What happened
On the afternoon of July 11, 2021, a Bell 4/12 helicopter, registration EC-MTS, operated by Rotorsun, was engaged in a firefighting mission near Ejea de los Caballeros, Spain. The aircraft was transporting a crew and fire brigade members to assist with a fire at a local waste recycling site. After dropping off the personnel, the helicopter proceeded to the Estanca el Gancho reservoir to load water using a helibucket.
While hovering over the reservoir to test the helibucket's operation, the crew noticed a drop in rotor revolutions (NR) and received low RPM and overtorque warnings. Although the pilots attempted to stabilize the aircraft by adjusting power settings, the rotor speed continued to decline. At 20:04 local time, the main driveshaft disconnected from the gearbox, causing the helicopter to plummet approximately seven meters into the water. The two crew members on board were able to evacuate the submerged aircraft unaided and sustained no injuries.
The investigation
The CIAIAC investigation focused on two primary issues: the helicopter's initial difficulty maintaining a hover and the mechanical failure of the transmission system. Investigators examined the wreckage and the maintenance history of the aircraft's main driveshaft and fuel control system.
Regarding the flight dynamics, the investigation found that while the helicopter did experience difficulty maintaining a stationary hover, this was not caused by the driveshaft failure. Technical anomalies in the fuel control system were identified, including unauthorized modifications intended to increase fuel delivery, though these were not ruled out as the primary cause of the loss of control.
Regarding the mechanical failure, the investigation established that the connection between the main driveshaft and the combining gearbox failed because the integrity of the six threaded joints holding them together had been lost.
Findings
- The primary cause of the accident was the in-flight disconnection of the main driveshaft from the gearbox.
- The failure occurred because at least one of the six threaded joints was operating without a self-locking nut.
- Three of the remaining joints showed evidence of operating below the required tightening torque, leading to their eventual rupture under stress.
- Maintenance practices contributed to the failure, specifically the reuse of threaded joint elements and the lack of a required tare torque check during the reuse of nuts.
- Unauthorized modifications to the fuel control system were present, which had been prohibited by the manufacturer for two decades.
- The crew did not utilize lifejackets or portable breathing devices during the water evacuation, although this did not impact their survival.