This chapter discusses the investigation of a helicopter tail rotor blade that fractured during a test flight. It includes images of the damaged blade along with close-ups of both sides of the blade tip showing that the tip tore off at the rivets. Based on their observations, investigators concluded that the rotor blade encountered a foreign object in flight causing the tip to shear off.

A helicopter lost the outboard rib on a tail rotor blade in flight and was forced to land because of the resulting vibrations. The investigation that followed is described in this chapter along with key findings. As shown in a sketch, the rib is held in place by a set of six rivets. All of the rivets on the failed blade were missing and sections of skin were torn from most of the rivet holes. One such rivet hole was examined in a SEM, revealing corrosion on one of the tear surfaces and dimples (characteristic of ductile overload failure) on the other. In addition, the inner surface of the skin nearest the rib was found to be coated with soot, the paint on the leading edge of the top skin was abraded, and the skin in that area had thinned. Based on their findings, investigators concluded that the outboard rib separated because of stress-corrosion cracking around the rivets, and erosion may have contributed.

The tail rotor blade of a helicopter developed a visible crack during service. The cracked region was removed from the blade and the fracture surface was examined in a SEM, revealing shallow pitting on the inside surface of the skin and a corresponding reduction in thickness. Based on these findings, investigators concluded that the failure was due to a fatigue crack initiated from a corrosion pit, which may have been caused by chemicals released by the burning of bonding resin.