Accident Overview
A helicopter pilot declared a mayday, shut off the fuel valve, and performed an autorotation into high grass after the single-engine helicopter experienced a loss of engine power shortly after takeoff. The pilot reported hearing a bang, followed by the illumination of the engine out warning light, and observed the engine beginning to smoke.
Autorotation and Landing
During the terminal phase of the autorotation, the pilot assumed the grass height was 3 feet and flared accordingly. However, the grass was at least 6 feet tall.
Post-Accident Engine Examination
Post-accident examination of the engine revealed that the No. 1 turbine wheel was missing. The No. 8 bearing thrust nut exhibited a locking indentation and a notch on its inside surface. Additionally, indentations were found on the threads of the threaded end of the first stage turbine wheel shaft. These marks suggest that the thrust nut was not fully engaged on the first stage turbine shaft. The location of the notch suggests that the edge of the nut protruded beyond the end of the turbine shaft. There were no indications of contact by the shaft threads on the inner surface of the deformed area to suggest that it had been displaced into a groove on the first stage turbine shaft.
Maintenance History
Review of the maintenance records and interviews with the mechanic who performed the most recent maintenance work revealed that the turbine section had been replaced approximately 30 hours prior to the accident. Due to excessive oil consumption, maintenance personnel removed the turbine and replaced the No. 5 carbon seal approximately 14 hours prior to the accident. The maintenance records indicated that the day before the accident, the sump cover had been removed for troubleshooting oil consumption. Though it is not detailed in the maintenance entry, it is interpreted that the "sump cover" referred to is the No. 8 bearing sump nut since the thrust nut was loose.