internal stringer

Knowler's design replaced the internal stringers with external corrugations in the hull plating which needed no riveting and avoided the stringer-frame intersection.

In October, with a total of 75 hours in the air the buckling was more evident and the planing bottom had to be reinforced locally with internal stringers.

The investigation disclosed "evidence of fatigue cracks in the right wing's lower surface skin panel, with origins beneath the forward doubler.... The origin points were determined to be in rivet holes which join the external doubler and the internal stringers to the lower skin panel.