What happened
On March 6, 2024, at approximately 2140 EST, a Eurocopter Deutschland Gmbh MBB-BK117 C-2 helicopter, registration N191LL, was involved in an accident at Purdue University Airport (LAF) in West Lafayette, Indiana. The aircraft was operating as a Part 135 air ambulance flight.
During a hover taxi intended to accelerate for takeoff, the pilot experienced a force against the pedals, and the helicopter began to yaw to the right. The pilot attempted to correct the yaw by applying full left pedal, but the pedals provided no resistance and had no effect on the aircraft's movement. The helicopter subsequently performed a hard landing. There were no injuries to the pilot or the two medical crew members on board.
The investigation
An examination of the aircraft revealed substantial damage to the fuselage, tailboom, vertical fin, horizontal stabilizer, tail rotor assembly, and one main rotor blade. Investigators found that the tail rotor pitch change bellcrank was not connected to the pitch change slider. The T-bolt and its two attachment bolts were missing from their proper position; one bolt was found on the ramp near the site, while the T-bolt and the second bolt were initially missing.
Post-accident analysis determined that the T-bolt attachment bolts had been installed "finger-tight" during recent maintenance and were not torqued or safety wired. This allowed the bolts to back out during flight. Once the bolts backed out, the T-bolt also separated from the pitch change slider, resulting in a loss of tail rotor pitch control. The T-bolt likely struck a tail rotor blade, causing damage that led to an imbalance and the subsequent separation of the upper vertical fin.
Maintenance records showed that between February 29 and March 6, 2024, the aircraft underwent troubleshooting for pedal feedback issues. During this process, mechanics removed the T-bolt and its hardware to facilitate a friction check of the tail rotor blade mounting forks. This removal was not documented in the official work order. One mechanic installed the T-bolt and bolts temporarily to assist with the installation of other components and was then reassigned to another aircraft without ensuring the hardware was properly secured.