What happened
On March 4, 2018, a Messerschmitt-Bolkow-Blohm BK 117 B-2 helicopter, registration N1174U, was operating as an air ambulance flight from Fond du Lac, Wisconsin, to HSHS St. Nicholas Hospital in Sheboygan. The flight was conducted under night visual meteorological conditions.
During the descent toward the hospital, the pilot experienced restricted cyclic control movement, limiting movement to approximately one inch in any direction, though collective and yaw controls remained normal. The pilot elected to abort the descent and divert to Sheboygan County Memorial Airport (SBM) for a precautionary landing.
While approaching runway 13, the pilot held the cyclic trim switch aft to attempt to gain additional aft cyclic authority. While maintaining continuous cyclic pressure against the aft stop, the pilot slowed the aircraft to 80 knots for final approach. Upon touchdown near the runway centerline, the helicopter skidded approximately 500 feet and exited the runway surface. The crew, consisting of a pilot, flight nurse, and flight paramedic, were not injured.
The investigation
Post-accident examination of the aircraft revealed that the top wire strike protection system was separated due to contact with the main rotor blades. The landing gear skids partially collapsed, and the main rotor blades sustained leading edge damage and separation of the blade skin and core.
Investigators found the cyclic trim was in the full forward and left position. Testing of the cyclic control trim system switch and motors, conducted under FAA supervision, showed no anomalies over 50 cycles in various directions. Additionally, the dual hydraulic pack was tested both on the aircraft and at an overhaul facility; no malfunctions were identified. Maintenance logs showed no recent repairs or reported problems regarding the flight control system.