Accident Summary
On June 19, 2014, at about 1030 central daylight time, a Robinson Helicopter Company R-22 Beta, registration N784SH, impacted terrain while performing a practice autorotation at Greensfield Airport (M71) in Moscow Mills, Missouri. The helicopter sustained substantial damage. The flight instructor and the student pilot on board received minor injuries. The helicopter was operated by Helicopter Services and Technologies, LLC (DBA HeliSat) under 14 CFR Part 91 as an instructional flight. Visual meteorological conditions prevailed at the time, and the flight was not operating on a flight plan. It had departed from the operator's facility in Moscow Mills around 0930 for a local flight.
Flight Details
According to the flight instructor, the training flight began about 0930 with normal takeoffs, landings, and autorotations at the airport. At approximately 10:30, the student pilot initiated a 180-degree autorotation. While turning toward the runway during the maneuver, with the rotor RPM just above the green arc and an airspeed of 65 knots, the helicopter began to "fall" through about 100 feet above ground level. The instructor took the flight controls and attempted to roll on the throttle and flare the helicopter before it touched down onto the ground.
Investigation Findings
An examination of the helicopter by a Federal Aviation Administration maintenance inspector revealed no mechanical anomalies that would have precluded normal operation. The sprag clutch assembly was removed and examined at Robinson Helicopter Company under the supervision of personnel from the Los Angeles Aircraft Certification Office and the Manufacture Inspection District Office.
Sprag Clutch Examination
The examination report detailed the following observations: The sheave was rotated by hand in the direction of normal rotation; the clutch shaft moved with the sheave. When rotated opposite the direction of normal rotation, the sheave freewheeled on the shaft. A slight roughness was felt when rotating the shaft. The yokes and support bearing were removed. The forward retainer plate was removed, and the oil was drained from the assembly. The oil appeared black but still had a red tint and did not smell burnt. Approximately 9 milliliters of oil was recovered (typical assembly contains about 17 milliliters). A small, thin metallic chip was recovered from the oil when scanned with a magnet. The bearings were pressed out of the sheave and off the shaft, and the sprag assembly was removed. Both bearings had rough spots when rotated by hand but rotated smoothly after being flushed with solvent. Several very small metallic slivers were recovered from the flushing solvent. The bearing contact surfaces of the inner and outer races showed no discoloration or damage. The inner contact surface of several sprags had small areas of the surface plating gouged or chipped away, with one sprag showing a larger area of damage. The outer surface of the sprags appeared normal. The sprag surface of the clutch shaft and the sheave had several scuff marks that appeared to match the size and spacing of the sprags, with two areas on the shaft matching the shape of the more damaged sprag. No other rotational scoring of the sprag surfaces was noted. The diameter of the sprag surface on the clutch shaft was measured and verified to be within production specifications.