Eurocopter Deutschland Gmbh MBB BK 117 C-2 N911MK incident near Sioux Falls, South Dakota

Probable cause

The gas generator rear bearing oil scavenge line blocked with coke, which resulted in a failed rear bearing due to an excess of oil lubrication. Contributing to the incident was the operator not completely performing the maintenance items and not utilizing an oil for higher temperatures.

NTSB analysis

The pilot and two crewmembers were conducting a night helicopter air ambulance flight. While en route, the No. 2 engine twist grip caution light illuminated. The pilot checked the instruments and did not notice anything anomalous. The twist grip light disappeared then reappeared and flickered a few more times. All parameters were in the proper range. Shortly after, he heard a "winding down sound," then " a hissing sound” and detected “the smell of exhaust in the cabin." He responded by turning off the cabin heat then lowered the collective and diverted toward an airport. During the turn, the No. 2 engine FIRE light illuminated, and he noticed an orange glow from the back-right side of the helicopter. He pressed the No. 2 engine emergency off switch, noticed that the FLI needles split, and continued with one engine inoperative. Subsequently the No. 1 fire bottle light illuminated, which, indicated there was a fire in the engine compartment, so he activated the bottle. Then the No. 2 fire bottle light illuminated, which indicated that the fire had not yet been extinguished, so he discharged the second fire bottle. The FIRE light extinguished, and he continued the flight and made a running emergency landing at the diversion airport.

A post-incident examination of the helicopter and inspection of both engines was completed. The No. 2 engine deck area was thermally damaged from an oil fire. The No. 2 engine exhibited significant internal coke buildup with blockages noted in the rear bearing scavenge line and the airframe's vent line 3-way union. The blocked scavenge line allowed the oil to escape the rear bearing chamber, which resulted in oil in the engine's main air path, in the rear bearing vent line, and onto the engine deck. A failure of the rear bearing occurred due to excessive oil lubrication in the rear bearing chamber. The No. 2 engine gas generator rear bearing was found severely damaged and seized; the seizure of the bearing was due to the bearing operating in excess oil.

The No. 1 engine rear bearing assembly also exhibited significant coke buildup in similar locations but continued to operate normally during the incident. The bearing was found covered with coke, but otherwise in good condition. The 3-way union was partially blocked with coke.

The operator would have benefitted in using a high thermal stability oil. The engine maintenance performed by the operator was lacking proper recording and was not completed exactly as prescribed in the engine maintenance manual. If performed properly, the rear bearing strainer maintenance check, the permeability test, and permeability trend analysis could have helped the operator recognize the impending clogging of the rear bearing scavenge line.

Text from the NTSB. Open the NTSB docket for CEN17IA094 for the full record.