Incident Overview
A helicopter sustained substantial damage during a hard landing following a loss of engine power and subsequent autorotation. The flight instructor reported that shortly after takeoff and within two miles of the departure airport, the helicopter suddenly yawed to the left and the engine began to uncontrollably increase and decrease speed. He noted that it sounded as if the throttle was being rolled on and off. No warning lights were illuminated.
Pilot Actions
The instructor took control of the helicopter and instructed the dual student to apply carburetor heat. However, the engine continued to rev and was "making a popping sound, like it was backfiring." The instructor observed a drop in rotor speed and elected to initiate an autorotation at an estimated altitude of 500 feet above ground level. He described the safest landing spot as a paved road in a subdivision, where the helicopter struck the ground hard, resulting in damage.
Post-Accident Inspection
A post-accident inspection revealed no anomalies. The engine was subsequently removed and torn down, but no abnormalities were found during disassembly and examination.
Weather Conditions
Weather conditions recorded at the departure airport approximately 20 minutes after the accident included a temperature of 9°C and a dew point of 1°C. Data concerning the probability of carburetor icing was obtained from Transport Canada. The conditions bordered regions of serious icing at any power setting and moderate icing at cruise power.