What happened
On February 4, 2022, a civil-registered EC135 T3 helicopter was conducting a training flight near the Munster Nord military training area in Germany. The flight was part of a pilot training program involving low-level navigation and basic maneuvers.
During the approach for a slope landing at the Heidehöfe training area, the aircraft transitioned from forward flight to a hover at approximately 100 ft AGL. During this phase, the helicopter lost forward airspeed and entered an uncontrolled rotation around its vertical axis. The aircraft completed approximately 6.5 rotations within 12 seconds before impacting the ground at an angle. Both pilots sustained serious injuries, and the aircraft was heavily damaged. An NH90 crew operating nearby heard the emergency beacon and provided immediate assistance, transporting the pilots to medical care.
The investigation
The BFU examined the flight data recorder, cockpit voice recorder, and cockpit camera footage. The investigation focused on the aircraft's yaw control systems, including the Fenestron tail rotor, the autopilot, and the Yaw Stability Augmentation System (YSAS).
Investigators analyzed the pilot's pedal inputs and the mechanical behavior of the SEMA (Yaw Smart Electro-Mechanical Actuators). The investigation also reviewed the training syllabus of the German military, specifically regarding tail rotor effectiveness and unanticipated yaw. Technical inspections of the wreckage confirmed that all mechanical and hydraulic systems, including the Fenestron drive shaft and engine components, were functioning correctly prior to the impact. No technical failures were identified as the cause of the loss of control.
Findings
- The primary cause of the accident was insufficient pedal correction to counteract the developing rightward rotation, combined with a counterproductive increase in engine power.
- The pilot's foot position on the pedals, as seen on camera, was unusual and potentially limited the ability to utilize the full range of left pedal travel.
- The transition from forward flight to a hover outside of ground effect created aerodynamic changes that increased the demand for active yaw control.
- There were significant training and knowledge gaps regarding the complex interactions between manual inputs, automated systems (such as the autopilot's 'Feet-ON' logic), and the aerodynamic effects of the Fenestron tail rotor.
- The pilot's decision to increase power during the rotation likely exacerbated the yaw rate due to collective yaw coupling.
- The presence of a left crosswind increased the control workload during the approach.