Accident Overview
On March 4, 2020, about 1842 central standard time, a Bell 206 L1 helicopter, registration N32AE, was substantially damaged during a hard landing near Clark, Missouri. The commercial pilot, two crewmembers, and one patient were uninjured. The helicopter was operating as a Title 14 Code of Federal Regulations Part 135 helicopter air ambulance flight.
During cruise on a patient transfer flight, the helicopter yawed to the right, the rotor rpm warning light illuminated, and the rpm aural alarm sounded. The engine continued to operate but produced less than 90% power. The pilot reduced collective pitch and turned toward a field for a precautionary landing. The helicopter touched down hard, resulting in substantial damage to the tail boom.
Examination Findings
Postaccident examination revealed a separation of a pneumatic line near its filter connection. The line and filter were sent to the engine manufacturer for analysis. The tube was fractured completely through at the toe of the weld between the tube and its filter fitting. Surface smearing was present on sections of the separation, but the separation exhibited surfaces consistent with fatigue that initiated near the top of the tube.
The outer diameter surface of the tube near the elbow separation showed rub damage. A photo of the engine bay indicated a red ribbed hose was present near the tube separation, and the hose exhibited discoloration on its outer surface.
Non-destructive examination measurements revealed that the height of the weld root reinforcement exceeded component specifications. Other tube and weld measurements met specifications. A polished cross-section through the weld on the fitting side showed the fatigue fracture located at the toe of the weld, initiating near the intersection of the weld heat affected zone (HAZ) and the weld metal. No material anomalies were found at the fatigue origin. The fatigue fracture progressed approximately along the intersection between the HAZ and weld metal near the outer diameter surface before proceeding into the HAZ to the inner diameter surface. The weld and base metal microstructures and materials met component specifications, as confirmed by energy dispersive x-ray spectroscopy.
The outer diameter of the filter housing had depressions and deformations consistent with tool marks. The line was sectioned near an intact connector weld for computed tomography (CT) examination, which revealed three voids within that weld area, ranging from 0.0019 to 0.0026 inch in diameter.
Maintenance and Operational History
A pneumatic leak check was performed 114.9 hours before the accident flight. The tube was last removed 238.9 hours before the accident flight. A leak in the pneumatic line can cause the engine control system to enter a sub-idle fuel flow condition, resulting in power loss. After the accident, the operator examined their fleet of helicopters with Rolls Royce 250-C30 engines and found no other pneumatic line leaks. According to the engine manufacturer, this was the first occurrence of failure involving this pneumatic line design, which was released in 1998. The fleet equipped with this line had accumulated 17 million flight hours since introduction.