What happened
Shortly after takeoff, approximately two miles from the departure airport, the helicopter experienced a sudden yaw to the aircraft's left. The flight instructor reported that the engine speed began to increase and decrease uncontrollably, creating a sensation as if the throttle were being moved on and off. Despite these fluctuations, no warning lights illuminated in the cockpit.
The instructor took control of the aircraft and instructed the student to apply carburetor heat. However, the engine continued to rev irregularly and produced a popping sound similar to backfiring. As rotor speed began to drop, the instructor initiated an autorotation at an estimated altitude of 500 feet above ground level. The crew attempted to land on a paved road within a subdivision, resulting in a hard landing that caused substantial damage to the aircraft.
The investigation
A post-accident inspection of the aircraft revealed no anomalies. The engine was subsequently removed and disassembled for a detailed examination; however, no abnormalities were found during the teardown process. Investigators also reviewed weather data from the departure airport recorded 20 minutes after the accident, which indicated a temperature of 9 degrees Celsius and a dew point of 1 degree Celsius. Data from Transport Canada regarding carburetor icing probabilities suggested that conditions were bordering on regions of serious icing at certain power settings.
Findings
- The engine experienced unstable power fluctuations and backfiring sounds.
- Rotor speed decreased, necessitating an emergency autorotation.
- Weather conditions were near the threshold for significant carburetor icing.