History of Flight
On February 23, 2016, at about 1145 Pacific standard time, a Robinson R66 helicopter, registration N266RH, sustained substantial damage to its tailboom during a precautionary autorotation at Zamperini Field Airport (TOA) in Torrance, California. The helicopter was an experimental research and development category test aircraft, registered to and operated by Robinson Helicopter Company (RHC) under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which originated from TOA at 1145.
The flight's purpose was to test the auxiliary fuel system, low RPM horn, autopilot, and sprag clutch. The pilot, the sole occupant, reported no injuries.
According to the pilot, he performed a takeoff to a hover, a hover taxi, and one hovering autorotation without incident. He then executed a maximum performance takeoff. At about 200 feet above ground level, the pilot heard a noise and felt a momentary vibration. Subsequently, he performed an autorotation to a grassy area on the airfield. After landing in soft grass with forward momentum, the helicopter experienced a fore-and-aft rocking motion, and the main rotor blades contacted the tailboom.
Aircraft Information
The five-seat helicopter, serial number 0002, was manufactured in 2009. Its maximum gross weight was 2,700 pounds, and it weighed 2,047 pounds at the time of the accident. The helicopter was powered by a 300-horsepower Rolls-Royce 250-C300/A1 engine. The last annual maintenance inspection was conducted on October 15, 2015, at a total engine time of 363 hours. The airframe had 400.04 total hours at the time of the accident, with 20 hours since the last inspection.
Tests and Research
A visual examination of the airframe components found no anomalies. No metal particles were found in the oil filter or chip detector. The Engine Monitoring Unit (EMU) data, covering the entire accident flight, was downloaded and reviewed by a Rolls-Royce representative. No exceedances were noted; however, at time stamp 32.27.1, the power turbine (N2) peaked at 107.76% and the torquemeter oil pressure peaked at 95.304% (275 ft/lbs). According to the Rolls-Royce 300 Operational and Maintenance Manual, the N2 limit is 110% for 15 seconds, and the torque limit during takeoff is 288 ft/lbs with a maximum transient of 430 ft/lbs.
With no pre-accident anomalies noted, the engine was test run on the helicopter at various power settings. No anomalies were observed, and the engine data downloaded after the runs showed no faults.
Following the engine run, the sprag clutch was inspected and found to have moved between 45 and 90 degrees from its original position. When removed, it required about 550 pounds of torque to loosen the clutch assembly, compared to the 300 pounds used during assembly. The clutch itself showed no anomalies.
A review of the maintenance logbook revealed that a new Revision E sprag clutch had been installed before the accident flight. Comparison with the previously installed Revision D sprag clutch showed wear patterns that appeared normal and consistent with those observed in the Revision E assembly.
The Pilot's Operating Handbook states that a vibration can indicate a drive system failure, but such failure is also accompanied by nose yaw and a decrease in rotor RPM while N2 RPM increases. RHC Safety Notice SN-39, issued in July 2003, states that an unusual vibration may indicate a main rotor blade crack, characterized as severe and making the helicopter difficult to control.