What happened
During a day visual flight rules training session approximately 20 nautical miles north of Toronto / Buttonville Municipal Airport, an instructor and a student pilot were practicing autorotations in a Robinson R22 Beta. At an altitude of 1700 feet, the instructor simulated an engine failure. The student pilot selected a landing field and began a forced approach.
As the aircraft descended to 40 feet above ground level, the student initiated a flare by moving the cyclic aft. Noticing tall trees at the edge of the field, the student applied aggressive upward collective and rearward cyclic input to avoid the obstacle. Although the instructor opened the throttle to attempt a recovery, the engine failed to respond because the rotor RPM had already dropped below safe limits. The helicopter continued its descent and struck the ground, causing the aircraft to pivot onto its right side. Both occupants evacuated the Robinson R22 Beta without injury, though the helicopter sustained substantial damage.
The investigation
Investigators examined the aircraft, which had 4699 total flight hours, and found no evidence of mechanical failure or pre-impact airframe malfunction. The drive train and flight control systems were intact. The investigation focused on the mechanics of the low-inertia rotor system and the execution of the autorotation procedure. It was determined that the student's aggressive control inputs at low altitude, combined with the low state of the rotor, prevented a successful recovery.
Findings
- The primary cause of the accident was that the main-rotor RPM dropped below safe limits, resulting in an aerodynamic stall of the rotor blades.
- The instructor allowed the training exercise to proceed into a landing area that was unsuitable for the maneuver.
- The instructor failed to place sufficient emphasis on the specific risks associated with low rotor RPM and rotor-blade stalls prior to the exercise.