Incident
While cruising at 900 feet above ground level, the pilot noted a heavy shudder followed by a grinding noise, a loud bang, and a low rotor RPM signal. The pilot initiated an autorotation and landed on a road. During touchdown, the main rotor blades flexed downward and severed the tail boom.
Examination
Post-accident examination revealed that the main power (short) shaft had failed. The failure was attributed to misalignment and high temperatures resulting from the misalignment. Further investigation showed that the misalignment was caused by the failure of the pylon isolator mount due to debonding between the upper plate and the rubber damper element.
Findings
The debonding occurred after the isolator mount was exposed to oil and grease over a prolonged period, which chemically deteriorated the bond. There was evidence that the deterioration had been ongoing for a long time. The unit had been inspected periodically, but the impending bond failure had not been detected.