Accident Overview
On June 27, 2017, about 1403 Pacific daylight time, a Eurocopter EC130 B4 helicopter, registration N151GC, sustained substantial damage following a loss of engine power and subsequent forced landing near Boulder City, Nevada. The helicopter was operated by Papillon Grand Canyon Helicopters as an air tour flight under Title 14 Code of Federal Regulations Part 135. Visual meteorological conditions prevailed, and a company flight plan had been filed. The flight originated from Boulder City Municipal Airport at 1348 and was destined for Grand Canyon West Airport in Peach Springs, Arizona.
Flight Sequence
The pilot reported that while executing an "S-turn" at about 3,000 ft mean sea level, the helicopter made a subtle yaw to the right accompanied by an audible "gong." The pilot lowered the collective and saw a yellow engine parameter failure indication on the instrument display. Moments later, the main rotor rpm warning horn activated, followed by warning lights on the caution warning display. The helicopter began to descend immediately. The pilot performed an autorotation toward a nearby helicopter pad. While trying to steer away from power lines, the pilot made a left cyclic input, and the retreating main rotor blade contacted a power line. The pilot continued maneuvering and executed an up-slope emergency landing on rising mountainous terrain. The helicopter was recovered to the operator's hangar for examination.
Maintenance History
The last engine maintenance was performed 109.6 hours before the accident at the Safran HE USA facility in Grand Prairie, Texas. On June 30, 2016, the facility received the accessory gearbox for an overhaul. The oil filter was removed and discarded during arrival inspection. The overhaul was not due, and the operator agreed to have the accessory gearbox returned as is. The accessory gearbox was returned with no tags and a log card entry stating "Equipment returned repairable, not repaired, not airworthy as is." This statement was later voided by Safran HE USA after the operator questioned it, reverting airworthiness to the previous entry. The manufacturer did not inform the operator that maintenance had been performed or that the oil filter had been removed. The engine was rebuilt with the existing accessory gearbox and operated for 109.6 hours without an oil filter installed.
Engine Examination
On June 28 and 29, 2017, FAA, Papillon Grand Canyon Helicopters, and Safran HE representatives examined the engine. No visual external damage was found. All fuel, oil, and air lines were secure; oil was present in the tank. The gas generator and power turbine were seized and could not be rotated manually. There was no evidence of foreign object debris on the axial compressor. A borescope inspection revealed significant heat damage to the high pressure turbine blades and thermocouples. The Vehicle and Engine Multifunction Display recorded engine temperature exceedances. Metal particles were found on the accessory gearbox magnetic plug, and sludge was present on the reduction gearbox magnetic plug and electric chip detectors. The engine oil filter was not installed.
Further Disassembly and Analysis
On July 6 and 7, 2017, at the Safran HE USA facility, the engine was disassembled, and Digital Engine Control Unit (DECU) data was downloaded. All magnetic sensors, strainers, and plugs were found clean except the accessory gearbox magnetic plug. The thermocouple harness sustained thermal damage. The reduction gearbox rotated freely; the free turbine module was seized at the blade tip/shroud section with thermal and impact damage on the blades. The accessory gearbox rotated freely with continuity. The axial compressor rotated roughly and noisily; the gas generator could not rotate. After removing the axial compressor, a hole was observed in the centrifugal compressor, and the axial compressor rear bearing was shattered. The oil jet for the rear bearing was obstructed and sent for further examination. The gas generator rear bearing appeared normal and rotated freely.
DECU data showed three faults during the accident flight: a P3 drift, a raw t4 fault likely from thermal damage to the thermocouple harness, and a helicopter t4 indication fault six seconds later. During those six seconds, gas generator and free turbine speeds decreased while fuel flow increased. Torque recorded at the last fault was 0.52%, consistent with the helicopter freewheeling.
On October 9, 2017, the obstructed axial compressor oil jet was examined at the Safran HE factory laboratory in France. The double oil jet was blocked on one side. Borescope examination revealed the obstruction was composed of carbon, oxygen, and phosphor, indicating oil cokefaction. Coke pollution had moved through the scavenge circuit to the oil tank and then through the oil filter (which was absent) to the lubrication circuit, ultimately migrating to the oil jet.
Analysis of the axial compressor bearing showed all balls deformed with flat zones. The two-half internal race had material transfer leading to seizure and jamming. The outside race and separating cage were stuck together with the outside housing.