History of Flight
On September 22, 2010, at 1552 eastern daylight time, a Bell Helicopter 412EP, registration 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, certificated airline transport-rated co-pilot, and four additional crew members sustained minor injuries. Visual meteorological conditions prevailed and no flight plan was filed. The public use aerial observation flight originated from NYPD Air Operations Heliport - Floyd Bennett Field (NY22), Brooklyn, at 1510.
According to written statements from three crew members, the flight departed NY22 about 1510 for a local patrol flight. About 30 minutes later, the flight was directed to return for refueling. While on final approach about half a mile from the heliport at 300 feet above ground level, the crew heard a loud mechanical bang from the engine compartment. No cautions or warnings were observed, but the helicopter began descending and the co-pilot noted a loss of rotor rpm. The co-pilot transmitted a distress call, and the crew confirmed 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 shut down the engines and were retrieved by boat.
Aircraft Information
The helicopter was equipped with one Pratt & Whitney Canada PT6T-3D turbo-shaft engine assembly, comprising two separate power sections coupled to a single reduction gearbox (RGB). The RGB reduced output speed through a 5.0:1 ratio to a nominal 6,600 rpm, further reduced by the airframe transmission. The RGB housing consisted of five cast magnesium housings. Internal components included main input drive shafts, idler drive spur gears, clutch drive gear assemblies, and intermediate drive helical gears driving a single output drive gear (ODG). The ODG drove the helicopter's transmission.
The helicopter was manufactured in January 2009, issued an airworthiness certificate on March 23, 2009, and delivered to NYPD on December 21, 2009, with 30.3 hours total time since new. Low oil quantity in the RGB was reported on January 9 and June 30, 2010. The most recent routine maintenance inspection was completed on August 16, 2010, at 303 hours total time. The helicopter had accumulated about 359 hours at the time of the accident.
Wreckage and Impact Information
Examination by a Federal Aviation Administration inspector revealed impact damage to all four main rotor blades, underside of the fuselage, tail boom, and tail rotor blades. A 3- by 9-inch hole was observed at the top of the RGB. The engine assembly was removed and examined.
Neither power section showed signs of fire, breaches, or impact damage. The RGB exhibited an approximate 3-inch axial by 9-inch lateral exit hole in the output housing, aligned with the ODG. The ODG was fractured, with about half of the helical gear outer rim detached. Internal examination found a hole in the wall dividing the output housing center section and the No. 1 power section input cavity, through which a section of the ODG outer rim protruded. Metallic debris was present. The No. 1 power section main input drive shaft support structure had rub marks consistent with contact with the fractured ODG piece. The oil transfer hole on the No. 1 input cavity was occluded with metallic debris.
Output Drive Gear Examination
The portion of the ODG remaining installed (piece 1) had fractures at two tooth locations. Features consistent with fatigue were found at one tooth location, with the crack initiating from the root near the chamfer, approximately 0.020 inches from the rear end face. Other fractures exhibited tensile overload features. Dark-bluish color at web arms indicated heat distress from heavy rubbing; reddish-brown color at the tooth location was consistent with oil coke deposits from elevated temperatures.
Scanning electron microscopy identified river lines converging at the fatigue crack origin. Evidence of intergranular cracking starting adjacent to the origin, transitioning to transgranular fatigue crack propagation, was observed. No anomalies or foreign material were found at the origin. The intergranular area measured approximately 0.025 inches by 0.008 inches, within the carburization layer.
Manufacturing and Material Analysis
The ODG was made of AMS 6265 wrought alloy steel, meeting specification. Micro-hardness traverses and chemical analysis showed no anomalies. Hydrogen content was 1 ppm, nitrogen 0.0052% by weight. TOF-SIMS analysis of secondary cracks found no significant detrimental elements. The ODG was part of a batch of eight forged in May 2008; one was scrapped during manufacturing, leaving seven in service. The manufacturing process included carburization, hardening, cold treatment, tempering, grinding, shot peening (which introduced compressive residual stress), and plating with dehydrogenization after each plating operation.
Additional Information
On October 7, 2010, the engine manufacturer issued Alert Service Bulletin PT6T-72-A5517, informing operators that certain ODGs from the same manufacturing batch as the accident gear may have a manufacturing anomaly that could cause fracture during operation. The six suspect ODGs were removed by end of December 2010. Fluorescent penetrant inspection and magnetic particle inspection found no cracks. A metallographic examination of one sectioned gear showed case hardness and core microstructure within specifications.