What happened
On a day in British Columbia, a Bell 214B-1 helicopter was performing heli-logging operations approximately 10 nautical miles southeast of New Denver. While the crew was in the process of lifting two logs, a loud noise originated from the rear of the aircraft. Following this bang, the helicopter began to rotate in a clockwise direction. Witnesses on the ground noted that the tail rotor had ceased spinning.
Recognizing a tail rotor failure, the pilot executed emergency procedures and attempted to land the aircraft at the edge of a logged area near a gary and a stand of small trees. During the landing, the main rotor blades struck the trees, causing the helicopter to roll over into the gully. The incident resulted in one serious injury to the co-pilot, while the pilot sustained minor injuries. The aircraft suffered substantial damage, and a small fire broke out in the engine area following the impact.
The investigation
Investigators examined the wreckage and found that the tail rotor drive shaft had sheared at a point where it enters a protective tunnel beneath the engine. Further analysis of the helicopter revealed that the left, upper transmission spindle mount had broken due to fatigue. Laboratory examination of the broken mount showed that a crack had developed at the radius between the barrel and the shoulder of the spindle. This crack progressed through alternating stages of fatigue and overload until the spindle finally failed.
The investigation looked into the maintenance history of the transmission. The transmission had been overhauled by Transwest Helicopters, but the process involved several complexities. The operator had purchased the upper transmission case and the spindles as separate units. Because the spindles were already attached to the upper case when sent for non-destructive testing (NDT), the required magnetic particle inspection (MPI) for the spindles was omitted. The investigation also noted that the overhaul was performed by three different technicians without a single point of supervision.
Findings
- The tail rotor drive shaft failure was caused by bending loads resulting from the misalignment of the transmission.
- This misalignment occurred because the transmission spindle mount failed due to fatigue.
- The magnetic particle inspection (MPI) required for the spindles was not performed during the last overhaul.
- The omission of the inspection was due to a lack of maintenance supervision and the complex nature of the overhaul process involving multiple technicians and separate component acquisitions.
- The origin of the initial fatigue crack in the spindle could not be determined, though the part showed evidence of previous rework.