On June 1, 2015, about 1948 mountain standard time, a Bell 206 L-4, N73AW, sustained substantial damage during an emergency landing following a loss of engine power, near Dewey, Arizona. The commercial pilot, the sole occupant of the helicopter, was not injured. The helicopter was registered to Air Medical Services LLC and operated by Airwest Helicopters as a Title 14 Code of Federal Regulations Part 91 ferry flight. Visual meteorological conditions prevailed and a company visual flight rules, company flight plan, was filed. The cross country flight, departed Valle Airport (40G), Grand Canyon, Arizona about 1905 with a planned destination of Glendale Municipal Airport (GEU), Glendale, Arizona.
The operator reported that several weeks prior to the accident, the helicopter's engine chip detector caution light momentary illuminated during a routine maintenance engine run. The engine was shut down and the magnetic chip detectors (MCDs) were examined. A small amount of debris, that was determined to be within the manufacturer's limits, was observed. The MCDs were cleaned and re-installed and then a 30-minute engine run was conducted. Afterward, no further debris was found and the helicopter was returned to service. During a subsequent flight, a couple of weeks later, the engine chip detector caution light momentary illuminated. The helicopter landed and the MCDs were once again inspected, and a small flake and metallic paste on the lower MCD was observed. The oil system was drained, flushed, changed, and a new filter was installed. A 30-minute ground engine run was accomplished, and the MCDs were inspected afterwards and free of debris. The helicopter was then flown about 30 minutes, to a location where an oil sample could be drawn from the original oil removed. The oil sample was sent out for analysis and the helicopter was not operated for about 2 weeks, while awaiting results. The oil analysis was received, and the operator was satisfied with the result of no metal detected, and attempted to ferry the helicopter back to its home base.
During the flight, the pilot reported observing a momentary illumination of a caution light that he could not identify, prior to it extinguishing. About 10 minutes later, the engine chip detector caution light briefly illuminated, while the helicopter was in cruise flight, about 750 ft above ground level. The pilot elected to make a precautionary landing and initiated a descent. During the descent, the engine chip detector light illuminated again and shortly thereafter, was followed by a loud bang. Immediately, the engine lost power and an emergency autorotation landing was accomplished. During the landing sequence, the main rotor blades struck the tail boom and resulted in substantial damage.
Postaccident examination of the helicopter, revealed control continuity with the cockpit controls to the engine and flight controls. Besides the tail boom damage, the remainder of the fuselage was relatively intact. Initial visual examination of the engine revealed no obvious damage. The inlet, exhaust section, and turbine blades, were clear of obstructions and observed to be undamaged. The engine was removed to facilitate an examination. The compressor could not be rotated by hand but the power turbine could be rotated by hand, with resistance. The magnetic chip detectors were removed and observed to have accumulated ferrous debris. The engine was shipped to Rolls-Royce for further examination.
The engine was examined and disassembled under the supervision of the National Transportation Safety Board, investigator-in-charge, at the Rolls-Royce facility near Indianapolis, Indiana. The compressor module was separated from the accessory gearbox by removing the compressor discharge tubes and the Compressor Turbine Drive Shaft (CT shaft). Thermal damage and cooked oil was observed on the CT shaft. The turbine module was removed for examination. On the first and second stages of the turbine, thermal and blade tip damage was observed. All the bearings were examined and were unremarkable, except for the No. 2 bearing.
The aft end of the compressor module revealed damage to the No. 2 bearing. The No. 2 bearing was fractured and exhibited thermal damage. Examination of the No. 2 bearing oil delivery tube revealed it was properly installed with no obvious blockage of the oil jets. The tube was removed and x-rayed for internal contamination of the oil passages with negative results. Further, the tube was flow checked and verified to flow oil at the specified rate.
The accessory gearbox was disassembled and observed to be intact and functional. Metal debris was observed throughout the gearbox. The oil pump screen, oil delivery tubes and engine mounted oil filter had minor contamination but no obstructions to flow.
A detailed examination of the No. 2 bearing revealed that its composition was consistent with the manufacture material types required. The bearing separator was fractured completely through both rails, at the forward and aft sides, of the bearing. All 10 of the ball bearings were retained in the separator, however half of them exhibited damage consistent with smearing and material transfer between the raceway and balls.
The No. 2 bearing is the primary means of support of the compressor impellor. Failure of the No. 2 bearing will allow the impellor to migrate forward, where it will eventually contact the compressor shroud. According to the manufacturer, only minor contact between the impeller and shroud is acceptable, however, examination of the compressor impellor revealed that the shroud and its vanes had considerable rubbing damage.