WPR23LA004 · Bell 206B

During a post-maintenance operational check flight, the pilot and mechanic heard a loud crack/pop sound, followed by the helicopter spinning to the right. The pilot initiated an autorotation, and the helicopter subsequently impacted a tree and landed hard, which resulted in a rollover and substantial damage to the helicopter

Postaccident examination of the helicopter revealed that the No. 4 segmented tail rotor drive shaft failed due to disbonding of the splined adapter from the tail rotor drive shaft tube, which resulted in a loss of tail rotor drive to the tail rotor gearbox and subsequent loss of directional control of the helicopter. Once the disbond occurred, the splined adapter continued to turn under power while the drive shaft tube slowed down, further damaging the disbonded surfaces, which prevented identification of a reason for the disbond.

The operator accomplished the recurrent proof-load test on their bonded segmented drive shafts, as outlined by Bell Alert Service Bulletin (ASB) No. 206-20-139, a total of 4 times since release of the ASB, with no anomalous findings. The ASB specified that within the next 75 flight hours or 3 months after release of the bulletin, and every 300 flight hours or 12 months thereafter, operators should conduct a visual inspection of the bond line of the bonded adapters, apply an index mark on each bonded adapter, and conduct a proof-load test using a bonded shaft tool. The operator had last performed the proof-load test about 77.7 hours before the accident. Accordingly, the frequency of the proof-load test in the ASB, which was subsequently required by Transport Canada Airworthiness Directive (AD) No. 2022-33 and Federal Aviation Administration (FAA) AD No. 2023-06-05, may not be adequate in discovering an impending failure of a bonded adapter. Testing by Bell on exemplar bonded segmented drive shafts, in support of the development of the proof-load testing (preceding issuance of the ASB), showed that a drive shaft [with a serviceable bond] would yield in excess of 200% applied torque, and that yielding of the shaft tube occurs first. Based on a review of the operator’s proof-load testing tools, the operator was using the correct tooling. Additionally, there was no evidence the operator applied excessive torque on the bonded segmented drive shafts during the recurrent proof-load tests. Therefore, the operator’s continuing maintenance and performance of the proof-load tests on the bonded segmented drive shafts were likely not a factor in this accident.

Metallurgical examination of the bonded tail rotor drive shaft found that the splined adapter tapered thin wall did not meet its thickness requirements; however, it’s likely this was not the sole factor in the disbond. The segmented drive shafts are not life limited and therefore are not required to be tracked after manufacture; accordingly, the age and time in service of the accident drive shaft could not be determined.

Verbatim NTSB analysis and probable cause from the NTSB dataset