ERA24LA263 · Rotorway EXEC 162F

The pilot of the experimental amateur-built helicopter stated that, as a part of the preflight inspection, he checked the tail rotor drive belt tension with a spring scale and it was within limits. Shortly after departing on the accident flight, while flying at an altitude between 200 and 300 ft mean sea level (msl), the pilot-rated passenger took over the controls. As they approached an airstrip for landing, at an altitude about 150 ft above ground level (agl) with an airspeed of 20-30 mph, which was just below effective translational lift (ETL), the helicopter made an uncommanded yaw to the left. The passenger was unable to maintain directional control with the application of right anti-torque pedal input. The pilot took over the controls from the passenger and applied forward cyclic in combination with full input of the right anti-torque pedal to correct the yaw. However, the helicopter descended in a sweeping left turn and the pilot was unable to regain full directional yaw control before the helicopter contacted the edge of an irrigation ditch. The helicopter rolled over to the right and came to rest, resulting in substantial damage to the airframe and rotor blades. Postaccident examination of the helicopter revealed that the tail rotor tensioner assembly was not secured. The right side of the tail rotor drive belt jam nut and the self-locking nylon nut were loose. The nuts, when properly secured, placed tension on the tail rotor drive belt. Even though the preflight tail rotor tension test was accomplished and within limits, there was no access to physically view these nuts during preflight inspection. Based on the available information, it is likely that the nuts loosened in flight, which would have allowed slack in the drive belt that prevented full tail rotor drive control

Verbatim NTSB analysis and probable cause from the NTSB dataset