Event Summary
On Thursday, 21 June 2007, at 18:40 local time, a Bell 412 helicopter with registration EC-HYM was participating in forest fire suppression efforts in the El Canton mountain range, north of Mascivenda, within the municipal boundary of Abanilla (Murcia). The pilot, the sole occupant, had been conducting water drops using a bambi bucket. After releasing several loads, the bucket was left on the ground near the fire. The helicopter then flew to Alcantarilla (Murcia) for refueling, with a maintenance technician on board. Upon return to retrieve the bucket, the helicopter landed on irregular, slightly inclined terrain. The mechanic exited and moved to the front to hook the bucket. At that moment, while the helicopter rested on the ground with some lift (collective not fully down and locked), vertical vibrations began. The pilot decided to climb to eliminate the vibrations, but when they persisted, he attempted to land again, moving laterally about 50 meters across terrain with an approximately 8° incline. During this maneuver, the blades of both rotors struck a tree, causing the helicopter to fall to the ground at coordinates 38° 16’ 37” N, 1° 0’ 20” W, with the nose facing northwest. Impact ignited a fire in the passenger cabin, which was quickly extinguished with on-site equipment. The pilot was not injured and exited the aircraft unaided.
Wreckage Examination
The main rotor sustained significant damage, with two blades (blue and yellow) severely affected. The tail cone detached in its aft half, and the landing gear skids were damaged. No damage was found to the main rotor hub or blade connections. The landing gear rear cross tube pivoted freely and was in good condition. The four pylon isolation mounts were disassembled; some bolts were incorrectly installed. The front left mount had its front bolt short instead of long, and both rear bolts long instead of short. The right aft mount had the right front bolt long instead of short. Despite these discrepancies, no fretting wear was observed among mating surfaces of the mounts, spacers, or pylon support channel.
Maintenance Findings
Post-accident inspection revealed that the collective control friction was near the minimum required limit. The operator reported no awareness of pre-existing vibrations or malfunction signs. The mounts, part number 204-031-927-105, had not been replaced since the helicopter began operations in Spain, though it is unknown if they were original. Analysis by the manufacturer showed that some fastening bolts on the front mounts were bent, though all met design hardness. Corrosion on the inner wall of the front left mount housing caused partial detachment of the elastomer outer cover. Only the front left mount had crack depths within specified limits (must not exceed 0.25 inches). None of the four mounts complied with design height (distance between upper ends of internal and external parts). Two mounts had static elasticity indices below required range in at least one direction, and three had dynamic elasticity indices outside design requirements. The manufacturer stated that mounts with low elasticity indices could resonate with main rotor vibrations, causing excessive lateral movement and control difficulties.
Analysis
The wind at the time of the incident was from the left aft quadrant at 14 knots with gusts to 25 knots, well below the maximum allowed by the flight manual (35 knots for density altitude ≤3,000 ft with wind from that quadrant). Thus, wind was not a determining factor in the vibrations. The incorrect bolt installation in the mounts, while important for proper attachment, likely did not directly cause the vibrations, as no fretting wear was found. The deteriorated condition of the mounts, particularly the substandard elasticity indices, is considered the primary factor leading to the vibrations that ultimately resulted in the loss of control.