Composite Stabilizer Lightning-Strike Damage
A composite horizontal stabilizer on a regional jet suffers a lightning strike near the leading edge. The strike entry point is in a lightning Zone 2 area. External damage is limited to a 2-in by 1-in area of burned paint, but tap testing reveals a 3-in by 2-in dull area. The stabilizer skin is a 12-ply carbon/epoxy laminate over 0.5-in honeycomb core. The maintenance manual permits bonded repair of Zone 2 skin damage up to 2 in in diameter if there is no core damage, no fastener involvement, and the repair is within the approved scarf ratio. Ultrasonic inspection indicates a 0.2-in-deep core crush beneath the skin disbond. The aircraft is away from its main base, and the operator has a replacement stabilizer available at the next station.
1. Determine whether the damage is within the approved bonded-repair limits and justify the disposition.
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The external paint damage is small, but the internal findings control the disposition. The tap-test disbond is 3 in by 2 in, which already exceeds the manual’s 2-in-diameter bonded-repair limit for Zone 2 skin damage. More importantly, ultrasonic inspection shows 0.2 in of core crush. The stated manual allowance requires no core damage. Therefore the damage is outside the approved bonded-repair limits and cannot be released under that allowance.
The correct disposition is to treat the stabilizer as unairworthy until an approved repair or replacement is installed. A simple skin patch would not restore the core crush or the load path. The operator should not fly the aircraft to the main base under a ferry permit unless the relevant authority and OEM approve a specific ferry limitation. The available replacement stabilizer at the next station is the practical route.
2. Outline the engineering and inspection requirements for a repair or replacement, including lightning-protection restoration.
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A repair in a lightning Zone 2 area must restore both structural capability and lightning protection. If the OEM approves a repair, it will normally require removal of the damaged skin and core back to sound material, a scarf ratio specified for the 12-ply laminate, core replacement or potting if allowed, and a bonded or bolted repair per the SRM. The repair must be vacuum-bagged and cured at the specified temperature and pressure, with cure records retained. Fastener holes, if any, must meet edge-distance and spacing requirements and be protected against galvanic corrosion.
Lightning protection is a separate airworthiness item. The conductive mesh, foil, or coatings must be restored across the repair and bonded to the surrounding structure with the specified electrical resistance. A repair that is structurally sound but leaves a lightning-protection gap can cause a subsequent strike to attach at the repair boundary. After installation, inspect the repair with tap testing and ultrasonic inspection as specified, verify electrical bonding resistance, and record the repair in the aircraft records. If the stabilizer is replaced, verify the part number, serial number, and lightning-protection configuration before installation.
3. What airworthiness decision should the certifying engineer make if the operator proposes a temporary patch and flight to the main base?
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The certifying engineer should reject the temporary patch as a normal return-to-service action because the damage exceeds the approved bonded-repair limits and involves core crush. A temporary patch does not restore the core or the load path in a primary stabilizer. Flight to the main base would require a specific ferry authorization with an approved engineering disposition and operating limitations, not a line-maintenance patch. The engineer should document the damage, the ultrasonic findings, and the reason for rejection. If no approved ferry or repair is available, the stabilizer must be replaced at the current location. The decision must be based on structural substantiation, not on the operator’s schedule.