CEN17IA094

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.

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.

Maintenance Information On December 27, 2016, a 400-hour periodic check was performed by the operator at 6,146 hours engine time since new (TSN), and 2,551 hours engine time since overhaul (TSO) on the engine's module 3. During the check they inspected the oil filter and the rear bearing's magnetic plug. There was no record of the inspection results. On December 14, 2016, a spectrometric oil analysis program (SOAP) was performed by the operator at 6,114 hours engine TSN, and 2,519 hours engine TSO on the engine's module 3. There was no record of the analysis results. On October 22, 2016, an 800-hour and a 1,200-hour periodic check were performed by the operator at 5,988 hours engine TSN, and 2,393 hours engine TSO on the engine's module 3. During the 800-hour check they drained the oil system and inspected the rear bearing's magnetic plug strainer (TU208). During the 1200-hour check they removed and cleaned the oil scavenge tube. There was no record of either inspection results. On July 25, 2016, a 600-hour periodic check was performed by the operator at 5,815 hours engine TSN, and 2,220 hours engine TSO on the engine's module 3. During the check, a rear bearing assembly permeability test was conducted. There was no record of the permeability test results or a trend analysis. On June 23, 2016, a 400-hour periodic check was performed by the operator at 5,739 hours engine TSN, and 2,144 hours engine TSO on the engine's module 3. During the check they inspected the oil filter and the rear bearing's magnetic plug. There was no record of the inspection results. On May 21, 2016, the oil level sight gauge was replaced by the operator at 5,637 hours engine TSN, and 2,042 hours engine TSO on the engine's module 3. The operator reported that the gauge was cloudy and dark and needed to be replaced. They also rinsed the oil system and conducted a rear bearing assembly permeability test; 100 milliliters of oil were collected during the test. A trend analysis was not completed. The most recent gas generator rear bearing scavenge oil flow rate (permeability) check was performed 383 flight hours (FH) before the incident. The result of this test and the amount of oil collected was not recorded. The last rear bearing scavenge oil tube and in-line strainer maintenance was performed 209 FH before the incident. The lubrication system was serviced, the oil filter replaced, and engine oil change was last completed 51 FH prior to the incident. The rear bearing scavenge magnetic plug was inspected 12 FH prior to the incident. Oil change intervals during this period averaged 168 FH. An oil system rinse and a permeability check were completed at 5,637 hours engine TSN. These tasks were unscheduled and were performed because of clouding of the oil tank's sight glass. Prior to and during the incident the operator was using Mobile Jet II oil, which was acceptable per the engine maintenance manual (EMM). After the incident they switched to a high thermal stability Eastman 2197 turbo oil, per the oil options in the EMM. The high thermal stability oils provide better performance especially when stopping the engine quickly. The pilot landed the helicopter along the centerline of runway 3 at FSD, with a running landing distance of less than 100 ft. The helicopter did not sustain any additional damage during the landing. A post-incident examination of the helicopter revealed fire damage limited to the aft outboard section of the No. 2 engine deck and the aft section of the engine compartment and cowling. The engine cowling, the central wall, and upper heat shield sustained thermal damage. Local blistering to the engine deck paint and oil splatter on the deck floor were observed. There was no evidence of a pool fire. The lower part of the engine was covered in oil residue with some thermal damage evident (Figure 1). Figure 1 – Engine deck thermal damage and oil residue The No. 2 engine oil tank was 1 to 1.5 quarts low. The right side of the fuselage was streaked with oil and oil was noted on the tailboom. The engine's vent line and all oil lines were checked and found tight, not broken, and not cracked. The elastomer vent tube was still attached to the engine's vent line and to the 3-way union deck fitting. The engines were removed and sent to the manufacturer's facility for a tear down and detailed examination under the supervision of the NTSB. The No. 2 engine displayed a light coat of soot aft of the combustor section and engine oil was observed on the bottom of the engine. There was light thermal damage to hose and harness insulation and attachment hardware along the lower right side and bottom of the engine. No other external discrepancies were noted. The gas generator rotor was seized. The end of the MO3 (high pressure gas generator module) rear bearing chip detector was coated with a black substance. No significant ferrous material was observed. The gas generator rear bearing oil scavenge pipe was clear at the rear bearing casing's exit and heavily contaminated with oily carbon material at the rear bearing strainer end. The rear bearing strainer housing union was obstructed by coke. The rear bearing scavenge strainer and magnetic chip detector housing were full of coke particles. The inner surfaces of the rear bearing casing were coated with oily soot and there was a large amount of loose carbon chunks and particles. Fragments of inner race were found inside. Soot marks showed that oil escaped from the rear bearing casing into the engine's main air path. There were no indications of fire in the rear bearing casing and adjacent parts. The rear bearing casing's support arms and the intermediate cavities around the rear bearing casing were clear of oil and showed no signs of internal fire. Oil also escaped from the rear bearing casing into the metallic vent line. The running surface for the small-diameter labyrinth on the piston shaft forward end was deeply grooved. The rear bearing was displaced on the turbine bolt. The retaining ring circlet was broken into two parts which were found during disassembly and each part did not show significant damage. A small amount of oil was present at the bearing. The bearing inner race was fragmented. The bearing color and texture were consistent with adhesion and subsequent seizure damage. The roller assembly exhibited significant rotational scoring with only a small amount of material remaining. The scored surface was battered. There were carbon deposits on the inner surfaces of the bearing support assembly cage outer race and vibration dampeners. All of the cage dampeners were corrosion damaged, and some were missing material. The bearing's lubrication hole was clear. The cage bolt had carbon deposits inboard of the lock feature. There was gouging on the inside consistent with contact with the turbine bolt. There were black deposits on the tip of the rear bearing chip detector. The oil scavenge pipe between the rear bearing housing and the in-line strainer/chip detector contained an oily black substance. The rear bearing strainer housing union where the rear bearing scavenge line attaches was partially obstructed by oily soot/coke. The rear bearing chip detector housing and strainer were obstructed by coke. The gas generator rear bearing chip detector was operationally tested using an engine installed on a test cell. Ultimately, the test indicated that the engine chip detector was operational and that the black debris on the detector tip impeded the detector function, until it was cleaned and retested. No. 2 engine 3-way union (deck fitting) There were outward opening, axially-oriented holes at two locations in the airframe elastomer tube that connected the engine vent pipe to the 3-way union deck fitting. A section of another elastomer drain tube, part of an airframe collector drain line, was lying on the engine deck. It was separated and melted at both ends. The outside surface of