Amateur-built 400 N326RW accident near Aguilares, Texas

Probable cause

The loss of tail rotor control due to the in-flight failure of the tail rotor pinion.

NTSB analysis

The private pilot departed in an amateur-built helicopter on a local flight for phase I flight test requirements. The helicopter was between 20 and 30 ft above ground level when it started to shudder. The pilot looked for a place to land and then heard a "pop"; the helicopter then lost tail rotor authority. The helicopter started to rotate counterclockwise, and the pilot attempted to stop the rotation without success. The helicopter impacted the ground and rolled onto its left side, which resulted in substantial damage.

Postaccident examination of the transmission revealed that the tail rotor pinion had failed. There was no damage to the tail rotor blades or the tail rotor driveshaft. Laboratory examination of the tail rotor pinion revealed that both halves of the pinion shaft fracture surface exhibited mechanical damage by recontact resulting in rubbing of the fracture surfaces. No evidence of a preexisting crack was observed. Microhardness measurement values were consistent with the specifications for the pinion shaft. There were no other mechanical anomalies with the helicopter or its systems that would have precluded normal operations.

The lack of damage to the tail rotor blades and tail rotor driveshaft is consistent with the failure of the tail rotor pinion in flight and not as a result of ground impact. Due to the recontact rubbing of the fracture surface, the investigation was not able to determine the exact reason for the pinion failure.

Text from the NTSB. Open the NTSB docket for CEN18LA247 for the full record.