What happened
A Bell Textron 212 helicopter, registration C-GNHX, was performing a ferry flight from Bolton, Ontario, to Richmond, British Columbia, when the crew encountered a sudden mechanical failure. While cruising at 1,500 feet above sea level, the pilot and two passengers experienced a series of loud bangs followed by intense airframe vibrations. The pilot initially struggled to maintain control for approximately 15 seconds but managed to stabilize the aircraft by lowering the collective, adjusting the cyclic, and reducing engine throttles to idle.
As the flight continued, the vibrations intensified, and the main rotor blades exhibited abnormal climbing behavior during each forward pass. To mitigate the risk, the pilot identified a large ploughed field for an emergency landing. The aircraft was successfully landed with no injuries to the three occupants, though the helicopter sustained substantial damage due to the extreme vibrations.
The investigation
Investigators examined the aircraft's maintenance history and the specific condition of the damaged rotor blade. A post-flight inspection identified that a 25-inch by 2-inch section of the lower skin near the blade tip had debonded from the spar doubler. The blade, which had 3,251 hours of service life, had previously undergone a repair in 2005 to address deep corrosion pitting. This repair involved using a bladder and heater blanket tool to bond a new skin to the spar.
Microscopic analysis of the debonded area revealed that while the adhesive had adhered well to the skin, it had failed to bond effectively to the spar surface. The investigation found that the original manufacturing adhesive was largely absent from the spar, leaving only small remnants. Furthermore, investigators discovered areas where the honeycomb inner core had separated from the spar, though manufacturer analysis suggested this specific separation was not the primary cause of the skin failure.
Findings
- The primary cause of the incident was the failure of the adhesive to bond to the spar surface during the previous repair process.
- The debonding of the rotor blade skin created severe aerodynamic imbalances, leading to intense vibrations and difficulty in flight control.
- While the repair procedure was performed according to standard protocols, the lack of adhesive on the spar surface allowed the skin to lift and curl.
- The extreme vibrations caused secondary damage to several main rotor head components and the transmission.