C-FRWI, a Robinson R44 Raven II helicopter operated by Synergy Aviation, had completed the days flying and was enroute to a fuel cache before returning to base. At approximately 1500 feet AGL while at cruising speed, the pilot experienced a yaw to the right, followed by a low rotor warning horn. The collective was lowered to regain rotor and engine speed. However, when power was brought back, a severe vibration developed, accompanied with a loud clacking noise. The pilot initiated an autorotation approach into a well site and, when power was brought back on short final, the vibrations increased. A full auto rotational landing was conducted.
After the landing, the pilot visually inspected the aircraft for damage; none was found.
The operator’s maintenance determined that an exhaust valve had failed on the Avco Lycoming IO-540-AE1A5 engine.