FTW03FA148 · Bell 407

The helicopter, which had a FADEC controlled turboshaft engine installed, was in cruise flight (about 800 feet AGL) over open ocean water when the FADEC FAIL aural warning sounded, followed closely by sound of the LOW ROTOR RPM horn. Simultaneously, the LOW ROTOR RPM, FADEC FAIL, and FADEC FAULT cockpit caution lights illuminated. The 8,300-hour helicopter pilot attempted to regain the RPM's with no result. The FADEC AUTO/MANUAL indicator light/button showed the engine control mode to be in the "AUTO" condition. The pilot recalls that the Ng was approximately 89%. The pilot stated that about 10 seconds elapsed from the onset of the event to cross checking the Ng. The pilot then depressed the AUTO/MANUAL button and switched to the MANUAL mode. He then increased the throttle above the 90% detent to try to regain rotor RPM's. He recalled that the light displayed "MANUAL", and that the FADEC FAIL aural warning ceased after the button was depressed. While descending, on three separate occasions, the pilot attempted to increase the throttle which were accompanied by three uncommanded right yaws, approximately 1-2 seconds apart. During the third uncommanded yaw, the ENGINE OUT audio sounded and the ENGINE OUT light illuminated (these occur when Ng drops below 55%). The pilot then entered a full autorotation, deployed the skid mounted emergency float system and landed upright on the water. Metallurgical examination of the 1st thru 4th stage turbine wheels and 1st thru 3rd stage turbine nozzles, revealed that all associated damage was due to extreme over-temperature operation. The manual mode schedule of the Hydro-mechanical Unit (HMU) was found within limits, and 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 revealed 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 (p/n CDR33BX104AKUR) on the IF board was found thermally distressed and was measured at .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. The C321 capacitor was removed from the IF board and there was no longer a short on the -15V power supply. The ECU was re-assembled, operated, and passed a functional acceptance test after removal of the C321 capacitor. The shorted condition of the C321 capacitor forced the -15V power supply to a low voltage condition and a significant current draw, resulting in the rectifying diode to overheat. According to the manufacturer, the -15V power failure of the ECU resulted in the HMU to revert to manual fuel metering. In the manual mode, the engine would have overspeed protection, but not overtemperature protection.

Verbatim NTSB analysis and probable cause from the NTSB dataset