CEN20LA113

The pilot of the helicopter air ambulance flight reported that, while en route on the night patient transfer flight, he received low rotor rpm warnings and engine power dropped below 90%. He reduced collective and turned toward a field to perform a precautionary landing, during which the helicopter touched down hard, resulting in substantial damage to the tail boom. A postaccident examination of the helicopter revealed that a pneumatic line, which had been leak-checked about 115 hours before the accident flight, had separated near its filter connection. A leak in the pneumatic line can cause the engine control system to enter a sub-idle fuel flow condition, resulting in a loss of engine power. An examination of the pneumatic line and filter revealed that the tube had 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; however, the separation exhibited surfaces consistent with fatigue that initiated near the top of the tube. The tube's outer diameter surface near the elbow separation exhibited rub damage. A red ribbed hose was present in the engine bay near the tube separation and the ribbed hose exhibited discoloration on its outer surface consistent with rubbing against the pneumatic tube. One weld exceeded its component specification; the remaining tube and weld measurements met their component specifications. The tube, fitting, and weld materials met specifications. According to the engine manufacturer, the accident was the first occurrence of a separation involving this revision of the pneumatic line over 17 million fleetwide flight hours. The cause of the fatigue could not be attributed to the hose and pneumatic tube rubbing or any other issue, and the reason for the failure could not be determined based on the available information.

On March 4, 2020, about 1842 central standard time, a Bell 206 L1 helicopter, N32AE, was substantially damaged when it was involved in an accident near Clark, Missouri. The commercial pilot, two crewmembers, and one patient were uninjured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 helicopter air ambulance flight. The pilot reported that during cruise on the patient transfer flight, the helicopter yawed right, the rotor rpm warning light illuminated, and the rpm aural indication sounded. The engine continued to operate but was producing less than 90% power. The pilot reduced collective and turned the helicopter toward a field for a precautionary landing. The helicopter touched down hard and the tail boom sustained substantial damage. A postaccident examination of the helicopter revealed that a pneumatic line, exhibited a separation near its filter connection. The pneumatic line and filter were shipped to the engine manufacturer for examination, which revealed that 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; however, the separation exhibited surfaces consistent with fatigue that initiated near the top of the tube. The tube’s outer diameter surface near the elbow separation exhibited rub damage. A photo of the engine bay showed that a red ribbed hose was present in the engine bay near the tube separation and the ribbed hose exhibited discoloration on its outer surface. Measurements taken during non-destructive examination revealed that the height of the weld root reinforcement exceeded its component specifications. The remaining tube and weld measurements met their component specifications. A polished cross-section through the weld on the fitting side fracture revealed the fatigue fracture was located at the toe of the weld, initiating at the approximate intersection of the weld heat affected zone (HAZ) and the weld metal of the tube. There were no material anomalies at the fatigue origin location. 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 at this plane of examination. Additionally, the weld and base metal microstructures were consistent with the materials required by the component specifications and energy dispersive x-ray spectroscopy revealed that the tube, fitting, and weld materials also met component specifications. The outer diameter of the filter housing exhibited depressions and deformations consistent with tool marks. The line was sectioned near its intact (non-failed) connector weld for a computed tomography (CT) examination, which revealed indications of three voids within that tube weld area. The void indications ranged in diameter from 0.0019 to 0.0026 inch. A pneumatic leak check was performed 114.9 hours before the accidentflight. 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 and result in an engine power loss. After the accident, the operator examined their fleet of helicopters equipped with Rolls Royce 250-C30 engines and no other pneumatic line leaks were found. According to the engine manufacturer, this was the first occurrence of failure involving this pneumatic line design, which was released in 1998. Since that time, the fleet equipped with the pneumatic line had accumulated a total of 17 million flight hours.