Accident Overview
On June 1, 2015, at approximately 1948 mountain standard time, a Bell 206 L-4 helicopter, registration N73AW, experienced a loss of engine power during a ferry flight near Dewey, Arizona. The commercial pilot, who was the sole occupant, was not injured. The helicopter sustained substantial damage during an emergency autorotation landing.
The helicopter was registered to Air Medical Services LLC and operated by Airwest Helicopters under Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a company visual flight rules flight plan was filed. The cross-country flight departed Valle Airport (40G) in Grand Canyon, Arizona, at about 1905, with a planned destination of Glendale Municipal Airport (GEU) in Glendale, Arizona.
Previous Maintenance Events
Several weeks before the accident, the helicopter's engine chip detector caution light momentarily 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, within manufacturer limits, was found. The MCDs were cleaned and reinstalled, followed by a 30-minute engine run with no further debris. The helicopter was returned to service.
During a subsequent flight a couple of weeks later, the chip detector caution light again illuminated momentarily. The helicopter landed, and the MCDs were inspected, revealing a small flake and metallic paste on the lower MCD. The oil system was drained, flushed, and refilled with a new filter. A 30-minute ground engine run showed the MCDs free of debris. The helicopter was then flown for about 30 minutes to a location where an oil sample could be drawn from the original oil. The sample was sent for analysis, and the helicopter was not operated for about two weeks while awaiting results. The oil analysis indicated no metal detected, and the operator decided to ferry the helicopter back to its home base.
Accident Flight
During the ferry flight, the pilot observed a momentary illumination of an unidentified caution light, which then extinguished. About 10 minutes later, the engine chip detector caution light briefly illuminated while the helicopter was in cruise flight at approximately 750 feet above ground level. The pilot elected to perform a precautionary landing and initiated a descent. During the descent, the engine chip detector light illuminated again, followed shortly by a loud bang. Immediately, the engine lost power, and the pilot conducted an emergency autorotation landing. During the landing sequence, the main rotor blades struck the tail boom, resulting in substantial damage.
Postaccident Examination
Postaccident examination of the helicopter revealed control continuity from the cockpit to the engine and flight controls. Aside from the tail boom damage, the remainder of the fuselage was relatively intact. Initial visual examination of the engine showed no obvious damage. The inlet, exhaust section, and turbine blades were clear of obstructions and appeared undamaged. The engine was removed for further inspection. The compressor could not be rotated by hand, but the power turbine could be rotated with resistance. The magnetic chip detectors were removed and found to have accumulated ferrous debris. The engine was shipped to Rolls-Royce for examination.
At the Rolls-Royce facility near Indianapolis, Indiana, the engine was examined and disassembled under the supervision of the National Transportation Safety Board investigator-in-charge. 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 were observed on the CT shaft. The turbine module showed thermal and blade tip damage on the first and second stages. All bearings were unremarkable except for the No. 2 bearing.
The aft end of the compressor module revealed damage to the No. 2 bearing, which was fractured and exhibited thermal damage. Examination of the No. 2 bearing oil delivery tube showed it was properly installed with no obvious blockage of the oil jets. The tube was removed and x-rayed for internal contamination with negative results; it was flow checked and verified to flow oil at the specified rate.
The accessory gearbox was disassembled and found intact and functional, with metal debris throughout. The oil pump screen, oil delivery tubes, and engine-mounted oil filter had minor contamination but no obstructions to flow.
Detailed examination of the No. 2 bearing revealed its composition consistent with required material types. The bearing separator was fractured completely through both rails at the forward and aft sides. All 10 ball bearings were retained, but half exhibited damage consistent with smearing and material transfer between the raceway and balls.
The No. 2 bearing is the primary means of support for the compressor impeller. Failure of this bearing allows the impeller to migrate forward, potentially contacting the compressor shroud. According to the manufacturer, only minor contact between the impeller and shroud is acceptable; however, examination of the compressor impeller revealed considerable rubbing damage to the shroud and its vanes.