What happened
On March 4, 2020, a Bell 206 L1 helicopter, registration N32AE, was performing a Part 135 air ambulance flight near Clark, Missouri, when the pilot experienced a sudden loss of engine power. During the cruise phase of the patient transfer, the pilot reported that the aircraft yawed to the right, the rotor RPM warning light illuminated, and an aural RPM warning sounded. The engine remained operational but was producing less than 90% power.
In response to the power loss, the pilot reduced collective pitch and maneuvered the aircraft toward a field to perform a precautionary landing. During the landing, the helicopter touched down hard, causing substantial damage to the tail boom. The pilot, two crewmembers, and one patient were not injured.
The investigation
Investigators examined the engine compartment and discovered that a pneumatic line had separated near its filter connection. A leak in this line can cause the engine control system to enter a sub-idle fuel flow condition, leading to reduced engine power.
Post-accident analysis of the pneumatic line and filter revealed the following:
- The tube had fractured completely through at the toe of the weld between the tube and its filter fitting.
- The fracture surfaces were consistent with fatigue that initiated near the top of the tube.
- Measurements showed that the height of the weld root reinforcement exceeded component specifications.
- The tube's outer diameter exhibited rub damage, and a nearby red ribbed hose showed discoloration consistent with rubbing against the pneumatic tube.
- Computed tomography (CT) of an intact section of the line revealed three small voids within the tube weld, ranging from 0.0019 to 0.0026 inch in diameter.
While a pneumatic leak check had been performed 114.9 hours prior to the accident, the engine manufacturer noted that this specific pneumatic line design had accumulated 17 million flight hours across its fleet since 1998 without a prior failure of this type.