Bell 412 N412PD accident near Brooklyn, New York

Probable cause

The fatigue fracture of the reduction gearbox output drive gear, which resulted in the loss of power output from the engines to the helicopter rotor blade system and a subsequent forced landing. The fracture of the output drive gear was caused by a fatigue crack that originated in a helical tooth root most likely as a result of hydrogen embrittlement during the manufacturing process.

NTSB analysis

The helicopter was on final approach to its home heliport following an uneventful local patrol flight. About 300 feet above the ground, the flight crew heard a loud sound from the engine compartment, which was immediately followed by a loss of main rotor rpm. The crew subsequently performed an autorotation to the water below, and upon contact with the water, the installed flotation devices deployed. Examination of the helicopter's reduction gearbox revealed an approximate 3-inch by 9-inch exit hole centered at the top of the reduction gearbox, and its output drive gear had fractured.

Metallurgical examination of the fractured pieces of output drive gear revealed features that were consistent with fatigue. The crack initiation site was identified along the outer rim section of the gear, at the root of one of the gear teeth. Even though the crack initiation site was heavily damaged, features indicative of intergranular cracking transitioning to a transgranular fatigue crack propagation were noted. No anomalies or foreign material were noted at the crack origin.

The location and size of the intergranular cracking area was within the carburization layer specified by the manufacturing print. Examination of the remaining output drive gear teeth revealed a total of six additional secondary cracks that were localized around an approximate 22-degree arc near the initial fracture location and located in the same general tooth root location as the initial fatigue fracture. The secondary cracks were examined in detail, revealing the presence of fatigue, plastic deformation, and oxidation near the origins of the cracks, which are clear indications that the cracks initiated and propagated prior to the complete failure of the output drive gear. The existence of multiple fatigue-type cracks implied that the cracking was likely the result of a systematic part anomaly or defect, rather than a localized defect, since no such defect was detected in the fracture surface of the secondary cracks.

The proper material composition, case hardness and depth, and grain structure along with no evidence of a material process issue indicated that the failed output drive shaft was manufactured as intended. Chemical analyses were conducted on the primary and secondary fracture surfaces to determine if any of these detrimental impurity elements were present and in quantities sufficient to cause a weakening of the material and lead to the initiation of the fatigue crack. Hydrogen content on the primary fracture surface was reported as 1 part per million. Hydrogen concentrations of a few parts per million dissolved in the steel could cause hairline cracking and loss of tensile ductility. Since hydrogen could diffuse out of the part easily under certain conditions, there was no way to definitively determine what the hydrogen level at the fracture surface was at the time of the crack initiation; however, the hydrogen concentration level found on the fractured tooth was in the general neighborhood where hydrogen embrittlement could occur. Thus, embrittlement was considered as a possible contributor to the fatigue failure. Embrittlement is a loss of ductility and/or toughness of a material and in steels could take various forms. Embrittlers such a hydrogen, phosphorus, and nitrogen, could be detrimental to the desired mechanical properties and are typically grain boundary embrittlers that produced low energy, intergranular ductile fractures.

Cracks caused by hydrogen embrittlement often originate near or at the surface, usually do not branch, and the crack path could be either transgranular or intergranular and could sometimes change from one plane to the other as it propagated. The output drive gear fatigue crack found on the primary fracture surface was a single crack located at the surface that initially propagated intergranularly then transitioned transgranularly prior to failing in overload. This was consistent with a hydrogen embrittlement induced fatigue crack. Hydrogen uptake could come from a various sources, including the electrochemical plating processes, which the output drive gears experienced three times. In order to prevent hydrogen embrittlement, hydrogen that was picked up during the plating process was driven out by a process called dehydrogenization. Review of the manufacturing production order showed that after each of the three plating operations, the output drive gears was subjected to a dehydrogenization process.

Text from the NTSB. Open the NTSB docket for ERA10TA493 for the full record.