What happened
On January 11, 2024, at approximately 1900 CST, a Eurocopter AS 350 B2, registration N204TX, was conducting an aerial observation flight near Spofford, Texas. After observing that the fuel gauge indicated 20% remaining, the pilot decided to return to base. During the flight, the aircraft experienced a violent yaw to the right, the engine low pressure annunciator illuminated, and the low rotor horn sounded. The pilot informed the copilot that the engine had been lost. After rotor RPM returned to the normal operating range, the pilot transferred controls to the copilot. The copilot performed an autorotation and landed the helicopter in a brush-covered field. The accident resulted in 2 minor injuries and 0 fatalities. The helicopter sustained substantial damage to the main rotor and tail boom.
The investigation
Post-accident testing of the engine in a test cell showed that the engine would not accelerate past 45% gas generator speed (Ng). Following the installation of a different fuel control unit (FCU), the engine operated normally. A bench test of the original FCU revealed it could not achieve maximum fuel flow per specifications. Upon disassembly of the FCU, investigators found that the FCU control lever arm was broken.
Examination by the NTSB Materials Laboratory of the broken lever arm revealed a crack with fine fatigue striations, characteristic of high-cycle or low-load fatigue. The fracture surface outside of this crack showed dimpled rupture, indicating a subsequent overstress fracture. The examination concluded that the lever arm initially fractured through approximately two-thirds of its cross-section, which then initiated fatigue cracking at multiple sites until the remaining cross-section failed via overstress.