Event Description
A helicopter equipped with a FADEC-controlled turboshaft engine was in cruise flight approximately 800 feet above ground level over open ocean water. The pilot, with 8,300 flight hours, heard a FADEC FAIL aural warning, followed shortly by the LOW ROTOR RPM horn. Simultaneously, the LOW ROTOR RPM, FADEC FAIL, and FADEC FAULT cockpit caution lights illuminated. The pilot attempted to regain rotor RPM without success. The FADEC AUTO/MANUAL indicator showed the engine control mode was in "AUTO". The pilot recalled the Ng (gas generator speed) was approximately 89%.
Approximately 10 seconds after the onset of the event, the pilot cross-checked the Ng and then pressed the AUTO/MANUAL button, switching to manual mode. He increased the throttle above the 90% detent to regain rotor RPM. The light displayed "MANUAL", and the FADEC FAIL aural warning ceased. While descending, the pilot attempted to increase the throttle on three separate occasions, each accompanied by an uncommanded right yaw, approximately 1-2 seconds apart. During the third yaw, the ENGINE OUT audio sounded and the ENGINE OUT light illuminated, which occurs when Ng drops below 55%. The pilot entered a full autorotation, deployed the skid-mounted emergency float system, and landed upright on the water.
Investigation Findings
Metallurgical examination of the 1st through 4th stage turbine wheels and 1st through 3rd stage turbine nozzles revealed that all damage was due to extreme over-temperature operation. The manual mode schedule of the Hydro-mechanical Unit (HMU) was found within limits, but the auto mode schedule had a flow shift of +10 to +15 PPH. Evaluation and testing of the Electronic Control Unit (ECU) and its sub-components showed a shorted condition on the ECU -15V power supply. Disassembly of the ECU and tests of the Interface (IF) and Power (PWR) circuit boards revealed that the C321 capacitor (part number CDR33BX104AKUR) on the IF board was thermally distressed and measured 0.58 ohms. The C321 is a high-frequency bypass capacitor from -15V to ground. The PWR board was also visually inspected, and the CR423 diode was found thermally stressed. According to the manufacturer, the CR423 diode provides rectification on the -15V power supply and was likely stressed as a result of the shorted C321 capacitor.
After removal of the C321 capacitor, there was no longer a short on the -15V power supply. The ECU was reassembled, operated, and passed a functional acceptance test. The shorted condition of the C321 capacitor forced the -15V power supply to a low voltage condition and caused significant current draw, resulting in the rectifying diode overheating. According to the manufacturer, the -15V power failure of the ECU resulted in the HMU reverting to manual fuel metering. In manual mode, the engine would have overspeed protection but not overtemperature protection.