What method is used to attach a seam where there is no underlying structure?

The method used to attach a seam where there is no underlying structure is a sewn seam. This method is particularly effective for fabric covering in aircraft, especially when there is no solid framework beneath the fabric to provide support.

Sewn seams involve stitching the fabric pieces together using a sewing machine or hand-stitching techniques, creating a durable connection that can withstand the stresses encountered during flight. This type of seam is widely used in fabric covering applications because it allows for flexibility and is compatible with the materials typically used in aviation fabrics.

While other options like bonded or adhesive seams can be effective in certain contexts, they are generally more suited for applications where the fabric can adhere strongly to a substrate or where a continuous, smooth surface is required. Heat welded seams can also be useful but are focused on thermoplastic materials and specific applications rather than standard fabric covering methods used in aircraft.