What happened
On July 30, 2005, an Eurocopter AS 350 B2, registration CS-HES, was engaged in firefighting operations near Lamas, Valpaços, as part of a mission for the National Service of Firefighters and Civil Protection. The aircraft, operated by Aeronorte, was performing water drops using the "Isolair" system.
While preparing for a third water drop, the helicopter encountered downward air currents that caused the aircraft to sink. In an attempt to recover altitude, the pilot applied sudden control inputs, increasing collective pitch and pulling back on the cyclic. This abrupt maneuver caused the heavy water refilling hose and pump assembly to swing pendularly, striking the lower part of the tail cone near the horizontal stabilizer.
The impact deformed the tail structure and caused the tail rotor drive shaft to disengage from its connection. Deprived of tail rotor thrust, the pilot could no and no longer counteract the engine torque, leading to an uncontrolled anti-clockwise rotation. The aircraft subsequently struck nearby trees and bushes before crashing into the ground. The pilot, the sole occupant, escaped the wreckage uninjured.
The investigation
The GPIAAF investigation focused on the mechanical failure of the tail rotor drive shaft and the dynamics of the "Isolair" water refilling system. Investigators examined the wreckage, noting severe damage to the tail cone, main rotor, and tail rotor, as well as the unrecoverable state of the engine due to overheating.
Technical analysis of the tail rotor blades indicated they were spinning at low or zero RPM during the impact with vegetation, suggesting the drive shaft connection had already failed. The investigation also examined the operational limitations of the water refilling system, specifically the 3-meter flexible hose and pump assembly, which adds significant mass and creates a pendulum effect during maneuvers.