What happened
While conducting a forest fire mapping mission north of Grand Lake, Labrador, a Bell 206L helicopter was flying at 400 feet above ground level when the pilot experienced a sudden, loud bang and a sharp leftward yaw. The pilot, accompanied by two passengers, immediately initiated an autorotative descent toward a nearby spit of land. During the descent, the pilot noted a potential momentary illumination of the engine out caution light, though no low rotor RPM warnings or horns were heard.
As the pilot attempted to flare for landing at approximately 50 feet, the main rotor RPM decreased, resulting in a hard landing on a surface covered in large boulders. The impact caused the helicopter to yaw uncontrollably to the left. While all three occupants escaped without injury, the aircraft sustained substantial damage, including structural deformation to the aft fuselage and tail boom.
The investigation
Investigators examined the engine and main transmission, finding that while the engine was functional, the transmission had suffered significant internal damage. Specifically, the sun gear splines were damaged where they engaged with the main ring gear shaft. The investigation revealed that the installed sun gear was actually a part designed for a Bell 206B, which was approximately 1.2 inches shorter than the required part for the Bell 206L.
Tracing the component's history, the TSB found that the part was ordered via telephone outside of normal business hours. A breakdown in communication occurred during the ordering or shipping process, leading to the delivery of the wrong part. Furthermore, a records clerk at the operator's main base noticed a discrepancy between the purchase order and the shipping invoice but chose to overwrite the purchase order to match the invoice rather than alerting maintenance personnel.
Findings
- The failure of the transmission was caused by the installation of an incorrect sun gear that was too short for the application.
- The insufficient engagement of the gear splines led to the uncoupling of the engine from the main rotor.
- A breakdown in communication between the operator and the supplier resulted in the wrong component being delivered.
- Inadequate oversight and a lack of physical verification by the maintenance engineer and supervisor allowed the error to go undetected during installation.
- The low rotor RPM warning system failed to alert the pilot because the engine continued to drive the rotor tachometer generator even after the main rotor had uncoupled from the engine.