Bell 206B N915B accident near Merced, California

Probable cause

The in-flight failure of the engine-to-transmission drive shaft due to a fatigue fracture of one of the KAflex flex frames caused by a loose bolt, which resulted in a total loss of power to the transmission and a subsequent hard landing.

NTSB analysis

The pilot reported that he was on an agricultural spray run. He had just performed a 180° turn and, about six feet above ground level (agl), he felt a vibration. He then felt a “snap” and the helicopter began to rotate left. He applied right pedal, entered an autorotation, and attempted to decrease the airspeed. He tried to perform a run-on landing in the 4-ft-tall corn, but upon touchdown, the helicopter rolled up on its nose and the main rotor blades severed the tail boom.

A postaccident examination revealed that the KAflex driveshaft was separated and there were several sheared bolts and pieces of the KAflex located underneath the engine. Subsequent metallurgical examination determined that the failure of the KAflex coupling resulted from fatigue observed in two flex plate segments and an adjacent attachment bolt, indicating these components were the initial points of failure. The fatigue cracking initiated at areas adjacent to wear and material loss. With the fracture of flex plates and bolts, the interconnect and end fitting could rotate, resulting in the assembly failure. One flex plate and bolt exhibited more fatigue, suggesting longer crack propagation and earlier fracture, while thumbnail cracks on a second flex plate indicated incipient propagation when other components failed in overstress.

The bolt and flex plate with fatigue features showed severe wear and loss of cross-sectional area, potentially concentrating stress at these locations. Although the investigation could not determine which fractured first, the joint had become loose, likely due to wear over time or inadequate assembly/maintenance. Most of the fractured bolts did not have discernable fracture features, likely lost to post fracture wear and battering. Visual inspections during maintenance might reveal a loose bolt, but fatigue cracks in these components would be more difficult to detect, leading to rapid propagation in rotary aircraft parts.

Text from the NTSB. Open the NTSB docket for WPR22LA236 for the full record.