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On June 24, 2008, at 0845 central daylight time, an Enstrom 480B, N480PD, was substantially damaged when during a test flight; the helicopter experienced a severe vibration. The helicopter was subsequently force landed in a field near Marinette, Wisconsin. The commercial pilot and passenger, a mechanic, on board were not injured. The helicopter was owned and being operated by Enstrom Helicopter Corporation, under the provisions of 14 Code of Federal Regulations Part 91. Visual flight rules (VFR) conditions prevailed and no flight plan was filed. The local flight originated from Menominee Airport, Michigan, approximately 0745.
The test flight was designed to record the helicopter’s characteristics at maximum gross weight and maximum aft center of gravity (CG). The helicopter departed and initially climbed to 2,500 feet. The pilot accelerated with trim to 95 knots indicated airspeed and then started to climb back up as they had lost some altitude. The pilot said as he applied power they noticed a moderate to severe vibration for a few seconds. The pilot reduced power a little and it went away. The pilot then said he reduced power to around 55 psi torque at about 90 knots indicated airspeed and trimmed forward again to do a reverse trim check while trying to maintain torque and altitude with slightly heavy collective. Then there was a shudder, followed almost instantly by a loud bang and crunching sound, the nose pitching down, and the helicopter turning 120 to 150 degrees to the left. The pilot said his headset came off when this happened. The pilot applied aft cyclic, right pedal, and increased collective. The shaking increased and a low rotor rpm warning came on. The pilot lowered the collective to increase the rotor rpm.
The pilot was able to regain control and selected a field for an emergency landing. He said he yelled for the mechanic to put his headset back on his head so he could make a mayday call. As the pilot applied collective to start the landing flare, the helicopter’s nose turned left again. The pilot said he realized he didn’t have pedal control. He rolled the throttle back intending to take manual control and pushed the cyclic forward, and decreased the collective slightly. This brought the nose around, and the pilot made a smooth level straight ahead landing.
The pilot held a commercial pilot certificate with a rating in helicopters and an instrument rating in helicopters. He was a former military helicopter pilot and had 7,409 hours of total flight time with 6,730 as pilot in command in helicopters, and a total time of 673 hours of flight time in the Enstrom 480B helicopter. He had flow 109 hours in the last 90 days.
The mechanic was not a rated pilot, but, had flown on numerous flights and test flights in Enstrom helicopters.
The helicopter was an Enstrom model 480B. The turbine powered helicopter was being flown in the experimental category as a future production model. The airframe had a total time of 707 hours and had a current annual inspection performed on March 28, 2008.
At 0755 the reported weather from the Menominee-Marinette Twin County Airport was clear skies with 10 miles visibility. The temperature was 62 degrees Fahrenheit (F). The winds were reported from 200 degrees magnetic at 4 knots. The pilot reported that there was no turbulence in the area.
At 2,500 feet pressure altitude and a temperature of 62 degrees F, the density altitude was 3,250 feet.
COMMUNICATIONS AND RADAR
The pilot was not in contact with any Air Traffic Control facility at the time of the accident.
WRECKAGE AND IMPACT INFORMATION
The pilot landed the helicopter in an open field. A postaccident inspection of the helicopter revealed that the tailcone was buckled in front of the horizontal stabilizer, displacing the stabilizer trailing edge downward and causing slack in the tail rotor control cables.
TESTS AND RESEARCH
The helicopter was examined at the Enstrom Helicopter Corporation manufacturing facility in Menominee, Michigan, on June 25, 2008. The examination showed primary buckling of the tailcone between 270 and 278 inches aft of datum. There was no evidence of reverse bending. All structural components in the tailcone were measured and compared to their specified dimensions. The tailcone was found to be in complete conformance with manufacturing designs.
The flight control system and mast head were inspected and no anomalies were found. No preimpact anomalies of the engine or drive line were observed that would have precluded normal operation.
The fuel in the main fuel tanks was measured and found to be more than what was indicated on the fuel gauges. It was also more than what the pilot said he thought was put on for the flight.
Taking into account the empty weight of the helicopter, the weights of the pilot, the mechanic and their equipment, and the weight of the fuel on board the helicopter at the time of the postaccident examination, it was determined that the helicopter weighed 3,081 pounds and had a CG approximately 140.8 inches aft of datum. According to the Operator’s Manual, the maximum allowable gross weight of the helicopter is 3,000 pounds.
According to the performance tables, the never-to-exceed airspeed (Vne) at an altitude of 3,250 feet and at 3,000 pounds at the maximum aft CG is 90 knots indicated airspeed. The estimated Vne would be 87 knots at 3,080 pounds.
In his Pilot/Operator Accident/Incident report, in the Recommendations section, the pilot stated, "Don't believe the fuel gauge when loading to max gross weight. When checked with a stick 2 days later there was more fuel in the tanks than the gauge indicated and the gauge indicated more than the 400 lbs it had prior to the incident. We were reminded that the section of tailcone that failed had a known small dent on the bottom side which had been checked some time ago. Don't consider dents in that section of tailcone as insignificant for operations at maximum gross weight and maximum airspeeds and those such as blade tracking which can produce significant shaking and vibrations."