Spar Carry-Through Corrosion and Elongated Rivet Holes
During an annual inspection of a 1978 low-wing trainer with 6,200 h total time, the engineer finds white powder and pitting on the wing-spar carry-through structure beneath the cabin floor. Twelve rivets in the lower wing skin aft of the rear spar are loose; one rivet hole is elongated about 0.8 mm and the skin around it is distorted. The aircraft maintenance manual states that rivet-hole elongation greater than 0.5 mm, or any corrosion of the spar carry-through beyond blend-out limits, requires repair by an approved scheme. The owner has a 40-hour flying school contract starting in three days and asks whether the aircraft can be released with the corrosion treated and the loose rivets replaced by oversize rivets. The aircraft has been hangared near the coast for 15 years; the battery box and nearby structure also show acid residue.
1. Classify the structural significance of the two findings and explain why they are not equivalent.
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The two findings are not equivalent. The spar carry-through is a primary load path: corrosion pitting reduces section thickness and can initiate fatigue cracks, and blend-out limits exist because the structure must carry flight and landing loads. The rivet-hole elongation is a local load-transfer problem, but 0.8 mm exceeds the manual’s 0.5 mm limit, meaning the hole has yielded and the joint is no longer transmitting load as designed; oversize rivets alone may not restore fatigue life. The loose rivets suggest the whole joint has been working. Both findings are structural, but the carry-through corrosion is the more serious because a failure there affects the wing attachment. Neither can be assessed by a visual “treat and fly” decision.
2. What should the engineer do before making an airworthiness decision?
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Before an airworthiness decision, the engineer should clean the area to bare metal, measure material loss against the structural repair manual, and perform NDT-dye penetrant or eddy current-on the carry-through and surrounding skin. The rivet holes should be measured with a hole gauge and the joint inspected for cracks; if elongation exceeds limits, the approved repair is likely a repair scheme or part replacement, not simply oversize rivets. The battery-box acid residue must be neutralised and the adjacent structure inspected for corrosion. Any repair must be approved and recorded, and the aircraft must be released by an appropriately approved maintenance organisation. A borescope or disassembly may be needed to inspect hidden faces.
3. The owner proposes treating corrosion, replacing rivets, and flying 40 hours. Give a reasoned response.
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The owner’s proposal is not acceptable. Treating visible corrosion does not restore the strength of pitted carry-through structure, and replacing loose rivets with oversize rivets without an approved repair scheme may leave the joint outside its fatigue and strength limits. The manual’s limits are airworthiness limits, not recommendations. The aircraft should remain grounded until the corrosion is blended out within limits or repaired, the rivet holes are repaired to an approved scheme, and the battery-box area is neutralised and inspected. A 40-hour school contract is a commercial pressure, not an airworthiness argument. If a ferry is contemplated, it would need a specific approval; it is not a way to avoid the repair.