5-2
5.3 Recommended Depth of Inspection. Generally speaking, inspection requirements ensure adequate inspection of all compartments and interior aircraft cavities. Adequate maintenance is assured for most operating conditions when such general requirements are observed, along with a periodic check of a list of common trouble areas. To assist in complete coverage, the following summary is included:
5.3.1 D aily and Preflight Inspection . Check engine compartment gaps, seams, and faying surfaces in the exterior skin. Check all areas not requiring removal of fasteners or panels, such as bilge areas, wheel and wheel well areas, battery compartments, fuel cell and cavity drains, engine frontal areas, including intake vents, and engine exhaust areas.
5.3.2 Indepth Inspections. Remove screw-attached panels, access plates, and removable skin sections, as necessary, to thoroughly inspect internal cavities. Remove questionable heavy, internal preservative coatings for at least a spot check of the area underneath the coating. Inspect the aircraft interior in corrosion-prone areas, such as around lavatories, galleys, under floors, or baggage compartments. See Figure 5-1, Corrosion Found After Removing Cargo Door Threshold Covers.
5.3.3 Annual Inspections. Conduct corrosion inspections during annual inspections or other scheduled indepth inspections during which areas of the aircraft not normally accessible will be available for corrosion inspection.
5.3.4 Corrosion Preventive Compounds. Use the Original Equipment Manufacturer (OEM)-recommended corrosion preventive or compounds, such as LPS3, Dinol AV5, equivalent products, or advanced developments of these compounds, to effectively reduce the occurrence of corrosion. Review the results of corrosion inspections to help establish the effectiveness of corrosion preventive compounds and determine the reapplication interval of them.
Figure 5- 1. Corrosion Found After Removing Cargo Door Threshold Covers
5-3
5.4 Primary Approach. Corrosion detection is primarily detected through corrosion inspections done on a regular schedule. Early detection and treatment reduce costs, out-of-service time, and the possibility of flight or flight-related incidents. All corrosion inspections should start with a thorough cleaning of the area to be inspected. Then, visually inspect the area with a flashlight, inspection mirror, and a 5X to 10X magnifying glass, looking for obvious defects and suspected areas. A detailed inspection follows of damage or suspected areas found during the general inspection. The detailed inspection can be one or more of the following items:
5.4.1 Nondestructive Inspection (NDI).
5.4.1.1 Visual Inspection. Visual inspection is the most widely used technique and is an effective method for detection and evaluation of corrosion. Visual inspection employs the eyes to look directly at an aircraft surface, or at a low angle of incidence, to detect corrosion. Using the sense of touch of the hand is also an effective inspection method for the detection of hidden, well-developed corrosion. Other tools such as mirrors, borescopes, optical micrometers, and depth gauges can be used for visual inspections. The following shows the type of corrosion damage detectable during visual inspection: Figures 5-2, Corrosion Under Chipped and Loose Paint on Wing Skin, and 5-3, Corrosion Indicated by Blistering of Paint in Fuel Cell, show chipped, missing, and lifted paint; Figure 5-4, Popped Rivet Heads Resulting from Corrosion Products, shows dished and popped rivets; Figure 5-5, Skin Bulging Around Fasteners Caused by Pressure from Corrosion Products, shows skin bulges or lifted surfaces; Figures 5-6, Corrosion Cracking Between Fasteners on a Wing Spar, and 5-7, Faying Surface Corrosion on Wing Spar Chord with Corrosion Cracking Also Visible, show cracks; and Figures 5-8, Severe Crown Stringer Corrosion, and 5-9, Spar Chord Corrosion, show corrosion products.
5.4.1.1.1 Indications of corrosive attack can take several forms depending on the type of metal and the length of time the corrosion has developed. Corrosion deposits on aluminum and magnesium are generally a white powder, while ferrous metals vary from red to dark reddish-brown stains.
5.4.1.1.2 Sometimes structural members or equipment installations block inspection areas, or for some reason, the areas are awkward to check visually. Ensure adequate access for inspection by removing access panels and adjacent equipment, by cleaning the area as necessary, and by removing loose or cracked sealants and paints. Mirrors, borescopes, and fiber optics are useful in providing the means of observing obscure areas. Figures 5-10 through 5-18 depict some of these conditions.
Figure 5-2. Corrosion Under Chipped and Loose Paint on Wing Skin Figure 5-3. Corrosion Indicated by Blistering of Paint in Fuel Cell 5-4
Figure 5-4. Popped Rivet Heads Resulting from Corrosion Products Figure 5-5. Skin Bulging Around Fasteners Caused by Pressure from Corrosion Products 5-5
Figure 5-6. Corrosion Cracking Between Fasteners on a Wing Spar Figure 5-7. Faying Surface Corrosion on Wing Spar Chord with Corrosion Cracking Also Visible 5-6
Figure 5-8. Severe Crown Stringer Corrosion 1060 Figure 5-9. Spar Chord Corrosion 5-7
5-8 Figure 5-10. Removal of Fillet Seal from Internal Edge of Lap Joint Exposed Full Extent of Corrosion Fi gure 5-11. Removal of Insulation Blankets Exposed Corrosion
5-9 Figure 5-12. Corrosion Developed Under Leveling Compound Installed During Previous Repair of Overwing Exit Fi gure 5-13. Corrosion Behind a Structural Component on a Wing Spar
Figure 5-14. Severely Corroded Lap Joint WiL 59. 24 outba 36r. 12-18 Figure 5-15. Severely Corroded Lap Joint 5-10
Figure 5-16. Local Paint Removal Required to Expose Full Extent of Corrosion Figure 5-17. Corroded Area After Paint Removal from Wing Skin 5-11
5-12 Figure 5-18. Bonderite 1200 Treatment Prior to Painting May Highlight Presence of Remaining Corrosion or Cracks
5.4.1.2 O ther Methods. There are several NDI methods which may be valuable in detecting corrosion, such as liquid penetrant, magnetic particle, eddy current, x ray, ultrasonic, and acoustical emission. These methods have limitations, and only qualified and certified NDI personnel should perform them. Eddy current, x ray, and ultrasonic inspections require properly calibrated per-use equipment and a reference standard to obtain reliable results.
5.4.1.2.1 Li quid Dye Penetrant. Inspect large stress-corrosion or corrosion fatigue cracks on nonporous ferrous or nonferrous metals using this process. The dye will enter cracks, fissures, or other small openings by capillary action when applied to a clean metallic surface. After absorption by any surface discontinuities, remove excess dye and apply a developer substance to the surface. A developer acts like a blotter and draws the dye from the cracks or fissures back to the surface of the part, giving visible indication of faults on the surface. The magnitude of the fault is indicated by the quantity and rate of dye on the surface after developer application.
5.4.1.2.2 Magnet ic Particle Inspection. Use this inspection method to detect cracks or flaws on or near the surface of ferromagnetic metals, or metals attracted by magnetism. A portion of the metal is magnetized and coated with finely divided magnetic particles, either in liquid suspension or dry. Surface faults will create discontinuities in the magnetic field and cause the particles to congregate on or above these imperfections, pinpointing their location.
5.4.1.2.3 Eddy Current Inspection. This method detects thinning due to corrosion and cracks in multi-layered structures. Low-frequency eddy current testing can, to
5-13 some degree, detect or estimate corrosion on the hidden side of aircraft skins because it can measure an uncorroded material’s thickness when used with a reference standard. Low-frequency eddy current testing can estimate corrosion in the underlying structure because the currents will penetrate through into the second layer of material with sufficient sensitivity for approximate results.
High-frequency eddy current testing is most appropriate for detection of cracks which penetrate the surface of the structure on which test personnel place an eddy current probe, including flat surfaces and holes.
5.4.1.2.4 X Ray Inspection. Because it is difficult to obtain the sensitivity required to detect minor or moderate corrosion, x ray inspection has somewhat limited use for detecting corrosion. Briefly, this method passes high-energy x rays through a material; a special film on the opposite side of the material registers the passage of the x rays. Areas of high density display as underexposed areas, while areas of 1ow density show up as overexposed areas. Moderate to severe corrosion or cracks using x ray inspection can be detected. However, this method, like other NDI methods, requires a qualified and certified operator to obtain reliable results.
5.4.1.2.5 U ltrasonic Inspection. Ultrasonic testing provides a sensitive detection capability for corrosion damage when access is available to a surface with a continuous bulk of material exposed to the corrosion. Personnel commonly use ultrasonic inspection to detect exfoliation, stress-corrosion cracks, and general material thinning. However, ultrasonic digital thickness gauges are not reliable for determining moderate or severe damage prior to removing corrosion. Trained personnel should conduct the examination to derive useful information from indicating devices.
5.4.1.2.6 A coustic Emission Testing. This method uses heat-generated emissions to detect corrosion and moisture in adhesive-bonded metal honeycomb structures. Acoustic emission testing can detect corrosion initiation as well as advanced corrosion.
5.5–5.99. RESERVED.