Agustawestland Philadelphia AW139 N639NA accident near Galliano, Louisiana

Probable cause

The fatigue failure of a tail rotor blade erosion shield extension, which resulted in in-flight damage to a main rotor blade.

NTSB analysis

During a business flight over the Gulf of Mexico, the crew felt a slight vibration of the tail rotor with no other anomalies noted and landed the helicopter uneventfully at their planned destination. A postflight inspection revealed that one of the tail rotor blade erosion shield extensions, also known as a lightning strip (LS), was missing and that only its side tabs remained attached to the blade. One of the main rotor blade tips sustained substantial damage when the LS separated from the tail rotor blade and impacted the bottom of the main rotor blade.

Metallurgical analysis of the fractured LS revealed microcavities in the metal, which resulted in a fatigue fracture. The helicopter manufacturer concluded that the microcavities were a direct result of the electroplating manufacturing process. Finite element modeling (FEM) of an exemplar LS revealed that the maximum stress values expected during flight were considerably lower than the fatigue limit specified for the electroplated nickel material. The microcavity observed on the accident LS could not be replicated by the FEM model, but the presence of a microcavity could adversely influence the in-flight stresses and reduce the overall strength of the LS. Additional flight testing with strain gauges did not reveal any load condition that would lead to similar LS failures that were experienced during the accident flight. It is likely the LS separated in flight due to a fatigue fracture that was the result of increased stress in the presence of a material flaw that had existed since its manufacture.

The helicopter manufacturer, in response to multiple reports of similar LS failures, changed the material specification from electroplated nickel to a higher strength steel. The steel LS will be installed on all newly built helicopters, and an optional service bulletin will be issued to operators to provide the steel LS upon request.

Text from the NTSB. Open the NTSB docket for CEN16LA401 for the full record.