What happened
On June 12, 2019, an AgustaWestland AW139, registration EC-NEH, was performing a positioning flight from Muchamiel to Gijón. While cruising over the Serranía de Cuenca, the aircraft experienced a sudden and severe destabilization. Within just three seconds, the helicopter entered an extreme flight state, reaching a roll angle of -140° and experiencing rapid yaw oscillations and pitch variations.
The extreme forces generated during this event caused two emergency windows to detach from the airframe. The pilot, attempting to recover the aircraft, inadvertently triggered the Force Trim Release (FTR) switches, disconnecting the automatic flight system and assuming manual control. During the maneuver, the engines experienced a turbine overspeed condition, which caused the Electronic Engine Controllers (EEC) to automatically switch from automatic to manual power control mode. The pilot, unaware of this mode change and perceiving the engine behavior as a failure, initiated an emergency descent.
Despite the loss of control and the structural damage, the pilot managed to stabilize the aircraft 11 seconds after the onset of the event. After a 46-second controlled descent, the helicopter performed a run-on landing in a clear area. The pilot was unharmed.
The investigation
The CIAIAC examined the flight data recorder and the physical wreckage to reconstruct the sequence of events. The investigation focused on the cause of the initial destabilization, the detachment of the windows, and the transition of the engine control modes.
Investigators analyzed the structural loads acting on the airframe, confirming that the extreme aerodynamic forces during the roll and yaw oscillations exceeded the design limits for the window assemblies, leading to their detachment. The investigation also scrutinized the engine performance, determining that the overspeed condition was triggered by rapid collective movements during the instability, which caused the EECs to switch to manual mode without the pilot's knowledge.