Corrosion in Wing Spar
During a routine inspection of a 20-year-old light aircraft, corrosion is found on the lower wing spar flange at the wing root. The corrosion appears as white powder and pitting, covering an area of about 2 square inches. The aircraft is hangared near the coast, and the owner reports that it is washed weekly but not rinsed with fresh water. The maintenance manual specifies that any corrosion on the spar requires assessment and possible repair. The aircraft is used for private flying and has a current annual inspection. The inspector has access to a corrosion removal guide, a magnifying glass, and a borescope.
1. What type of corrosion is likely present, and what steps should be taken to assess its severity?
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The white powder and pitting on an aluminum alloy spar indicate probable exfoliation corrosion or pitting corrosion. The coastal environment and lack of fresh water rinse exacerbate this. Assessment steps: 1) Clean the area with a mild abrasive and inspect with a magnifying glass to determine the depth of pitting. 2) Use a borescope to inspect internal areas of the spar if accessible. 3) Measure the depth of pits using a depth gauge or by comparing with the allowable limits in the maintenance manual. 4) Refer to the corrosion removal guide for allowable blend-out limits. If the corrosion exceeds the allowable limits, the spar may need repair or replacement.
2. If the corrosion is within allowable limits, what corrective action is required, and what preventive measures would you recommend to the owner?
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Corrective action: Remove the corrosion by careful blending using approved abrasives, following the maintenance manual. After blending, apply a protective coating (e.g., primer and paint) to the affected area. Ensure that the blend does not exceed the allowable material removal. Preventive measures: 1) Rinse the aircraft with fresh water after each flight, especially in coastal areas. 2) Apply corrosion preventive compound (e.g., LPS-3) to the spar and other susceptible areas. 3) Increase inspection frequency for corrosion. 4) Consider hangaring in a humidity-controlled environment. Document the maintenance action and advise the owner on the importance of regular rinsing.