History of Flight
On November 12, 2010, at 0748 Pacific standard time, a Eurocopter AS350B2, registration N1231A, departed from Whiteman Field in Pacoima, California. Shortly after takeoff, the pilot noted a slight upward change in engine revolutions per minute (rpm) during the turn from upwind to downwind leg. A second, more pronounced rise in engine rpm occurred while the helicopter was abeam midfield. The pilot lifted the collective to arrest the rpm rise, but within less than a second, the engine speed rapidly decreased. The pilot initiated an autorotation, lowering the collective fully and establishing a 60- to 65-knot attitude. Unable to find a suitable landing area ahead without endangering homes, the pilot turned the helicopter 90 degrees toward the airport. During the approach, the pilot raised the collective to extend the glide, clearing powerlines at the airport boundary. The helicopter touched down hard on the runway centerline, then began to rock and yaw right despite full left anti-torque pedal input. The helicopter came to rest upright on the skids, facing 160 degrees from the landing heading. The tail boom had separated, and white smoke emanated from the exhaust. The commercial pilot and one passenger were not injured. The helicopter sustained substantial damage.
Aircraft Information
The helicopter was a Eurocopter AS350B2, serial number 3682, with a total airframe time of 3,456.5 hours. The engine was a Honeywell LTS101-700D2 turboshaft, serial number LE-46130C, with 4,042 total hours and 546.5 hours since major overhaul. The helicopter had a 100-hour inspection completed on October 5, 2010.
Examination and Testing
Investigators examined the engine at Honeywell facilities in Phoenix, Arizona, from January 25-27, 2011. The engine was successfully run in a test cell, reaching takeoff power. During the power turbine (PT) governor check, the PT governor did not govern PT speed, allowing NPT to reach 107 percent. An overspeed protection system check was performed successfully. Leaks were found in pneumatic lines, including at Px bleed orifice threads and Pc filter connections. The PT governor (PTG) and fuel control unit (FCU) were removed for further testing.
Testing of the PTG revealed no PR-PG pressure signal due to metal debris on the throttle cam and excessive movement of the spool bearing cap. Disassembly showed the spool bearing was not properly supporting the cap, with bearing retainer pieces and two loose balls in the drive body cavity. The FCU passed an air leak test but several parameters fell outside test limits during functional testing.
Honeywell Materials Lab examined bearing debris using a scanning electron microscope. Hard particles, including diamond particles, were found embedded in the races and on a ball. The race damage was caused by these hard particles, which were used in the manufacturing process of the ball bearings. No external contamination source was identified.
Findings
The testing and examination disclosed a degraded power turbine governor spool bearing, which rendered the PT governor incapable of modulating governor servo pressure to the fuel control. No evidence of other mechanical malfunctions or failures that would have precluded normal operation was found.