What happened
On October 6, 2005, a EUROCOPTER AS 350 B2, registration EC-ILD, was performing aerial work near Canfranc, Huesca, to install avalanche protection netting. The aircraft was transporting metal netting using a long external cargo-holding system consisting of a sling, chains, and a metal bar.
After depositing a load at the top of Pico del Águila, the pilot began a descent toward Coll de Ladrones. During this descent, the pilot felt a heavy impact near the tail rotor area but noticed no immediate change in the aircraft's flight attitude or yaw. The pilot subsequently released the cargo hook to jettison the sling and the remaining apparatus. However, upon attempting to manipulate the pedals, the pilot discovered that the tail rotor controls were non-responsive, despite the pedals moving through their full mechanical range.
While attempting to navigate toward the Rio Seta valley for an emergency landing, the pilot reduced the descent rate. At this point, the helicopter entered an uncontrollable spin and crashed violently into the north face of a ravine. The pilot suffered fatal and serious injuries, including multiple spinal fractures.
The investigation
The CIAIAC investigation focused on the mechanical failure of the tail rotor and the sequence of events leading to the loss of control. Investigators examined the wreckage and found that the tail rotor transmission shaft had broken. This damage was consistent with an impact from the metal cargo-holding device.
Testing revealed that at the time of the impact, the aircraft was flying at approximately 50 knots with very low engine torque (under 10%). Under these specific flight conditions, the aerodynamic efficiency of the aircraft's vertical stabilizers was sufficient to offset the remaining torque, meaning the loss of tail rotor thrust did not produce a noticeable yaw. This lack of immediate physical feedback prevented the pilot from recognizing the severity of the failure.
Findings
- The primary cause of the accident was the use of an external cargo-holding device of excessive length that was flying free of any load.
- The impact of the long, unloaded cargo apparatus with the tail rotor caused the breakage of the power transmission shaft.
- The pilot's ability to recognize the emergency was compromised because the aircraft's vertical stabilizers neutralized the yaw that would normally signal a loss of tail rotor thrust at low torque levels.
- The emergency management was inadequate, as the pilot did not follow the flight manual's specific procedures for tail rotor pitch control failure, partly due to a lack of prior assessment of how the aircraft would behave in such specialized mountain flying conditions.