Accident Overview
On January 11, 2024, at 1900 central standard time, a Eurocopter AS 350 B2, registration N204TX, was substantially damaged when it was involved in an accident near Spofford, Texas. The pilot and copilot received minor injuries. The helicopter was operated as a public aircraft conducting an aerial observation flight.
Flight Sequence
During the flight, the pilot decided to return to base after observing that the fuel gauge indicated 20% fuel remaining. The engine instruments showed no anomalies. Subsequently, the pilot felt a violent yaw to the right, saw the engine low pressure annunciator illuminate, and heard the low rotor horn. The copilot asked the pilot what happened, and the pilot stated that they had lost the engine. After rotor rpm returned to 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 helicopter sustained substantial damage to the tail boom and main rotor.
Postaccident Examination
The engine was placed in a test cell for a postaccident engine run. During the run, the engine would not accelerate past 45% gas generator speed (Ng). The fuel control unit (FCU), part number 0164851320, serial number 937B, was removed, and another FCU was installed. The engine then operated normally during the second test run. The FCU from the accident helicopter was bench tested, revealing that it could not achieve maximum fuel flow per test specifications. The FCU was disassembled, and the FCU control lever arm, part number 0164042540, was found broken.
Examination Findings
The National Transportation Safety Board Materials Laboratory examined the broken FCU control lever arm. The examination revealed a crack with fine fatigue striations, typical of high-cycle or low-load fatigue. Outside this crack, the fracture surface exhibited dimpled rupture, consistent with subsequent overstress fracture in the remaining lever arm. According to the examination report, these features were consistent with the initial fracture of the lever arm through about two-thirds of the cross-section. This crack or fracture edge then initiated fatigue cracking at multiple sites. Once the fatigue cracks had propagated deep enough, the remainder of the lever arm cross sections fractured via overstress.