The perfect composite airframe would consist of a one-piece lay-up using laminate and sandwich constructions. Because of tooling and material limitations, this is not yet possible and so like their metal counterparts, composite aircraft are an assembly of parts. Where metal airframes use mechanical fasteners to join parts, composite airframes use structural adhesive (or in some applications, combinations of mechanical fasteners and adhesives).
Remember building plastic model airplanes as a kid? This is approximately how composite aircraft are built except for the part about using advanced composite materials instead of polystyrene plastic and glue. Because of their excellent adhesion and strength properties, epoxies are the most commonly used adhesives in aerospace. The bond mechanism created between adhesive and composite parts is referred to as a secondary bond. It is a complex molecular union which is nearly as strong as the composite material itself. When applied within allowable bond thickness limits, epoxy adhesives have very high shear strengths (shear is parallel but opposing loads applied to the joint).
Whenever one is faced with the prospect of assessing damage to composite airframes, it is important to be familiar with the unique strengths and weaknesses of these constructions. Due to composite’s very high strength, one must be cognizant of “point of impact damage” as well as “load path damage”. It is not uncommon to have minor point of impact damage while sustaining major load path damage.
Damage assessment nearly always begins with a thorough visual inspection either as part of a scheduled inspection or as a special condition inspection which may arise from a reported (known) incident or event. It is important to discriminate between these two types of inspections. The amount of energy associated with damage from a known event may be used to determine the extent of the inspection(s), e.g., a fuel truck hitting a wing will have much more energy than a golf club hitting the wing. Ironically, both may result in similar point of impact damages. In the fuel truck scenario, analysis of the energy’s load path would be reason to inspect interior wing structural components, bonded joints as well as attach points and adjacent structures.
Visual inspections of painted surfaces are best accomplished by using a high intensity flashlight held at a low incident angle to the structure. Keeping your eyes at a low angle perpendicular to the light beam, sweep the structure with the beam. Circle all cracks, punctures, abrasions, and obvious shadows with a nonpermanent marker or grease pencil and complete the visual inspection. Using OEM publications or technical assistance, determine the structure of the encircled areas (laminate or sandwich).
Defects to laminates may be further investigated by visually inspecting the reverse or interior side. This may require the use of optical instruments such as flashlight/mirror or borescopes. Because of their translucent nature, all types of damage to fiberglass laminates will appear white. If you cannot make a determination of damage in this manner, it may become necessary to remove the exterior paint finish.
Sandwich basesheet punctures and cracks will be obvious and must be repaired. Less obvious defects may be further investigated by use of the “coin tap” test. Coin tapping can be highly subjective and requires practice as well as a consistent tap frequency and intensity to be effective. Nonpuncturing impacts often cause the basesheet and core to disbond. An area of disbond will have a distinctively dull timbre when tapped. It is important to tap in all directions around the suspect area for confirmation, shape, and size of the disbond. Surfaces adjacent to punctures and cracks should be similarly investigated. Damage to secondary bonds will appear as a crack in the adhesive joint. The crack will appear evident from both sides of the joint and will produce a dis-resonant sound when tapped.
Training and support
FAA AC No; 65-33; Composite Maintenance and Repair: The maintenance of composites is complex and requires knowledge and skills to assure the continued airworthiness of these products … Experience, classroom training, hands-on on-the-job training, and assessments all work together to ensure that the skills are developed to perform safe composite maintenance and repair.
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