Summary of Aircraft Fabric Covering and Maintenance

## Introduction Fabric-covered aircraft remain common in light aviation and vintage restorations. Proper maintenance of fabric coverings ensures structural integrity, flight safety, and longevity of the airframe. This guide breaks down key inspection, repair, and finishing practices for fabric-covered wings and surfaces. > Definition: Fabric-covered aircraft use textile materials (cotton, linen, or synthetic polyester) bonded or sewn to the airframe and treated with finishes (dope, varnish) to create a taut, aerodynamic skin. ## Materials and treatment ### Common fabrics - **Natural:** Cotton and linen — traditional, require mildewcide in finishing coats. - **Synthetic:** Polyester — durable, heat-shrinkable with an iron or heated blower. > Definition: Dope is a finishing liquid (often nitrate or butyrate cellulose) applied to fabric to tighten and seal the surface. ### Preparing structure before covering - **Dope-proofing:** Apply to parts of the structure that the fabric may contact. This prevents the first coat of dope from bonding the fabric to the structure when the fabric sags before it tightens. - **Wood spars:** Finish with **transparent varnish** so rot or decay can be visually detected. - **Steel tubing interiors:** Protect with **hot linseed oil** to inhibit corrosion. ## Applying finishing tape and seam repairs ### When to apply finishing tape - Apply the finishing (reinforcing) tape after the second coat of dope has dried and the fabric nap has been sanded off. This ensures a smooth adhesive surface and strong bond. ### Hand-sewing panels - Use a **baseball stitch** and lock the stitch every 8–10 stitches when sewing in a panel of new fabric. This provides secure attachment and distributes load. > Definition: Finishing tape (also called reinforcing tape) is a strip applied over fabric seams and rib areas to strengthen and seal sewing lines. ## Tears and repair methods ### Assessing L-shaped tears - For an L-shaped tear where each leg exceeds 14 inches, a repair method depends on aircraft speed: - If the **never-exceed speed (V_NE)** is less than 150 mph, a **doped-on repair** may be acceptable. - Faster aircraft require more robust repairs (patching, resewing, or replacement) per manufacturer or regulatory guidance. ### Anti-tear strips - An **anti-tear strip** is a strip of the same fabric laid over the rib between the reinforcing tape and the fabric to prevent tear propagation. - Required when the aircraft has **V_NE > 250 mph**. > Definition: Anti-tear strip is a reinforcing fabric element installed over rib areas to stop tears running along structural members. ## Drainage and moisture control - **Drainage grommets** should be installed at the lowest point in each wing bay. Commonly one grommet is placed on each side of a wing rib on the underside near the trailing edge. These allow moisture to escape and reduce corrosion and mildew risk. > Definition: Drainage grommets are small openings fitted with corrosion-resistant eyelets that allow trapped water to drain from wing bays. ## Testing and acceptable deterioration ### Fabric strength testing - Approximate test: use an **FAA-approved fabric punch tester** to estimate strength. - Definitive test: **pull-test** a one-inch-wide sample to measure actual tensile strength. ### Minimum allowable strength - Fabric is considered unairworthy if it has deteriorated to less than **70% of the required strength** for that aircraft. Inspectors must verify strength by approved methods. ## Surface finishing and paint removal - For **cotton and linen**, mix a **mildewcide** into the first coat of dope to protect against mildew. - When using **paint remover**, cover the treated surface with **polyethylene sheeting** (e.g., a paint drop cloth). This prevents solvent evaporation and gives remover time to penetrate the old finish. ## Practical examples and applications 1. Example — finishing tape application: - After recovering a wing and applying two coats o