What happened
On November 14, 2007, an MD 900 helicopter, operated by the police, was performing an emergency landing at Hannover Airport following a mechanical malfunction. The flight crew, consisting of a pilot, a flight technician, and a systems operator, had been conducting a search mission. During the approach to landing pad number 2, the pilot noted that the aircraft began rotating to the right around its vertical axis. Despite attempts to compensate using left pedal inputs, the rotation persisted, prompting the pilot to abort the landing and climb to a safer altitude.
While circling south of the airport to analyze the issue, the crew observed that the aircraft continued to yaw right whenever the airspeed dropped below approximately 90 knots. This maneuver was accompanied by intense vibrations throughout the airframe. After several unsuccessful landing attempts, the crew attempted a final landing on runway 09L. During the seventh attempt, the helicopter touched down with the left skid first and began sliding. After approximately 380 meters, the aircraft veered sideways and off the runway. The left skid broke, the aircraft tipped onto its side, and the main rotor struck the ground. The three persons on board sustained light injuries, but the aircraft was heavily damaged.
The investigation
The BFU investigation focused on the mechanical failure of the NOTAR (No Tail Rotor) system's thrust vectoring mechanism. Investigators examined the connection between the Bowden cable and the thrust rod, discovering that the connection had severed. A metallurgical analysis performed by the Technical University of Braunschweig confirmed that the failure was a fatigue fracture (Schwingungsbruch) caused by high local stress concentrations at a machined notch in the threaded bolt.
Further examination revealed that the aircraft had recently undergone a modification to extend the tail boom by 53 cm. This change in the boom's length likely altered the structural stiffness and vibration characteristics of the control assembly. The investigation also reviewed the aircraft's flight data, onboard computer logs, and video recordings of the landing attempts. The investigators also tested the behavior of an identical aircraft type to determine how the thruster position influenced flight characteristics during low-speed maneuvers.
Findings
- The primary cause of the accident was the separation of the mechanical control for the steerable thruster due to a fatigue fracture.
- Following the breakage, the thruster remained in a position that was highly unfavorable for directional control during low-speed flight and landing, specifically when the pilot applied left pedal inputs.
- The existing emergency procedures in the flight manual were insufficient for this specific failure mode, as they did not account for the fact that pedal inputs could inadvertently increase fan performance and exacerbate the yawing effect due to the fixed thruster position.
- The recent extension of the tail boom likely contributed to the failure by altering the vibration and stress levels at the fracture site.