What happened
On September 6, 2010, a Bell 412 HP helicopter, operated by a German air rescue company, was preparing for a patient transfer at the Berlin-Marzahn hospital. The aircraft was positioned on a landing platform on the hospital roof, with its nose facing west. As the crew secured a patient in the cabin and began pre-flight preparations, a loud bang occurred.
The rear cross-tube of the elevated landing gear suddenly fractured. This caused the helicopter to tilt approximately 15 degrees to the right and pitch backward, with the nose rising about 10 degrees. Despite the heavy damage to the aircraft, there were no injuries to the four crew members or the patient.
The investigation
The BFU examined the wreckage and the fractured component. The aircraft was equipped with an elevated landing gear design to provide greater ground clearance. Microscopic and scanning electron microscopy conducted by the Technical University of Braunschweig revealed that the fracture was a fatigue crack. The crack had developed at the most highly stressed point of the component: the area where the rear cross-tube is attached to the fuselage via a central mounting bracket. This specific area experienced a stiffness change and maximum bending moments.
Investigators noted that the aircraft's weight-bearing distribution had changed with the evolution of the Bell 412 model. Unlike earlier models like the Bell 212, which used two attachment points, the 412 uses a single central mounting point, which increased the local load on the tube. The investigation also found that the fatigue crack developed during normal operational stresses, such as landings, loading, and even stationary periods in windy conditions, rather than from a single high-impact event.
Findings
- The primary cause of the failure was a fatigue crack in the rear cross-tube of the landing gear.
- The design of the elevated landing gear for the Bell 412 HP created a localized area of high stress due to the transition to a single central mounting point.
- The increased mass of the 412 model compared to earlier Bell series placed greater structural demands on the landing gear.
- Existing inspection protocols outlined in the manufacturer's Alert Service Bulletin (ASB) were insufficient, as they did not mandate inspections of the specific area where the crack originated.
- The component had completed 17,690 landings, which was below the 20,000-landing life limit specified by the manufacturer.