When a composite aircraft skin is damaged — whether from a ground strike, hail, delamination, or impact — simply patching over the surface is rarely acceptable for structural repair. Unlike sheet metal, composite materials derive their strength from the fiber orientation and the matrix that holds those fibers together. Any repair must restore both the load path and the fiber continuity across the damaged region. Two of the most widely used techniques to accomplish this are the scarf repair and the stepped repair, each of which removes damaged material in a controlled geometry and replaces it with new plies bonded into the parent structure. Understanding the geometry, the ply-by-ply sequencing, and the quality control requirements for each method is critical for any Aviation Maintenance Technician (AMT) working on composite airframes.
Why Composite Repairs Are Different
In metallic structures, stress travels through a relatively isotropic material that spreads loads in all directions. Composites, by contrast, are anisotropic — their strength is directional, defined by the orientation of reinforcing fibers within each laminate ply. When material is removed to prepare a repair zone, the load-carrying fibers are severed. The repair must not only fill the void but must also re-establish a continuous load path with minimal stress concentration at the bondline. The key tool for achieving this is a gradual transition — either a smooth angled slope (a scarf) or a series of discrete flat steps — that maximizes the bonded surface area and reduces peel stresses at the edges of the repair.
The strength of an adhesive joint is proportional to the area over which the load is distributed. A butt joint, which simply abuts two pieces of material at a 90-degree edge, concentrates almost all stress at a single interface and fails at a fraction of the parent material strength. By sloping or stepping the interface, these techniques spread stress over a much larger bonded area, allowing a well-executed repair to approach or match original design strength.
Scarf Repair: Geometry and Process
A scarf repair tapers the repair zone into a smooth, continuous slope, like sharpening a pencil. The damaged area and all surrounding delaminated or compromised material is removed, and then the laminate is sanded or ground down at a shallow angle until a smooth conical or sloped surface is exposed. New repair plies, cut to match the original ply orientations, are then laid up into this tapered pocket and cured in place.
Scarf Ratio
The most important parameter in a scarf repair is the scarf ratio, which is the horizontal run of the taper divided by the depth of the laminate. FAA Advisory Circular and manufacturer structural repair manuals typically specify scarf ratios in the range of 20:1 to 50:1, depending on the criticality of the structure and the materials involved. A 20:1 ratio means that for every 1 millimeter of laminate depth, the scarf extends 20 millimeters outward from the damage boundary. This very shallow angle — only about 2 to 3 degrees from the surface — is intentional: it minimizes the abrupt load transfer at any single point and maximizes fiber overlap between the original and repair plies.
In practice, achieving a perfect scarf angle requires patience and precision. Technicians use pneumatic sanders with progressively finer grits, constantly checking the taper with templates or depth gauges. The surface must be clean, dry, and free of any contamination before repair plies are applied, because adhesive bond strength drops dramatically when moisture, oils, or release agents are present. Many composite repair operations require surface testing with a water-break test or solvent wipe to verify cleanliness.
Ply Sequencing in a Scarf
Each repair ply must be sized to step slightly beyond the previous one, matching the taper geometry. The innermost ply — the one closest to the tool surface — is the smallest and covers only the core of the damage. Each successive ply is larger, extending farther up the slope to overlap a fresh area of the original laminate. When stacking is complete, the top surface ply covers the entire scarf zone and feathers into the original skin. The fiber orientations of each repair ply must match the corresponding original ply exactly; reversing or skipping an orientation introduces imbalance and can cause warping or reduced strength in a specific load direction.
Stepped Repair: Geometry and Process
Where a scarf repair uses a smooth continuous taper, a stepped repair creates a series of flat, level shelves — each one corresponding to a single ply of the original laminate. The outer ply is removed first, exposing the ply beneath; then that ply is removed around a slightly smaller perimeter, and so on through the laminate stack, creating a staircase profile when viewed in cross-section. Each step provides a flat, well-defined bonding surface for the corresponding replacement ply.
Stepped repairs are especially common in thicker laminates and in repair stations with CNC routing capability, because a router can be set to precisely controlled depths to cut each step without inadvertently damaging the plies below. Manual step cutting with a sharp chisel or router requires considerable skill to avoid cutting too deep, which would introduce new damage into the underlying material.
Step Width and Ply Overlap
Each step in a stepped repair must be wide enough to provide adequate bond area for that ply. A commonly referenced minimum step width is 0.5 inch (approximately 12–13 mm) per ply, though structural repair manuals for specific aircraft may call for greater widths on primary structure. Narrower steps reduce bond area and concentrate peel stress at the step edge, which can initiate delamination under cyclic loading. The repair plies are then laid into the steps from the inside out, each one sized to fit its shelf and extend slightly beyond to feather into the next step up.
Curing: Wet Layup vs. Prepreg
Both scarf and stepped repairs can be executed with wet layup (dry fabric saturated with resin on the bench or in the repair zone) or with prepreg materials (fabric pre-impregnated with partially cured resin that must be stored at low temperature). Prepreg repairs typically produce higher fiber volume fractions and more consistent mechanical properties, but they require controlled heated tooling — usually a heat blanket and vacuum bag assembly — to achieve proper cure temperatures, often around 250°F (121°C) for elevated-temperature prepregs. Wet layup repairs may cure at room temperature or with mild heat, but achieving adequate structural properties depends heavily on correct resin-to-fiber ratios and the absence of voids.
Vacuum bagging is used in both cases to consolidate the repair plies, remove excess resin, and prevent voids. A typical vacuum bag assembly includes the repair plies, a perforated release film, breather cloth to distribute vacuum across the surface, and an outer bag film sealed to the surrounding skin with vacuum tape. A vacuum level of at least 22 inches of mercury is commonly required to ensure adequate compaction.
Inspection After Repair
Once cured, the repaired area must be inspected to verify bond quality and the absence of voids or delaminations. Common non-destructive inspection (NDI) methods used on composite repairs include coin tap testing (a change in tap sound indicates a disbond), ultrasonic inspection (pulse-echo or through-transmission), and in some cases, thermographic inspection. The coin tap, while simple, is useful for initial screening but has limitations in detecting deep or small defects. Ultrasonic methods provide more reliable detection of internal disbonds and are required for many structural repairs. The allowable void content in a bonded repair area is typically very small — often less than 2–3% total void area per the applicable Structural Repair Manual (SRM).
Key Numbers and Rules
- Scarf ratio: typically 20:1 to 50:1 (horizontal run to laminate depth); shallower ratios for more critical structure.
- Step width: minimum approximately 0.5 inch per ply for stepped repairs; follow the SRM for exact dimensions.
- Vacuum level: at least 22 inches of mercury for consolidation during cure.
- Elevated-temperature prepreg cure: commonly around 250°F (121°C), requiring heat blankets and controlled ramp/soak cycles.
- Allowable void content: typically less than 2–3% in the bonded area per NDI after cure.
- Ply orientation matching: each repair ply must exactly reproduce the fiber direction of the ply it replaces.
- Damage removal boundary: all delaminated, disbonded, and moisture-contaminated material must be removed before the repair taper begins.
Common Test Traps
- Confusing scarf ratio direction: a 20:1 ratio means 20 units of horizontal distance for every 1 unit of depth — not the other way around. A steeper (lower number) ratio means less bond area and lower strength.
- Skipping ply orientation matching: test questions may imply that any fiber direction will restore strength. In reality, every repair ply must match the original orientation of its corresponding layer exactly, or the repair will be mechanically unbalanced.
- Assuming room-temperature cure is always acceptable: prepreg materials require elevated-temperature cure to achieve rated properties; substituting a room-temperature wet layup on a prepreg structure is not an approved substitution without engineering data.
- Overlooking moisture removal before repair: composite laminates can absorb moisture, especially around damage sites. Moisture trapped under repair plies will vaporize during elevated-temperature cure and create voids. Drying the repair area in a low-temperature oven before commencing the repair is typically required.
- Treating coin tap as a complete NDI method: while useful for initial screening, coin tap alone does not satisfy the inspection requirements for most structural repairs. Ultrasonic inspection is almost always required to verify bond integrity of the completed repair.