What happened
On August 12, 2004, at approximately 20:45 local time, a Bell 407 helicopter, registration EC-IMZ, was performing aerial firefighting duties near Candasnos, Huesca. The aircraft, operated by Helicópteros del Sureste, S. A., had departed from the Peñalba Fire Base to assist in extinguishing a fire involving stubble and tires.
During the operation, the pilot performed a reconnaissance flight and identified two existing power lines in the area. After delivering a forest brigade to the site and preparing the aircraft, the pilot began water drops using a "bambi bucket." While flying a north-bound course parallel to one power line, the pilot observed a second line crossing perpendicularly in front of the aircraft. In an attempt to clear the obstacle, the pilot increased the aircraft's pitch to climb.
During this maneuver, the suspended bucket struck the electrical cables. The impact caused a sudden downward movement of the aircraft's nose and a physical jolt. The pilot subsequently landed the aircraft to inspect the situation. While the pilot remained uninjured, the impact caused damage to one of the main rotor blades and rendered the bucket unusable. Additionally, one of the electrical towers was bent by the impact, though no cables were severed.
The investigation
The investigation focused on the flight path and the pilot's awareness of the obstacles. It was established that the aircraft was flying approximately six meters away from a north-south power line. During the water drop, the bucket was positioned at an altitude lower than the parallel power line, which effectively restricted the use of lateral release mechanisms for the load.
Inspectors examined the aircraft's cargo hook system, noting it featured both electrical and manual release options. The investigation also reviewed the pilot's actions, noting that although the pilot had observed the power lines during the initial reconnaissance, the perpendicular line was not identified in time to prevent the collision.
Findings
- The primary cause of the incident was that the pilot failed to perceive the presence of the perpendicular power line in time to avoid the collision.
- The positioning of the bucket below the height of the parallel power line meant that the lateral release gates were closed, limiting emergency options.
- The pilot did not activate the cargo hook release mechanism prior to the impact, an action that could have mitigated the damage to the aircraft and increased operational safety.