History of Flight
On January 26, 2017, about 2145 central standard time, a Eurocopter Deutschland GMBH MBB-BK 117 C-2 helicopter, registration N911MK, sustained minor damage during an inflight engine fire. The helicopter was operating as a Title 14 Code of Federal Regulations Part 135 air ambulance flight. The airline transport pilot and two crewmembers were not injured.
According to the pilot's statement, while en route at night, the No. 2 engine twist grip caution light illuminated. The pilot verified the throttle in flight mode and checked the first limit indicator (FLI) and tachometer, which were normal. The twist grip light intermittently reappeared. Engine instrumentation remained within proper ranges. Shortly after, the caution advisory display (CAD) showed ENG PA DIS cautions for both engines; both FLI needles were consistent, and all parameters were normal. The pilot then heard a winding-down sound, a hissing, and smelled exhaust in the cabin. He turned off the cabin heat, lowered the collective, and turned toward Joe Foss Field Airport (FSD). During the turn, the No. 2 engine FIRE light illuminated. At over 100 knots airspeed, he saw an orange glow from the back right side. He slowed below 100 knots for one engine inoperative (OEI) procedures, then broke the red plastic switch guard and pressed the emergency off switch (EMER OFF SW). The FLI needles split, and ACTIVE and BOT 1 lights illuminated; he pressed BOT 1, then BOT 2 when it lit. About two seconds after pressing BOT 2, the FIRE light extinguished. He completed all emergency checklist items. The No. 1 engine FLI never exceeded 10.5, and mast moment remained in normal range. The helicopter descended about 300 ft. With the fire out, he continued to FSD, maintaining 500 ft altitude. About 13 nm southwest of FSD, he declared an emergency to the tower and was cleared to land on runway 3. He executed a low approach with a running landing, stopping in less than 100 ft.
Aircraft Information
The helicopter was equipped with two engines. The incident involved the No. 2 engine. Maintenance records from the operator showed periodic checks on the No. 2 engine's module 3, including oil filter and rear bearing magnetic plug inspections at 400-hour intervals, a spectrometric oil analysis program (SOAP), and rear bearing permeability tests. Results of several inspections were not recorded. The most recent permeability check was performed 383 flight hours before the incident; the rear bearing scavenge magnetic plug was inspected 12 flight hours prior. The operator used Mobil Jet II oil before the incident; afterward, they switched to Eastman 2197 turbo oil.
Post-Incident Examination
Post-incident examination revealed fire damage limited to the aft outboard section of the No. 2 engine deck and aft engine compartment and cowling. The engine cowling, central wall, and upper heat shield had thermal damage. Local blistering of engine deck paint and oil splatter were observed. The lower engine was covered in oil residue with thermal damage. The No. 2 engine oil tank was 1 to 1.5 quarts low. Oil streaked the right fuselage and tailboom. All oil lines were tight and intact. The elastomer vent tube was attached.
The No. 2 engine was removed for teardown. It showed a light coat of soot aft of the combustor and oil on the bottom. The gas generator rotor was seized. The MO3 rear bearing chip detector had a black coating; no significant ferrous material was observed. The rear bearing oil scavenge pipe was clear at the exit but heavily contaminated with oily carbon at the strainer end. The rear bearing strainer housing union was obstructed by coke. The scavenge strainer and magnetic chip detector housing were full of coke particles. The rear bearing casing had oily soot and loose carbon chunks; fragments of the inner race were found. Soot indicated oil escaped into the engine's main air path. The rear bearing running surface was deeply grooved. The bearing was displaced; the retaining ring circlet was broken. The bearing inner race was fragmented; color and texture indicated adhesion and seizure. The roller assembly had rotational scoring. Carbon deposits were on the support assembly cage outer race and vibration dampeners; dampeners were corrosion damaged. The scavenge pipe contained oily black substance. The rear bearing chip detector was operational but impeded by debris.
The airframe elastomer tube connecting the engine vent pipe to the 3-way union deck fitting had two outward-opening holes. Another elastomer drain tube was separated and melted at both ends. The 3-way union was clogged with coke.
The No. 1 engine was inspected for comparison. It had no fire or component failure. Its rear bearing chamber contained significant coke; the 3-way union was partially blocked. The rear bearing was covered in coke but undamaged and rotatable. Hard carbon adhered to the oil supply and scavenge tube interior surfaces. The rear bearing support assembly was intact.
Additional Information
Engine manufacturer data from 2000 to 2017 (excluding this incident) showed 16 cases of rear bearing coking on Arriel 1 engines in over 22 million flight hours; only two involved an Arriel 1E2 on the same helicopter model. None led to bearing failure.
After the incident, the FAA issued Special Airworthiness Information Bulletin SW-18-04. Safran Helicopter Engines released Service Letter SL 1642/96/Arriel/25 version 3. Airbus issued Information Notice No. 3213-I-71.