What happened
During an aerial crop spraying operation in Ontario, a Bell 47-G2 helicopter experienced a catastrophic mechanical failure. While the pilot was performing a 180-degree turn at approximately 100 feet above the ground, a loud bang was heard, followed by an immediate loss of control. The aircraft rolled left and entered an uncontrollable pitch sequence before striking the ground. The impact triggered a post-crash fire that consumed much of the engine and cabin area. The pilot sustained serious injuries and was rescued from the burning wreckage by bystanders passing on a nearby road.
The investigation
Investigators examined the wreckage and identified that one of the two main rotor blades—distinguished by its blue tip—had separated from the rotor hub. The separation occurred at the inboard end within the internal threaded region of the blade grip. Notably, the main rotor head, mast, and transmission detached from the helicopter entirely before it hit the ground.
Technical analysis of the components revealed that the blue blade grip failed due to fatigue cracking. This cracking originated in the internal threads of the grip. Examination of the second blade grip, identified by its yellow tip, revealed similar, though less advanced, cracking. While the threads met standard specifications, the root radii were significantly smaller than those of a known good spare, and extensive pitting was present. These features acted as stress concentrators that likely facilitated the crack's development.
Findings
- The primary cause of the accident was the separation of a main rotor blade caused by fatigue cracking within the blade grip threads.
- The failure occurred 992 hours before the component was scheduled for its first mandatory inspection.
- The presence of sharp root radii and pitting on the blade grips provided a plausible mechanism for the initiation of the fatigue cracks.
- No abnormal operational or installation conditions were found to explain why the failure occurred so far ahead of the required inspection interval.