Why should one hole be left undrilled at each end of the stringer when drilling pilot holes?

Leaving one hole undrilled at each end of the stringer during the drilling of pilot holes is primarily a matter of practical assembly mechanics. Specifically, the undrilled holes help avoid potential interference issues with C-clamps used in securing the parts together during the assembly process. By not drilling through the ends, there is enough material to provide a solid grip for the clamps without risking damage to the stringer or affecting its alignment and structural integrity when the assembly is held in position.

While other considerations can be important, such as avoiding structural weakness or adhering to standard practices, the interference with tools like C-clamps directly addresses a common problem faced in assembly work, thereby ensuring a smoother and more efficient drilling and assembly process.