What happened
On February 25, 2016, a three-person crew of the Federal Police was conducting a planned night training flight near Bimöhlen, Germany. The mission was designed to test the effectiveness of the aircraft's thermal imaging camera for identifying personnel using specialized blinking lights. The crew utilized night vision goggles (NVG) for the operation.
During the flight, the crew attempted to reposition the EC 135T2+ to improve the thermal camera's view of ground personnel. While performing a low-altitude hover-out-of-ground-effect (HOGE) maneuver with a lateral shift to the left, the aircraft began an uncommanded rotation around its vertical axis (yaw) to the right. The copilot noted the deviation with the exclamation, "oh, oops," but the rotation accelerated rapidly. Within approximately 12 seconds, the helicopter completed about four and a half rotations around its vertical axis before striking a field at a high sink rate. The impact resulted in two fatalities and one serious injury, and the aircraft was destroyed.
The investigation
The BFU examined flight data recorder (FDR) and cockpit voice recorder (CVR) data, as well as thermal imaging footage. The investigation confirmed that there were no mechanical or technical failures in the aircraft's engines, controls, or Fenestron tail rotor system. Testing by the manufacturer and BFU simulations demonstrated that the aircraft's yaw could have been controlled with sufficient pedal input.
The investigation also reviewed the crew's training and operational procedures. It was noted that the pilot in command was heavily focused on the mission objectives (the thermal imaging quality), while the copilot, who was the pilot flying, had limited experience with night operations using NVGs. Furthermore, the investigation found that the crew was operating in violation of certain internal flight manual guidelines, including flying below the minimum required altitude and occupying the incorrect seats for their specific training status.
Findings
- The primary cause of the accident was a loss of control over the aircraft during an uncommanded rotation around the vertical axis.
- The crew's focus on the thermal imaging mission objectives led to significant task saturation and distraction.
- The copilot's limited experience with night vision goggles and night flight operations contributed to an inability to maintain adequate monitoring of flight parameters.
- The pilot in command failed to provide sufficient monitoring and assistance to the pilot flying.
- The crew failed to implement the correct emergency recovery procedures, such as reducing power and increasing forward airspeed, once the rotation became uncontrollable.
- The use of the thermal imaging display on the cockpit's multi-function display (MFD) obscured critical compass information, making it harder to perceive the slow onset of the yaw.