ERA10TA493

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.

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.

HISTORY OF FLIGHT On September 22, 2010, at 1552 eastern daylight time a Bell Helicopter 412EP, N412PD, operated by the New York City Police Aviation Unit, was substantially damaged during a forced landing to Jamaica Bay, near Brooklyn, New York. The certificated commercial pilot, the certificated airline transport-rated co-pilot, and 4 additional crewmembers sustained minor injuries. Visual meteorological conditions prevailed and no flight plan was filed for the flight. The public use aerial observation flight originated at New York City Police Department (NYPD) Air Operations Heliport - Floyd Bennett Field (NY22), Brooklyn, New York, at 1510. In separate written statements, three members of the flight crew recounted the events that transpired prior to and during the accident flight. The flight departed NY22 about 1510 for a local patrol flight. About 30 minutes later, the flight was directed to return to NY22 for refueling. While on final approach to, and about 1/2-mile from the heliport at an altitude of 300 feet agl, the crew heard a loud, mechanical "bang" sound from the engine compartment of the helicopter. None of the crew heard or saw any cautions or warnings, but noted that the helicopter immediately began descending, and the co-pilot noted a loss of rotor rpm. The co-pilot transmitted a distress call to the operator's base, and the crew confirmed that the emergency flotation system was armed. About 8 to 10 seconds after the event began, the helicopter impacted the water, the floats deployed, and the windscreens shattered after a portion of the main rotor blade struck the top of the cockpit cabin. The crew subsequently shut down the engines, secured the helicopter, and were retrieved from the scene via boat. AIRCRAFT INFORMATION The accident helicopter was equipped with one Pratt & Whitney Canada PT6T-3D turbo-shaft engine assembly. The engine assembly was comprised of two separate power sections, which were coupled to a single reduction gearbox (RGB). Each PT6T-3D power section featured a three-stage axial and single-stage centrifugal compressor driven by a single stage compressor turbine, a reverse-flow annular combustor, and a free turning power turbine coupled to the RGB. The RGB drove the helicopter's main rotor transmission and various engine sensing and indicating components. The PT6T-3D assembly had a takeoff and continuous power limit of 1,800 shaft horsepower (SHP) and was flat-rated to 22 degrees C. The purpose of the RGB was to reduce the power turbines' output speed to a speed suitable for helicopter operation. The RGB had a reduction ratio of 5.0 to 1, resulting in a nominal output shaft speed of 6,600 revolutions per minute (rpm), and rotated the output drive gear clockwise. The rotational speed was further reduced, at a ratio of 20.4 to 1, through an airframe transmission to a nominal speed of 324 rpm. The RGB assembly outer structure was comprised of five cast magnesium housings: output housing (most forward), diaphragm, input housing (aft), and 2 covers mounted on the input housing (left and right). The internal components of the RGB featured a pair of main input drive shafts (left/right) that received power from each of the power sections, idler drive spur gears, clutch drive gear assemblies, and the intermediate drive helical gears (also referred to as the 2nd stage helical gears) that drove a single output drive gear (ODG) from its circumference. The output from the ODG drove the helicopter's transmission. The accident helicopter was manufactured by Bell Helicopter in January 2009, issued a standard United States Airworthiness Certificate on March 23, 2009, and delivered to the NYPD on December 21, 2009. When the NYPD took procession of the helicopter, both the helicopter and engines had accumulated 30.3 hours total time since new (TSN) and the engines had accumulated 60 cycles since new (CSN). According to the helicopter's discrepancy log, low oil quantity within the RGB was reported on January 9, 2010 and on June 30, 2010. The helicopter and engines had both accumulated 39.2 hours TSN and 226.7 hours TSN, respectively, on those dates. The helicopter's most recent routine maintenance inspection was completed on August 16, 2010, at 303 hours TSN, and consisted of the 25, 100, and 300-hour inspection requirements. The only engine maintenance actions during the inspection were standard oil replenishment and filter replacement. The helicopter had accumulated approximately 359 hours TSN at the time of the accident. WRECKAGE AND IMPACT INFORMATION The helicopter was examined by a Federal Aviation Administration inspector after it was recovered from Jamaica Bay. The helicopter had sustained impact-related damage to all four main rotor blades, underside of the fuselage, tail boom, and tail rotor blades. Additionally, a 3- by 9-inch hole centered at the top of the RGB was observed. The engine assembly, including RGB, was subsequently removed from the helicopter and forwarded to the manufacturer's facility for further examination. Engine Assembly and RGB External Examination Visual examination of the engine assembly revealed that neither power section exhibited any signs of fire, breaches, or impact damage. The accessory gearboxes of both power sections were attached and their respective accessories were in place and undamaged. The inlet screens were removed from each power section and reaching through the inlet case, the compressor on each power section rotated freely by hand in both directions. Tactile examination of the leading edges of the first stage compressor blades revealed no nicks, tears or damage consistent with foreign object ingestion. The RGB exhibited an approximate 3-inch axial (longitudinal) by 9-inch lateral exit hole in the output housing in-line with the axial position of the ODG. Pieces of the output housing in the areas of hole were either missing or were fractured and pushed outwards. Looking into the RGB, the ODG was fractured with about half the helical gear outer rim portion no longer attached to the remainder of the gear assembly. RGB Disassembly The power sections were removed from the RGB exposing the input cavities of the output housing. Pieces of output housing debris fell from the No. 1 power section input cavity and were collected. The output housing exhibited an approximate 2-inch axial by 6.5-inch circumferential internal hole in the wall that divided the output housing center section (output cavity) with the No. 1 power section input cavity. A section representing about 1/2 of the circumference of the ODG outer rim was found sticking through the hole in the wall that divided the output housing center section with the No. 1 power section input cavity. All of the gear teeth were present and there were three distinct areas where the helical teeth were pushed and smeared in the direction of rotation. This section of the ODG had three stubs of the web arm still attached; each exhibited varying degrees of impact damage. The No. 1 power section main input drive shaft support structure exhibited four distinctive rub marks, located radially outward from the center cavity exit hole and in the vicinity of the No. 7 flanged roller bearing, consistent with contact with the fractured piece of the ODG. The oil transfer hole on the No 1. power section input cavity was occluded with metallic debris. The No 1. power section main input drive shaft appeared undamaged and the spiral retaining ring was still engaged. The No. 2 power section input cavity was clean and undamaged, and the oil transfer tube was unobstructed. The No. 2 power section main input drive shaft appeared undamaged and the spiral retaining ring was slightly dislodged. The output housing was examined with the remaining portion of the ODG and both intermediate drive helical gears still installed. A large amount of metallic debris was found within the center section of the output housing, includi