NTSB Factual Report

On June 19, 2014, at 1030 central daylight time, a Robinson Helicopter Company R-22 Beta, N784SH, impacted terrain during a practice autorotation at Greensfield Airport (M71), Moscow Mills, Missouri. The helicopter sustained substantial damage. The flight instructor and student pilot sustained 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 of the accident. The flight was not operating on a flight plan and departed from the operator's facility in Moscow Mills, Missouri, about 0930 on a local flight

The flight instructor stated that training flight began about 0930 with normal takeoffs, landings, and autorotations at M71. At approximately 10:30, the student pilot began to perform a 180-degree autorotation. While turning towards the runway during a 180-degree autorotation, with the RPM just above the green arc and with an airspeed of 65 knots, helicopter began to "fall" through about 100 feet above ground level. The instructor took the flight controls and tried to roll on the throttle and flare the helicopter before it touched down onto the ground.

Examination of the helicopter by a Federal Aviation Administration maintenance inspector revealed no mechanical anomalies that would have precluded normal operation. The sprag clutch was removed and examined at Robinson Helicopter Company under the supervision of personnel from the Los Angeles Aircraft Certification Office and Manufacture Inspection District Office.

The report of the sprag clutch examination (A166-1 clutch shaft serial number 9795, A188-2 sprag assembly serial number 10453, and the A184-1 support bearing) stated, in part:

The sheave was rotated by hand in the direction of normal rotation, the clutch shaft moved with the sheave, as the sheave was rotated opposite the direction of normal rotation, the sheave freewheeled on the shaft. A slight roughness was felt when rotating the shaft.

Both 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 to it. The oil did not smell burnt. Approximately 9 milliliters was recovered from the assembly (typical clutch assembly contains approximately 17 milliliters). The recovered oil was scanned with a magnet and a small, thin metallic chip was recovered.

The bearings were pressed out of the sheave and off of the shaft and the sprag assembly removed. Both bearings had rough spots when rotated by hand. Both bearings were flushed with solvent and afterward the bearings rotated smoothly. The flushing solvent from both bearings was scanned with a magnet and several very small, metallic slivers were recovered. The bearing contact surface of the inner and outer races were visually examined, no discoloration or damage was noted.

The inner contact surface of several sprags of the sprag assembly had small areas of the surface plating (proprietary to Formsprag) gouged/chipped away with one sprag having a larger area damaged. 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 that match the shape of the more damaged sprag. There was no other rotational scoring of the sprag surfaces. The diameter of the sprag surface on the clutch shaft was measured and verified to be within production specifications.

NTSB Probable Cause

The flight instructor’s delayed remedial action to correct the student pilot’s use of an excessive descent rate during a practice autorotation, which resulted in a hard landing.