Eagle Pass, TX, USA
N747R
Eurocopter AS350B3
The commercial pilot was in cruise flight with the auto-pilot engaged when the red "GOV" light illuminated and the audio sounded. He disengaged the autopilot and resumed manual control of engine RPM. As the pilot was turning toward his home base, the pilot re-engaged the autopilot, and the engine RPM began to increase and decrease. The pilot was unable to control the RPM and initiated an autorotation to a field. The helicopter landed hard separating the tail boom and fracturing two arms of the main rotor starflex. The helicopter was equipped with a Turbomeca Ariel 2B series engine, which incorporated an electronic engine control unit (EECU) that electronically controlled engine fuel flow via a Hydromechanical Metering Unit (HMU). Data retrieved from the helicopter's flight data computer revealed that the fuel-metering unit valve had malfunctioned. Further investigation revealed that there had been 10 previous incidents worldwide where there was an internal failure in the EECU, which resulted in a red "GOV" warning light illuminating during flight. When this light illuminated, it caused the fuel metering valve to freeze in the position it had at the time of the detection, and therefore engine power could no longer be automatically controlled. The HMU consisted of a resolver and a stepper motor. The resolver measured/sensed the position of the fuel metering valve. The stepper motor took commands from the EECU and physically moved the metering valve. In automatic mode, the HMU compared the resolver information and stepper motor information, as to the position of the fuel -metering valve. When a cumulative mismatch of greater than 7 degrees occurred, the red "GOV" light illuminated and the pilot had to revert to manual mode for throttle operation. Further examination of the Hydromechanical Metering Unit (HMU) revealed that the stepper motor rack was too short and needed to be lengthened.
On September 2, 2002, at 1440 central daylight time, a single-engine Eurocopter AS350B3 helicopter, N747R, was substantially damaged during a forced landing following a loss of automatic engine governing near Eagle Pass, Texas. The commercial pilot, sole occupant of the helicopter, was not injured. The helicopter was registered to Sage Well Services, Incorporated, and was being operated by Lewis Energy of Encinal, Texas. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 business flight. The cross-country flight departed Del Rio, Texas, at 1400, and was destined for an oil drilling rig near Encinal. In a written statement, the pilot reported that he was traveling southbound at 104 knots at an altitude of 1,800 feet with the auto-pilot engaged, when he heard a "gong" noise "going off." He looked at the caution light panel and observed that the red governor light was "on." The pilot also reporting hearing the rotor RPM speeding-up and then heard the high RPM audible alarm. The pilot added that he proceeded to loosened the "knurled lock nut at the base of the twist grip" and pushed the red lock latch to adjust the RPM manually. The autopilot was still engaged and the RPM returned to an acceptable limit. The pilot continued his flight and encountered "moderate" turbulence, during which time, the RPM became erratic and he was constantly trying to correct for it by adjusting the twist grip. The pilot then turned the twist grip to reduce RPM and the RPM continued to increase. He then disconnected the autopilot and turned the twist grip throttle position to reduce the rotor RPM. The RPM came back into range and the pilot set it to 390 RPM +/- and added friction to the collective to help keep the RPM in control and then continued to his destination. While en route with the rotor RPM under control, the pilot elected to land at a maintenance facility. Before the pilot dialed in the new airport frequency into the GPS, he put the auto pilot (altitude /heading hold) back "on." Shortly after, the engine RPM began to rapidly increase and decrease. The pilot was unable to control the RPM and initiated an autorotation to a field. The helicopter landed hard separating the tail boom and fracturing two arms of the main rotor starflex. The helicopter was equipped with a Turbomeca Ariel 2B series engine, which incorporated an electronic engine control unit (EECU) that electronically control engine fuel flow via a hydromechanical metering unit (HMU). Data retrieved from the helicopter's flight data computer revealed that the fuel-metering unit valve in the HMU had malfunctioned. Further investigation revealed that there had been 10 previous incidents worldwide where there was an internal failure in the HMU, which resulted in a red "GOV" warning light illuminating during flight. When this light illuminated, it caused the fuel metering valve to freeze in the position it had at the time of the detection, and therefore engine power could no longer be automatically controlled. The HMU consisted of a resolver and a stepper motor. The resolver measured/sensed the position of the fuel metering valve. The stepper motor took commands from the EECU and physically moved the metering valve. In automatic mode, the HMU compared the resolver information and stepper motor information, as to the position of the fuel metering valve. When a cumulative mismatch of greater than 7 degrees occurred, the red "GOV" light illuminated and the pilot had to revert to manual mode for throttle operation. This fault was originally believed to have occurred on flights lasting 2 to 3 hours in duration and was later found to have occurred as a result to the number of cycles the collective was moved up and down. On November 24, 2000, Turbomeca issued Alert Letter No. 2064/00/AR2B/15 alerting operators of the illumination of the red "GOV" warning light and how to handle this critical flight situation via the Helicopter Flight Manual. On December 3, 2002, Turbomeca issued Service Bulletin No. 292 73 2066 titled Engine Electronic Unit Control with version V7.01 software / Incorporation of Modification TU66C, which introduced an interim software fix that increased EECU tolerance to the rotor pitch drop phenomenon. On April 3, 2001, Eurocopter issued Service Letter No. 1510-71-01, titled Total Governor Failure, and informed maintenance and flight personnel that if the red "GOV" light illuminated then the pilot must adjust "the engine rating using the manual fuel flow control" and follow procedures in the Emergency section of the Helicopter Flight Manual. In addition, "..to reduce suspected risk, we have decided on a more frequent re-initialization of the computer. Re-initialization is performed automatically at each engine shutdown. As a precautionary measure, we are temporarily implementing the following measures: It is forbidden to replenish the fuel tank with the rotors spinning." On June 6, 2002, Eurocopter issued Service Letter No. 1556-71-02, which addressed the failures of the fuel control unit as a result of the "discrepancy between the theoretical and effective positions of the stepper motor in the fuel metering unit", and as a precaution had asked operators to completely shut down the engine prior to refueling to "reduce the suspected risk of discrepancy by resetting the computer more frequently. The computer is automatically reset at each engine shutdown." It also stated that Turbomeca had issued Service Bulletin 292 73 2066, which provided data for installing software modification TU66C, that "...basically increases FADEC tolerance to the rotor pitch drop phenomenon...Once modification TU66C is introduced, the prohibition of refueling while the engine was running is no longer necessary." On April 17, 2004, Turbomeca issued Service Bulletin (SB) 292 73 2076/TU76, titled Hydromechanical Metering Unit (HMU) / Rack is guided inside the fuel metering unit body / Incorporation of modification TU 76. The purpose of the bulletin was to "...eliminate a possible cause of losses of actuator pitch which could lead to a transition into manual mode." The operator was provided instructions to remove the existing HMU and install a modified HMU. This modification included a longer stepper motor rack and a modified cover so the new rack would fit. The pilot held a commercial pilot certificate for airplane single and multi-engine land, rotorcraft-helicopter, and instrument airplane. He reported a total flight time of 3,398 hours of which, 726 hours were in rotorcraft-helicopter. According to the AS350B3 Flight Manual, section 3.3, page 4, Red Lights, the following actions should be taken by the pilot when the red "GOV" light illuminates. These actions are: - Store the flight data. -Unlock the "FLIGHT" notch, the fuel flow can be increased or decreased by turning the twist grip. - Only apply small amplitude adjustments, synchronized with the collective pitch control in order to maintain MR in the green range. - Fly the approach at 40 knots and adjust the fuel flow rate to maintain NR within the upper section of the green range. Slowly reduce the speed if necessary. Adjust the fuel flow rate slightly on the twist grip to maintain NR within the green range. On final approach, when the collective pitch is increased on reaching a hover, let the NR drop for touchdown. After touchdown, reduce the fuel flow rate before lowering the collective pitch. The autopilot was part of a supplemental type certificate. According to the helicopter's Flight Manual, it did not address the use of the autopilot when a governor fails. According to Eurocopter, although not recommended in the Emergency procedures, engagement of the autopilot after a governor failure should not have resulted in the pilot's inability to manually control the fuel flow and RPM. `
The insufficient length of the stepper motor rack inside the HMU, which caused the red "GOV" light to illuminate and the fuel metering valve to freeze causing the engine to automatically engage the manual mode.
Source: NTSB Aviation Accident Database
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