What happened
On June 21, 2007, a Bell 412 helicopter, registration EC-HYM, was engaged in aerial firefighting operations in the El Canton mountain range near Abanilla, Murcia. The aircraft, operated by Helicópteros del Sureste, S. A., was performing water drops using a bambi bucket. After several drops, the pilot left the bucket on the ground to allow a maintenance technician to board the aircraft for a refueling trip.
Upon returning to the site to retrieve the bucket, the helicopter landed on irregular, inclined terrain. While the aircraft was on the ground, the pilot noted significant vertical vibrations. The collective lever was not in the fully down and locked position, which may have allowed vibratory interactions between the rotor and the airframe. In an attempt to eliminate the vibrations, the pilot attempted to climb, but when the shaking persisted, he attempted a lateral landing maneuver approximately 5/0 meters away. During this maneuver, the rotor blades struck a tree, causing the aircraft to crash. The impact triggered a fire in the passenger cabin, though it was quickly extinguished. The pilot escaped without injury, but the aircraft sustained significant damage.
The investigation
Investigators examined the wreckage and the maintenance history of the aircraft. While the investigation found that several bolts in the pylon isolation mounts had been installed incorrectly (swapping long and short bolts), no fretting or wear was found that would suggest these errors directly caused the accident.
However, the inspection of the four pylon isolation mounts revealed critical issues. The mounts were found to be degraded; specifically, one mount showed corrosion that had caused the elastomer material to detach. Furthermore, the static and dynamic elasticity indices of the mounts were outside of the required design specifications. The investigation also noted that the aircraft had previously been out of service for an extended period, which likely allowed water to penetrate the mounts and cause this degradation.
Findings
- The primary cause of the accident was the onset of vibrations that degraded flight conditions and hampered control, forcing the pilot to perform an immediate landing that resulted in the tree strike.
- The vertical vibrations were likely caused by the simultaneous occurrence of three factors:
- The collective stick was not in the full down position, potentially allowing rotor-to-airframe vibratory interactions.
- The degraded mechanical characteristics of the pylon mounts, which had lost their ability to effectively absorb vibrations due to corrosion and improper elasticity rates.
- The aircraft was operating on irregular, inclined terrain under adverse wind conditions during the preceding landing.