Bell 206 N32AE accident near Clark, Missouri

Probable cause

The fatigue failure of the pneumatic line for reasons that could not be determined, which resulted in a loss of engine power and subsequent hard landing.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN20LA113 for the full record.