What happened
On January 26, 2017, at approximately 2145 CST, a Eurocopter MBB BK 117 C-2 helicopter, registration N911MK, was conducting a night air ambulance flight when the crew experienced an engine fire. The flight crew, consisting of an airline transport pilot and two crewmembers, was operating under 14 CFR Part 1 35.
During the flight, the pilot noted the No. 2 engine twist grip caution light flickering. Shortly after, the pilot heard a winding down sound and a hissing sound, followed by the smell of exhaust in the cabin. While turning toward Joe Foss Field Airport (FSD) in Sioux Falls, the No. 2 engine fire light illuminated, and the pilot observed an orange glow from the rear right side of the aircraft.
Following one engine inoperative (OEI) procedures, the pilot reduced airspeed below 100 knots and activated the emergency off switch. After the fire light remained illuminated, the pilot discharged both fire bottles, which eventually extinguished the fire. The pilot maintained an altitude of 500 feet and declared an emergency with the tower. The helicopter completed a running landing on runway 3, stopping within 100 feet of the threshold. There were no injuries to the three crewmembers.
The investigation
Post-incident examination of the aircraft revealed fire damage to the No. 2 engine deck, engine compartment, and cowling. The No. 2 engine oil tank was 1 to 1.5 quarts low, and oil residue was found on the fuselage and tailboom.
Detailed inspection of the No. 2 engine revealed that the gas generator rotor had seized. Investigators found significant internal coke buildup (solid residue from thermal breakdown of oil) within the engine. Specifically, the rear bearing scavenge pipe was heavily contaminated with oily carbon material, and the rear bearing strainer housing union was obstructed by coke. The inner surfaces of the rear bearing casing contained large amounts of loose carbon chunks. The investigation also found that the running surface of the piston shaft labyrinth was deeply grooved and the rear bearing was displaced on the turbine bolt.
While the No. 1 engine did not experience a malfunction, it also exhibited significant coke buildup in the rear bearing assembly and the 3-way union.