Incident Overview
On June 10, 2005, at 1220 eastern daylight time, a Bell Textron 212 helicopter (registration C-GNHX, serial number 30983) was being ferried from Bolton, Ontario, to Richmond, British Columbia. The helicopter was flown by the company’s chief pilot with two passengers on board. At an altitude of 1500 feet above sea level and an airspeed of 100 knots, a series of loud bangs was heard, followed immediately by severe airframe vibrations.
Pilot Actions and Emergency Landing
The pilot lowered the collective, pulled back on the cyclic control, and reduced the engine throttles to idle. Control was regained after 10 to 15 seconds, but the banging and vibrations persisted. Each advancing main rotor blade was observed to climb off track abnormally. The vibrations intensified as the flight continued. The pilot proceeded toward a large ploughed field for an emergency landing. As airspeed decreased, the helicopter became more controllable, and a successful landing was executed. No injuries were reported. The helicopter sustained substantial damage due to the in-flight vibrations.
Post-Flight Inspection
A post-flight inspection revealed damage to one main rotor blade. A small section of skin near the blade tip, aft of the spar doubler, on the lower surface had debonded. The skin was raised and curled but had not separated from the blade. The debonded area measured 25 inches by 2 inches between stations 263 and 288. Additionally, several main rotor head components and the transmission were damaged by the severe vibrations.
Blade History and Previous Repair
The damaged main rotor blade (part number 212-015-501-115, serial number A-3257) had accumulated 3251 flight hours since new, with a total service life of 4000 hours. Manufactured by Bell Helicopter, it entered service in December 1996. In early 2005, the blade was damaged while the helicopter was parked in a hangar and was shipped to an authorized rotor blade repair shop.
During paint stripping for repair, deep corrosion pitting was discovered on the lower skin surface between stations 243 and 262, just inboard of where the debonding later occurred. Bell Helicopter approved a repair procedure that involved removing the damaged skin and replacing it with a bonded external doubler. The trailing edge trim tab was also replaced. The repair used a bladder and heater blanket tool to apply heat and pressure for adhesive curing. This tool covered the blade from the tip to a point inboard of the repair area, including the location of the eventual debonding. The repair process was performed according to standard procedures, and the blade was inspected by tap hammer before being reinstalled on C-GNHX.
Examination of the Debonding
After the occurrence, the debonded section was examined. A cohesive bond of the adhesive to the skin substrate was present, but the original bonding adhesive was not uniformly adhered to the spar doubler; only small remnants remained. The adhesive had eroded due to airflow and environmental elements entering the debond area between the initial debonding and the emergency landing. Scanning electron microscopy showed that the adhesive remained firmly attached to the skin and replicated sanding marks from the spar, indicating good adhesion to the skin but poor adhesion to the spar. Little adhesive was found on the spar surface. The sanding marks were consistent with standard surface preparation procedures. During subsequent cleaning for further analysis, the spar surface was altered, preventing additional examination of the bond.
Other sections of the blade showed sporadic cohesive separation on the skin side in the previous repair area, while other samples exhibited even adhesion between skin and spar. Two areas of crushed honeycomb core separated from the rear of the spar were found between stations 211–224 and 263–287. Foam adhesive was present on the spar but had not contacted the core. Bell Helicopter’s Engineering Department determined that the skin-to-spar bondline would not have experienced static or fatigue failure during the blade’s service life. No records indicated additional damage to the blade.
Aftermath
The helicopter was substantially damaged, but the pilot’s actions ensured a safe landing with no injuries. The investigation did not identify a specific probable cause for the skin debonding, though the previous repair and adhesive bond quality were examined in detail.