Accident Details
On October 1, 2022, about 0955 Pacific daylight time, a Bell 206B helicopter (registration N284CA) was substantially damaged during a hard landing in the front yard of a residence near Fresno, California. The pilot and passenger sustained serious injuries. The flight was conducted under 14 CFR Part 91 as an other work use flight.
The purpose of the flight was to perform an operational check following a recent maintenance inspection. After completing the check items and while returning to the airport, the passenger heard a loud crack/pop sound. The pilot reported experiencing a tail rotor failure. The helicopter began to yaw to the right, prompting the pilot to execute an autorotation. During the landing, the helicopter impacted a tree, landed hard, and rolled onto its left side.
Postaccident Examination
Examination of the helicopter revealed that the No. 4 segmented tail rotor drive shaft and its forward splined adapter had separated from the drive shaft tube. The separation area was consistent with a disbond between the splined adapter and the tube. The No. 4 segmented drive shaft was retained for further analysis.
The NTSB Materials Lab examined the shaft. No anomalous damage was found within the splined connection. The serial number "VN12657" was observed on the splined adapter. The outer diameter of the adapter at the aft end was tapered, with thickness measurements below specified limits. The adhesive material between the tube and adapter showed evidence of heating and resolidification. The main wall thickness of the tube (0.051 inch) and its outer and inner diameters (1.004 and 0.9 inches, respectively) were within specified limits.
Maintenance History
The Bell 206B maintenance manual required a recurrent inspection of segmented tail rotor drive shafts during the 100-hour or 300-hour progressive inspections. The inspection criteria included examining the shafts and aft splined adapter for condition and security, and checking lubrication and freedom of movement.
Bell Alert Service Bulletins (ASBs) 206-20-139 and 206L-20-184 required initial and recurring visual inspections and proof-load testing of installed segmented drive shafts with bonded adapters. On June 15, 2022, Transport Canada released AD CF-2022-33, mandating compliance with ASB 206-20-139. The FAA subsequently released AD 2023-06-05 on March 16, 2023, requiring compliance with aspects of the Transport Canada AD.
The accident helicopter's last 100-hour inspection was completed on September 30, 2022, at an airframe total time (ATT) of 26,191.4 hours. The accident flight was the post-maintenance check. On August 11, 2022, at ATT 26,113.7 hours, multiple inspections were performed, including the 100-hour and 300-hour inspections and Part I of ASB 206-20-139. The operator had accomplished Part I of the ASB four times total. About 77.7 flight hours elapsed between the last Part I accomplishment and the accident.
Drive Shaft History
Segmented drive shafts are not life-limited and are not tracked after manufacture, so the accident shaft's time in service could not be determined. Bell could not locate a record for the serial number "VN12657" on the splined adapter, preventing estimation of the shaft's manufacture date.
The operator's director of maintenance stated that a commercially sourced bonded shaft tool was used to accomplish Part I of the ASB. A second tool with internal splines was installed on the oil cooler drive shaft to hold the tail rotor drive inline during proof loading. Examination of the operator's bonded shaft tool showed the distance between the square drive and bolt hole centers was about 3.019 inches, compared to the specified 3.00 inches in the ASB.
Historical Context
According to Bell, between 1986 and 2023 there were 16 known occurrences of in-service disbond of segmented tail rotor drive shafts with bonded adapters, including N284CA. Of these, 11 had no known cause. The remaining five were attributed to reuse after a tail rotor sudden stoppage (3), damage from media blasting (1), and inadequate adhesive at manufacture (1).