Dent With a Dull Tap on a Composite Flap Trailing Edge
After a ground-handling incident, a carbon/epoxy flap trailing-edge skin 2 mm thick shows an 8 mm dent, 0.4 mm deep, with cracked paint but no visible broken fibres or puncture. The aircraft then stood outside in tropical humidity for three weeks; skin temperature at inspection is 45 degrees C. Tap testing with a coin gives a dull response over roughly 15 mm around the dent compared with crisp tapping nearby. The SRM defines damage categories and allowable damage limits and mandates NDT for any dent, but the illustrated damage chart page is missing from the hangar copy. The technician suggests filling the dent with body filler, painting it, and signing it off as 'dent within limits'.
1. Set out the assessment sequence required before any disposition, and explain why the tap test cannot be the basis for a decision.
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Sequence
- Record and protect: mark, photograph with a scale, note location relative to edges, fasteners and substructure, measure dent depth and diameter.
- Clean only with approved materials - no aggressive solvents, no abrasion that destroys evidence.
- Map the damage: visual plus tap test plus NDT (pulse-echo or through-transmission ultrasonic, or a bond tester as the SRM specifies) to size delamination, establish depth, check the back face and the bond to substructure.
- Compare the measured dent and any delamination with the allowable damage limits / damage categories in the current SRM.
- Disposition: within limits (no repair, monitor), repairable (bonded or bolted per SRM), or beyond limits (replace).
Why tapping is not enough: it is subjective and operator-dependent, it only responds to fairly large near-surface disbonds, and it cannot size depth or area. A thin skin over trailing-edge substructure changes the response anyway. It misses matrix cracking, fibre failure and tight delamination - which is exactly what a 0.4 mm dent (20% of the 2 mm skin) may hide. A dull tap is a trigger for NDT, never a disposition.
2. Explain why filling and painting is the wrong action here, and what the three weeks in tropical humidity mean for any later bonded repair.
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Filling and painting
- It hides damage from the next inspection, adds mass, may be incompatible with the epoxy matrix, and restores no strength whatsoever.
- 'Within limits' cannot be asserted when the limits are not available. Signing off without approved data is an airworthiness documentation failure, not a shortcut.
Moisture
- Epoxy laminates absorb moisture (of the order of 1% by weight). If a bonded repair is cured on a wet laminate, trapped water turns to steam at cure temperature, producing porosity and blowholes and a weak bondline. Moisture also lowers the glass transition temperature of the matrix.
- The repair scheme will therefore require controlled drying (heat blanket, vacuum bag, specified temperature and time), a moisture check before bonding, correct surface preparation, and a workspace with controlled temperature and humidity.
- Skin temperature of 45 degrees C matters: adhesives have work-life and cure limits, and the structure must be within the specified bonding window. Ambient soak conditions must be checked, not assumed.
3. What data and records must exist before the aircraft can be released?
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- The current, complete SRM including the damage category charts and allowable damage limits (obtain the correct revision - do not work from a partial copy).
- An NDT report naming the method, equipment, calibration and the qualification of the technician, plus a map of any delamination.
- Engineering disposition if the damage falls outside the limits, with an approved repair scheme.
- Repair records: materials with batch and shelf life, cure records (temperature and time), surface preparation, and post-repair inspection (tap and/or NDT).
- Weight and balance update if a doubler or repair patch is added, and a check of the effect on adjacent structure and fastener rows.
- Formal return-to-service certification by authorised staff, with the defect and the disposition entered in the aircraft records.