Corrosion and a Loose Rivet at the 100-Hour
During a 100-hour inspection on a 1979 two-seat trainer with 11,400 hours total time, an inspection panel in the left wing rear spar bay reveals white powdery deposits and flaking between the aluminium spar cap flange and the skin. After cleaning to bright metal, pitting on the 1.2 mm 2024-T3 flange measures 0.18 mm deep. The adjacent 1/8 in rivet is loose; with the rivet removed, the hole measures 3.30 mm against the nominal 3.20 mm. There is a second patch of light, uniform surface staining further outboard with no measurable depth. The manufacturer's structural repair manual permits blending of corrosion to a maximum of 10% of material thickness, and one oversize rivet step. The owner wants the aircraft back for the weekend and suggests 'cleaning it up and treating it'.
1. Assess the corrosion and decide whether it can simply be blended out.
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Two things control the decision: the type of attack and its depth. Flaking and white powder between a spar cap and the skin, with pitting rather than uniform staining, suggests intergranular or exfoliation corrosion, which runs along grain boundaries and is far more serious than surface attack - the depth you can measure is rarely the whole story. On a 1.2 mm 2024-T3 flange, a 10% blend allowance is 0.12 mm; the measured 0.18 mm is already beyond a simple blend-out. So this is not 'clean, alodine and go'. Clean to bright metal to establish the true extent, compare with the SRM allowable damage, and if it exceeds limits the part is repaired to an approved scheme or replaced, with engineering input. The uniform staining outboard is a separate, likely acceptable, item.
2. Specify the correct repair for the loose rivet and the limits you must respect.
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The loose rivet needs a proper repair, not a bigger rivet and hope. The hole is 3.30 mm against a nominal 3.20 mm for a 1/8 in rivet - within the range for the next oversize (5/32 in), which is one step only. Ream true, set the oversize rivet, and check edge distance (at least 2D) and pitch. If the hole were beyond one oversize step, or edge distance marginal, do not keep stepping up: that is where an approved repair (hi-lok or bolted doubler per the SRM or an engineer's scheme) is used. Do not fit a self-tapping screw, a nut-and-bolt in thin skin, or a rivet of the wrong alloy, and inspect under the head for corrosion before closing up.
3. What does releasing the aircraft require, and what else would you inspect before signing?
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Release to service requires the paperwork to match the work: a log entry describing the discrepancy, the reference used (SRM section or approved repair scheme), the parts fitted, and the signature of the appropriately licensed engineer. If the corrosion is beyond SRM limits you need a repair scheme approved by the manufacturer or the authority - the owner's weekend is not a factor in that decision. Before signing, inspect the mating surface under the spar cap, because the corrosion you can see usually has a partner you cannot; inspect the same area on the other wing; and check whether the area carries a corrosion-prevention treatment interval in the maintenance programme. Corrosion found once in a structure of this age is a sample, not an event.