History of Flight
On June 24, 2008, at 0845 central daylight time, an Enstrom 480B, N480PD, was substantially damaged during a test flight when the helicopter experienced a severe vibration. A forced landing was subsequently made in a field near Marinette, Wisconsin. The commercial pilot and a mechanic passenger were not injured. The helicopter was owned and operated by Enstrom Helicopter Corporation under 14 Code of Federal Regulations Part 91. Visual flight rules 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 after losing altitude. As power was applied, a moderate to severe vibration occurred for a few seconds. Reducing power slightly caused the vibration to cease. The pilot then reduced power to around 55 psi torque at about 90 knots indicated airspeed and trimmed forward for a reverse trim check while attempting to maintain torque and altitude with slightly heavy collective. 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’s headset came off. Aft cyclic, right pedal, and increased collective were applied. The shaking increased, and a low rotor rpm warning activated. The pilot lowered the collective to increase rotor rpm.
The pilot regained control and selected a field for an emergency landing. He called for the mechanic to put his headset back on to make a mayday call. As collective was applied for the landing flare, the nose turned left again. Realizing a lack of pedal control, the pilot rolled the throttle back to take manual control, pushed the cyclic forward, and decreased the collective slightly. This brought the nose around, and a smooth level straight ahead landing was made.
Personnel Information
The pilot held a commercial pilot certificate with ratings in helicopters and an instrument rating in helicopters. He was a former military helicopter pilot with 7,409 hours total flight time, 6,730 hours as pilot in command in helicopters, and 673 hours in the Enstrom 480B. He had flown 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.
Aircraft Information
The helicopter was an Enstrom model 480B, turbine powered, flown in the experimental category as a future production model. The airframe had a total time of 707 hours and a current annual inspection performed on March 28, 2008.
Meteorological Information
At 0755, the reported weather from Menominee-Marinette Twin County Airport was clear skies with 10 miles visibility. Temperature was 62 degrees Fahrenheit. Winds were from 200 degrees magnetic at 4 knots. The pilot reported no turbulence in the area. At 2,500 feet pressure altitude and 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 helicopter was landed in an open field. A postaccident inspection 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. No evidence of reverse bending was found. All structural components in the tailcone were measured and compared to their specified dimensions, and the tailcone was found to be in complete conformance with manufacturing designs. The flight control system and mast head were inspected with no anomalies found. No preimpact anomalies of the engine or drive line were observed that would have precluded normal operation.
Fuel in the main fuel tanks was measured and found to be more than indicated on the fuel gauges and more than the pilot thought was put on for the flight. Considering empty weight, pilot and mechanic weights, equipment, and fuel weight at postaccident examination, the helicopter weighed 3,081 pounds with a CG approximately 140.8 inches aft of datum. According to the Operator’s Manual, the maximum allowable gross weight is 3,000 pounds. Per performance tables, the never-to-exceed airspeed (Vne) at 3,250 feet and 3,000 pounds at maximum aft CG is 90 knots indicated airspeed. The estimated Vne at 3,080 pounds was 87 knots.
Additional Information
In his Pilot/Operator Accident/Incident report, the pilot recommended: "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."