Bell 412HP crash near Ejea de los Caballeros, Zaragoza, Aragon, Spain

The Bell 412 was mobilised from its base at Ejea de los Caballeros to a landfill fire nearby, taking off with ten brigade members and two pilots aboard. Having dropped the brigade at the north of the fire, it flew to the Estanca el Gancho reservoir to load water. During the loading and release test the crew received over-torque and low main rotor RPM warnings; these cleared after the water was released and they lifted a few metres above the reservoir, at which point the low rotor speed warning returned and the helicopter descended into the water. It floated briefly before rolling onto its left side. Both pilots escaped unaided and uninjured. The investigation found the cause was in-flight loss of control following disconnection of the main driveshaft from the combining gearbox, after a loss of integrity in the six threaded joints holding them together: at least one had been operating without a self-locking nut and the remainder had degraded through loss of tightening torque. Contributing maintenance factors were the reuse of threaded joint elements, the absence of a tare torque check as a requirement for reusing nuts, and the operator's tare torque verification practices. Final report A-031/2021.