Composite Repair Decision on a Wing Spar Cap
A light training aircraft has a delaminated carbon-fibre wing spar cap found during inspection. The damaged zone measures 42 mm along the span and lies 18 mm from the spar cap edge. The approved structural repair manual permits a bonded scarf repair only when the damage is at least 25 mm from any free edge and no longer than 35 mm in the spanwise direction. A bolted doubler is permitted for damage up to 60 mm provided edge distance stays above 20 mm. The operator needs the aircraft flying in four days. The bonded repair kit has a 72-hour cure at ambient temperature plus a 24-hour post-cure inspection hold. The bolted doubler adds 0.6 kg to the wing and requires rebalancing the aileron. The chief engineer must choose a repair path that is both airworthy and defensible to the regulator.
1. Evaluate whether the bonded scarf repair is permissible under the manual, and explain the consequence for the operator's four-day timeline.
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The bonded scarf repair is not permissible. Two independent limits are breached. First, the manual requires damage to be at least 25 mm from a free edge, but the damage is only 18 mm from the spar cap edge, so the edge-distance condition fails. Second, the manual caps spanwise length at 35 mm, and the damage is 42 mm, so the length condition also fails.
Because either breach alone disqualifies the method, the engineer cannot approve the bonded repair by arguing the damage is 'close enough'. The timeline then collapses: the cure alone takes 72 hours, plus a 24-hour post-cure inspection hold, meaning 96 hours (four days) before the aircraft is even inspectable. Even if the geometry had passed, the aircraft would not be released inside the operator's window. The engineer must therefore reject the bonded option on both structural and scheduling grounds.
2. Assess the bolted doubler option against the stated constraints and recommend a course of action.
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The bolted doubler is geometrically acceptable: 42 mm is below the 60 mm limit, and the 18 mm edge distance is above the 20 mm minimum? It is not. The edge distance is 18 mm, which is below the 20 mm minimum, so the doubler also fails its edge-distance rule. This is the trap: the length limit passes, but the edge limit does not.
Even setting edge distance aside, the doubler adds 0.6 kg and requires aileron rebalancing, which is additional work and a new approval step. So neither manual repair path is available as written.
The defensible action is to stop and escalate: seek an engineering disposition or approved deviation from the manufacturer, or route the aircraft to a heavier repair scheme with substantiation. The engineer should document both failed limits and not release the aircraft on an unapproved field repair just to meet the four-day deadline.