To ensure proper alignment along the curved surface of the doubler, which type of scale should be used?

Using a flexible scale is ideal for ensuring proper alignment along the curved surface of a doubler because its adaptability allows it to conform to the shape of the surface being measured. Unlike a rigid scale, which maintains a straight edge, a flexible scale can easily wrap around curves and contours, providing precise measurements and allowing the mechanic to accurately assess alignment without forcing the tool into an unnatural position. This flexibility is critical in aerospace assembly, where even small misalignments can lead to significant structural issues.

A metal scale, while durable, does not possess the flexibility needed for curved surfaces. A rigid scale, like the metal scale, also cannot accommodate the contours required for accurate alignment on a doubler. A digital scale can be useful for some measurements, but it, too, may not offer the adaptability needed for curved surfaces. Therefore, a flexible scale emerges as the most appropriate tool for the task at hand.