Accident
On June 23, 2014, about 1345 central daylight time, an Airbus Helicopters AS-350B2, registration N407EM, was substantially damaged following an autorotation near Texarkana, Texas. The commercial pilot, two crew members, and a passenger were not injured. The helicopter was operated by EagleMed LLC as an emergency medical services flight under 14 CFR Part 135. Visual meteorological conditions prevailed, and the flight operated on a visual flight rules flight plan. The flight originated from Idabel, Oklahoma, about 1320, and was destined for Texarkana, Texas.
According to statements from the pilot and medical crew, the helicopter was in cruise flight at about 1,000 feet above ground level, five minutes from landing at the destination hospital, when the pilot noticed the rotor RPM rapidly increase accompanied by a high rotor aural warning. The pilot attempted to troubleshoot the malfunction before deciding to perform an autorotation to a farm field. During the run-on landing, the helicopter skipped and settled firmly to the ground. The main rotors contacted and partially severed the tail boom, resulting in substantial damage. Damage to the front of the left skid was also found.
Investigation
The engine was removed and sent to Honeywell's laboratories in Phoenix, Arizona. Under FAA oversight, the engine was examined and placed on a test bed for an engine run. The engine started and idled for 8 seconds before performing an uncommanded acceleration, at which point the run was terminated. Three subsequent runs isolated various connections to troubleshoot the uncommanded acceleration. Only when the pg line from the fuel control unit (FCU) to the power turbine (PT) was removed did the engine not experience the uncommanded acceleration.
The FCU was removed and inspected. The drive bearing appeared degraded and no longer provided axial or radial positioning of the FCU drive shaft. The splined fuel pump to FCU coupler spun freely on the FCU drive shaft. The fuel pump was inspected, and debris and three bearing balls from the FCU were removed from the fuel pump overboard drain passage. The fuel pump was then sent to Triumph for additional testing, which found no defects.
Disassembly of the FCU revealed that the drive bearings had failed. Metallurgical analysis of the FCU bearings and particulates found evidence of polyimide material in particles located during FCU removal. The bearing separators are constructed of a porous polyimide material. Particles rich in silicon and oxygen were found embedded in ball contact surfaces of the drive bearing inner and outer races, as well as in undamaged bearing balls. Bearing balls recovered from the drain passage were dented and covered with aluminum-rich material. No material anomalies were detected in the components. The reason for the failure of the drive bearing could not be determined.
On December 18, 2014, Honeywell Service Bulletin LT 101-73-20-0272 was amended to change a one-time replacement of the main drive bearings to every 600 hours. A search of the FAA's Service Difficulty Reporting Site did not find any reports of part failure after the issuance of the updated service bulletin.