Bell 407 accident near Gulf Of Mexico, GM, December 7, 2019

The NTSB's narrative final · quoted from the NTSB record

The helicopter was flying over the Gulf of Mexico between two oil platforms. Flight track data showed the helicopter in cruise flight at 700 ft mean sea level (msl) before it gradually descended to 375 ft msl. Just before the data ended, the helicopter entered a left turn, descended to 150 ft msl, and slowed from about 115 knots (kts) groundspeed to about 75 kts groundspeed. The helicopter impacted the water and sank to the sea floor. The main wreckage was eventually located on the sea floor about 350 ft southwest of the last recorded flight track point. Examination of the engine revealed that the power turbine pinion gear teeth were significantly worn and smeared, which ultimately resulted in the engine’s inability to provide power to the rotor system. While the majority of the No. 3 bearing was not recovered, the significant thermal damage and loss of material of the power turbine pinion gear shoulder, to which the No. 3 bearing inner race was installed, indicated that the bearing likely failed first, but the cause of the No. 3 bearing failure could not be determined because the bearing was not recovered. The failure of the No. 3 bearing could result in misalignment of the gear mesh between the power turbine pinion gear and the torquemeter gear, which likely caused the pinion gear teeth to rapidly degrade and/or fracture. The initial failure of the No. 3 bearing would have resulted in the failure of the No. 4 bearing and the power turbine pinion gear. The reasons why damage to the Nos. 3 and 4 bearings and the power turbine pinion gear did not result in an engine chip light or oil filter contamination, leading to a filter bypass indication, could not be determined based on the available evidence. The blockages observed in the oil delivery tube and oil nozzle were consistent with corrosion, likely due to postaccident saltwater immersion; therefore, whether there were any preexisting anomalies of the oil delivery tube jets and/or the oil nozzle, such as oil coking, could not be determined. However, the remainder of the recovered engine bearings and gears did not show evidence of damage associated with a widespread lubrication issue. Analysis of the caution and warning panel bulb filaments showed that several annunciator lights were illuminated at the time of impact, consistent with the effects of a loss of engine power due to a failure of the power turbine pinion gear. The main rotor rpm warning light would have illuminated within a second of the loss of engine power as main rotor speed (Nr) decreased, and the hydraulic system and transmission oil pressure annunciator lights would have illuminated once Nr decreased to critical levels. The data downloaded from the helicopter’s engine control unit was also consistent with a loss of engine power and subsequent loss of Nr. While collective pitch was reduced for a few seconds after the loss of engine power, collective pitch was subsequently increased, which led to a quicker decay of Nr. The substantial decrease in Nr would have affected controllability of the helicopter and ultimately resulted in a loss of control. The pilot’s cyclic and pedal inputs, as well as the helicopter’s attitude and airspeed, at the time of the loss of engine power were not known. Therefore, it could not be determined if the pilot successfully entered autorotation or the reasons why the pilot increased collective control about 3 seconds after the loss of engine power. There was no evidence that the emergency flotation system was deployed. While flight track data for the accident flight was available, the last few data points were discontinuous and could not be reliably matched with the recorded engine data; therefore, it could not be determined if the pilot was in a position to deploy the emergency flotation system following the loss of power.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.