defect.
Postcleaning instructions if required.
Instructions for disposition of the article if it cannot be returned to service.
EVALUATION Procedure for establishing inspection reliability.
Accept/reject criteria.
Reporting requirements.
202. ACTIVITIES APPLYING TO ALL VISUAL INSPECTIONS. Some activities which represent good practice may not be specifically called out in the procedures, but should be completed if they are relevant. Examples of such good practice are:
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8/14/97 AU 43-204
a. Preliminary Inspection. A preliminary inspection of the overall general area should be performed for cleanliness, presence of foreign objects, derormed or missing fasteners, security of parts, corrosion, and damage. If the configuration or location of the part conceals the area to be inspected, it is appropriate to use visual aids such as mirror or borescope.
b. Precleaning. The areas or surface of parts to be inspected should be cleaned without damaging any surface treatment which may be present. Contaminants that might hinder the discovery of existing surface indications should be removed.
cleaning methods may remove indications of damage; care should be used if the cleaning tends to smear or hide possible indications of trouble. Surface coatings may have to be removed at a later time if other NDI techniques are required to verify any indications that are found. Some typical cleaning materials and methods used to prepare parts for visual inspection are detergent cleaners, alkaline cleaners, vapor degreasing, solvent cleaners, mechanical cleaning, paint removers, steam cleaning, and ultrasonic cleaning.
Corrosion Treatment. Any corrosion found in the preliminary inspection should be removed before starting a close visual inspection of any selected part or area. Manufacturers' handbooks, when available, are a good general guide for treatment of corrosion. Recommendations of AC 43-4A, if appropriate, should be used on aircraft for which the manufacturer has not published a recommended corrosion inspection schedule or treatment program.
The AC contains a summary of current available data regarding identification and treatment of corrosion on aircraft structure and engine materials. Examples of types of corrosion damage detectable by the visual method are also given.
d. Use of Visual Aids. When inspecting the area required, visual aids should be used as necessary. An inspector normally should have available suitable measuring devices: a flashlight and a mirror. Chapter 3 provides information on lighting techniques that can be used when inspecting for defects in different types of materials.
RECORD KEEPING. All defects found should be documented by Maintenance Record entry, written report, squawk sheet, photograph, or video recording for appropriate evaluation. Depending on the rules governing the facility and person performing the inspection, the report may be limited to simpiy reporting findings without any rejection or acceptance disposition. The type, location, and approximate size of any defects present should be documented. Based upon the particular inspection process specification (i.e., AD, Service Bulletin, Maintenance Manual requirement, or normal inspection discovery) the mechanic, repairman, or other authorized person should determine the particular acceptance, re- Chap 2 Par 202 Page 45
work, repair or rejection status of the part or structure being inspected.
a. Findings. One of the most important elements of the visual inspection process is the nature of the report of findings.
As a guide, the record of findings for any defects should be kept in a manner as to permit others who may be doing the same inspection to have the benefit of any previous information found and experience gained.
b. Maintenance of Records. The full value of visual inspection can be realized only if records are kept of the conditions found on parts inspected. The size and shape of the finding and its location should be recorded along with other pertinent information, such as rework performed or disposition. The inclusion of some permanent record of a defect on a report makes the report much more complete.
c. Illustrations as Records. It should be stressed that frequent and effective use of illustrations not only will enhance the effectiveness of the procedure but is indispensable in communicating to the inspector the nature of the defects to be found.
In addition to textual and anecdotal data, the following types of
(1) Sketches. The simplest record is a sketch of the part showing the location and extent of the defect. On large areas it may be sufficient to sketch only the critical area.
(2) Photography. Photographs (still or video recording) of defects can be taken for visual record purposes. Photographs produce a permanent and highly descriptive record since they show both size and location on the part. They are permanent, reproducible, and the required equipment is readily available. It is good practice to include a scale in the photo when practicable as well as some marking for identification. This is particularly necessary if the photograph is likely to become an exhibit involved in litigation.
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204. FOUR LEVELS OF VISUAL INSPECTION.
a. Inspection Tasks. Visual inspection tasks are divided into four categories relating to their difficulty and degree of effectiveness as follows:
• Walkaround Inspection.
General Visual Inspection.
• Detailed Visual Inspection.
• Special Detailed Visual Inspection.
b. Additional Special NDI. An additional category may be used when visual inspection is supplemented by specialized NDI equipment.
05. ACCEPTABLE PRACTICE FOR VISUAL INSPECTION. Acceptable prac ice for the four levels of visual inspection is defined in the following paragraphs. For each level, the practice is divided into the four parts of the inspection procedure described in Table 2-1. Details of the inspection procedures will vary for different types of operators and aircraft; for example, the walkaround for a general aviation aircraft and a commercial jet will have major differences. It is important however, that the procedure for each aircraft type in a given service be carried out with the same care and detail every time the aircraft is inspected. Also, in all inspections described herein, the inspector should have available and be familiar with the documentation and reporting forms and the ground rules mandating the inspection. Refer to Appendices A through F for sample visual inspection procedures and equipment.
Level 1. Walkaround. The walkaround inspection is a general check conducted from ground level to detect discrepancies and to determine general condition and security.
NOTE: This is the only one of the four inspections that may be accomplished by either flight or maintenance personnel. The focus and perspective will vary based on the relation of the inspection to flight or maintenance operations.
Basis for Inspection. Most maintenance instructions mandate walkaround inspections on a periodic basis. The overall purpose is to serve as a quick check to determine if detectable inconsistencies exist which would affect the performance of the aircraft.
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Preparation for the Inspection. Aircraft history should be used to gain information useful in inspecting the aircraft (e.g., are there recurring problems or have there been hard landings?). In addition the aircraft should be clean enough for an effective inspection to take place, the necessary tools and equipment should be available (e.g., flashlight, rag, notebook), and other aids, tools, and procedures may be necessary (e.g., inspection of some aircraft is easier if already on jacks, but this is not always necessary).
(3) Implementation. Beginning the inspection at the nose, the inspector should approach the inspection from the perspective of the pilot and mechanic by, first, determining general condition and, second, identifying maintenance items. A good rule is to check for items affecting safety, legality, efficiency, and comfort. The following paragraphs give a partial listing of things to look for. Each aircraft will be different, but a surprising number of different aircraft have similar struc-
• Observe left and right side of fuselage and left and right wings. (Does the aircraft list?) Work to your left. Check general condition of paint.
Check exterior surface components within reach.
Check windows. Check engine, propellers or fan blades, exhaust area, and pylons. Check leading edges everywhere. Check control surfaces for slop, wear, and security. Check each gear well. Check all entry and exit points to the aircraft.
static dischargers. Observe surfaces at different angles using available light to enhance surface evaluation.
• Examine according to what the standard condition (Question the existence of any unusual condition.) Look for anything different from one side to another. (It is important to shake, push, pull, listen, and feel when possible.) Run your hand over skin junction areas or composite surfaces.
• Walk around twice. (Observations during the first cursory walkaround the aircraft should be used to determine the general condition. On the second time around revisit the areas noted on the initial walk and look for other discrepancies that may be revealed through closer scrutiny of a specific This is when specific locations should be observed.)
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8/14/97 A0 43-204
• Are there major dents or intrusions in the skin?
Look for evidence of flexing parts, waves in the skin, weave or bubble in fiber glass or composite components, eroded fairings, and bulging or flattened seals. Are any external components bent?
there evidence of damage? Check windows for crazing, dirt, and pitting.
• Rivet characteristics should be noted. Observe rivets for damage. Look for loose broken or missing rivets. Localized chipping of paint, cracked paint on sealant, or fretting corrosion are indicative of movement. Look for smoked or stained riv-
• Check all venting for leaks. Check all input tubing to insure it is clear. Check all antennas for chipping and strikes. Check for obstructions in pitot tubes, static and engine pressure orifices, and temperature venturies.
(4) Findings. Initial findings should be recorded in a personal notebook as soon as discrepancies are discovered. Do not rely on memory: Findings should be transferred to the aircraft log or official record of discrepancy as soon as possible.
Recommendations for action will depend on whether the inspection precedes flight operations or maintenance and whether the discrepancy is safety or flight critical.
b. Level 2. General. A general inspection is made of an exterior with selected hatches and openings open or an interior, when called for, to detect damage, failure, or irregularity.
(1) Basis for Inspection. When a specific problem is suspected, the general inspection is carried out to identify, if possible, the difficulty. General inspections are also routinely used when panels are open for normal maintenance.
(2) Preparation for the Inspection. Ensure cleanliness of the aircraft. The necessary tools and equipment required may include flashlight, mirror, notebook, droplight, rolling stool, tools for removal of panels, ladders stands, or platforms. Other aids such as jacking of the aircraft may or may not be discretionary; knowledge of a specific aircraft may be essential; and common problems may require information, even if not on the inspection card.
Implementation. General looking is not enough. As this pictures, You should continually ask Be inquisitive. Question whether you have seen "What is wrond with chap 2 Par 205 Page 49