CEN24LA088

The flight crew had competed their aerial observation and were enroute to the destination airport. While enroute, the pilot felt a violent yaw to the right, saw the engine low pressure annunciator illuminate, and heard the low rotor horn. After the engine lost power, the pilot transferred the flight controls to the copilot, who performed an autorotation and landed the helicopter in a brush-covered field. Postaccident examination and testing of the engine revealed that the engine’s fuel control unit (FCU) control lever arm was broken. Metallurgical examination of the control lever arm revealed it had failed due to a fatigue fracture which resulted in a loss of fuel control to the engine and fuel starvation.

On January 11, 2024, at 1900 central standard time, a Eurocopter AS 350 B2, 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. During the flight, the pilot decided to return to his base when he saw that the fuel gauge indicated 20% fuel remaining. The engine instruments did not indicate any anomalies. The pilot then 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 said that they had lost the engine. The pilot transferred controls to the copilot after the rotor rpm returned to the normal operating range. The copilot then performed an autorotation and landed the helicopter in a brush-covered field. The helicopter sustained substantial damage to the tail boom and main rotor. 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 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 airplane was bench tested. The test revealed that it could not achieve maximum fuel flow per the test specifications. The FCU was then disassembled, and the FCU control lever arm, part number 0164042540, was found broken. National Transportation Safety Board Materials Laboratory examination of the broken FCU control lever arm revealed a crack with fine fatigue striations, typical of high-cycle or low-load fatigue. Outside of 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 of this early fracture 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.