Metro Aviation Eurocopter Deutschland Gmbh MBB-BK 117 C-2 accident

The pilot reported that while in a hover taxi to accelerate for takeoff, he felt a force against his feet from the pedals and the helicopter began to yaw to the right. The pilot was unable to arrest the right yaw with pedal inputs. The helicopter landed hard, which resulted in substantial damage to the fuselage, tailboom, vertical fin, horizontal stabilizer, tail rotor assembly, and one main rotor blade. Postaccident examination revealed that the T-bolt and its attachment bolts, which connects the pitch change bellcrank to the pitch change slider, became disconnected during operation due to inadequate installation. Specifically, the T-bolt attachment bolts were installed but not torqued and safety wired to maintenance manual requirements, allowing them to back out during operation. Once the T-bolt attachment bolts backed out, the T-bolt also backed out and separated from the pitch change slider, resulting in a loss of tail rotor pitch control. The T-bolt likely impacted one of the tail rotor blades, resulting in the separation of the outboard portion of its leading edge and afterbody. The subsequent imbalance of the tail rotor, due to the damaged tail rotor blade, led to overload separation of the upper portion of the vertical fin. During maintenance of the accident helicopter in the days leading up to the accident, the T-bolt and its attachment bolts were removed by a mechanic at the direction of a lead mechanic to facilitate troubleshooting of adjacent components for the tail rotor control system. That mechanic assumed that the T-bolt’s removal was temporary and quick, so he did not follow established procedures by recording the removal in the discrepancy sheet for the work order. The T-bolt attachment bolts were subsequently temporarily installed “finger-tight” by another mechanic to assist a mechanic who was installing the tail rotor blade mounting forks and pitch change links. However, the first mechanic was tasked to another company helicopter shortly after, and while he stated he relayed to the other mechanics that the T-bolt attachment bolts were finger-tight, no one followed up on the installation of the T-bolt attachment bolts to ensure they were torqued correctly and safety wired. Additionally, no one from the maintenance team found that the safety wiring for the T-bolt attachment bolts was missing during final checks of the helicopter before it was released for an operational check flight. Since there was no maintenance discrepancy entry for the removal of the T-bolt, there was no formalized task for the maintenance team to track and verify the complete T-bolt installation, including torquing and safety wiring the T-bolt attachment bolts. Therefore, the lack of an entry in the work order discrepancy sheet for the T-bolt’s removal contributed to this accident.

Verbatim NTSB analysis and probable cause from the NTSB dataset