FTW96LA395
FTW96LA395

On September 21, 1996, at 1425 central daylight time, a Bell 407 helicopter, N1114S, registered to and operated by Bell Helicopter Textron Inc., as a Title 14 CFR Part 91 maintenance test flight, was substantially damaged during a forced landing near Kerrville, Texas. Visual meteorological conditions prevailed, and a flight plan was not filed for the local flight. The commercial pilot and one passenger were seriously injured, and the other passenger was not injured. The flight departed Kerrville on a local test flight, about 20 minutes prior to the accident.

According to the operator, the helicopter departed the Bell Helicopter plant in Fort Worth, Texas, the day prior to the accident. The helicopter was being ferried to South America, and was en route to Kerrville, Texas, for it's first fueling stop. When the flight was 20 minutes from Kerrville, after flying through a light rain, the RESTART FAULT light illuminated on the caution and warning light panel. The flight continued to Kerrville, and landed without further incident.

The pilot reported that he elected to troubleshoot the discrepancy prior to engine shut down. He increased and decreased the throttle from idle to 100% Nr in the Full Authority Digital Electronic Control (FADEC) AUTO mode with no anomalies noted. The FADEC was switched from the AUTO mode to MANUAL mode, and all engine indications were normal. He then increased the throttle from idle to 75% Nr and back to idle with no anomalies noted. He repeated this procedure, going to 85% Nr the second time and then back to idle. With the throttle at idle, the FADEC was switched from the MANUAL mode to the AUTO mode. The FADEC warning horn sounded, and the engine began to accelerate at a rate he did not feel comfortable with so the FADEC was switched back to the MANUAL mode. This procedure was repeated with the same results, so he shut the engine down. Maintenance was performed on the helicopter to correct moisture in the Hydromechanical Unit (HMU) P4 connector, and the HMU and Electronic Control Unit (ECU) J1 and J2 connectors that were found loose at the engine firewall.

The pilot further reported that the day of the accident he "motored the engine and parked the piston." The helicopter's engine was started and ground run two times with all systems normal. A 10 minute test flight was performed and no discrepancies were noted. While returning from the test flight, during the approach to Kerrville Airport, at 300 feet AGL, in a right turn, approximately 60 knots, he noted the FADEC FAIL (red) light and warning horn. He did not hear the Engine Out or Low Rotor warning horns. The AUTO RELIGHT, FADEC FAULT and ENGINE OUT caution lights were noted. The rotor RPM was between 90% and 95% and the Np was decreasing through 60%. During the autorotation, to avoid trees and houses, he extended the glide by increasing collective pitch. After clearing the obstacles, he leveled the helicopter and "used all remaining collective for landing." The helicopter "landed hard and remained upright."

According to the Bell 407 Rotorcraft Flight Manual, when the FADEC FAIL warning light illuminates in flight, the pilot should accomplish the FADEC FAILURE procedure as prescribed in paragraph 3-3-K. The procedure is, immediately retard the throttle and hold it to the 90% throttle bezel position; maintain Nr (rotor) with collective only; depress the FADEC MODE switch one time regardless of switch indication, FADEC will switch to MANUAL mode 2 to 7 seconds after this action if it is not already in manual mode; maintain Nr 95% to 100% with throttle and collective; land as soon as possible, and perform a normal shutdown if possible. There is a warning that 2 to 7 seconds after the FADEC FAIL warnings, FADEC may be in MANUAL mode without any pilot action. Nr may increase very rapidly and overspeed to 110% which will result in an engine flameout unless the pilot takes immediate manual control of the FADEC with the throttle. See the enclosed excerpts from the flight manual.

Examination of the helicopter by the FAA inspector at the accident site revealed that, the left skid was buckled, and the right skid was partially separated. The lower right forward portion of the fuselage was damaged, and the fuselage at the tailboom attaching point was buckled. One main rotor blade sustained damaged in the area of the trim tab. The battery was connected and it was verified that the "auto light" was on. The throttle position in the cockpit was found open approximately 80 degrees.

An examination of the FADEC system was completed on September 26, 1996, under the supervision of the investigator-in-charge at the Bell Textron plant in Fort Worth, Texas. With a notebook computer connected to the FADEC download port, the wiring harness from the ECU to the HMU was flexed by hand. It was found that when the aft portion of the harness from the forward firewall to the HMU was flexed by hand near the HMU connector, the voltage from the HMU metering valve position sending pot became erratic. The harness was disconnected from the ECU; the FADEC warning horn sounded and the FADEC FAIL warning light illuminated. The harness was removed and an insulation resistance check was performed using a high voltage tester (Megger). The test revealed that the pin N on the HMU end of the harness indicated a low resistance to the connector back shell (approximately 10,000 Ohms). This aft HMU harness, P/N 23062796, S/N NX0020 was sent to Simmonds Precision for a detailed examination. The HMU and ECU were removed and sent to Chandler Evans Corporation for further examination. The engine was removed and sent to Allison Engine Company for examination. See the enclosed report from Bell Helicopter and the excerpts from the Allison report for further details of the aircraft examination.

The examination of the engine, HMU, and ECU revealed that they performed within the manufacturer's specifications. The examination of the aft HMU harness revealed a manufacturing defect. See the detailed reports of these examinations which are in the enclosed excerpts from the Allison Engine Company report.

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