Corrosion Found Beyond the Inspection Zone
During a scheduled 200-hour inspection on a light twin, a B1.1 engineer removes an inspection panel on the lower wing skin and finds intergranular exfoliation corrosion around 6 adjacent rivets, covering roughly 40 mm × 25 mm. The approved maintenance data defines this area as a non-critical inspection zone and gives a blend-out limit of 10% of skin thickness, with re-inspection at the next 100-hour check. The skin thickness is 1.2 mm. The engineer's blend-out attempt reaches 0.14 mm removal before the corrosion is fully gone, which is about 11.7% of skin thickness. The aircraft is booked on a revenue flight in 6 hours. The structural repair manual is available, but the engineer has not used its blend-out chart for this skin before.
1. Determine whether the blend-out is acceptable under the approved data and what the engineer must do next.
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The approved data permits blend-out up to 10% of skin thickness, which is 0.12 mm on a 1.2 mm skin. The engineer has removed 0.14 mm, about 11.7%, so the blend-out exceeds the limit and the repair is no longer covered by the routine allowance. The engineer cannot sign the task off as a simple blend-out. The next step is to consult the structural repair manual's blend-out chart for this specific skin and rivet pattern; it may permit a larger local blend with additional rivets or a doubler, or it may require engineering disposition. The engineer should not continue removing material to chase the corrosion without that authority. The 6-hour flight deadline is irrelevant to the airworthiness decision; if approved data cannot be satisfied in time, the aircraft does not fly with that defect.
2. Explain how the engineer should assess the structural significance of the remaining material and the inspection interval.
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With 1.06 mm of skin remaining, the engineer must confirm from the structural repair manual that the residual thickness still meets the required minimum for the zone, accounting for the rivet load transfer around the 6 rivets. If the manual gives a minimum allowable thickness or a doubler requirement for this blend depth, that governs. The engineer should also check whether the exfoliation has propagated under the rivet heads, since intergranular corrosion can run along grain boundaries beyond the visible area; a tap test or local removal of rivets may be needed. The re-inspection interval of 100 hours in the original data assumed a blend within limits, so if a larger blend is approved, the manual may specify a shorter interval or a permanent repair. The engineer must document the actual depth, the disposition source, and the revised inspection requirement.