Accident Overview
On July 26, 2012, at 1445 Pacific daylight time, a Hughes 369E helicopter, registration N427WC, operated by Wilson Construction Company under 14 Code of Federal Regulations Part 133, experienced a partial loss of engine power during external load operations. The helicopter was in an 80-foot hover, lifting the ends of winch lines and dragging them across sloping agricultural terrain, when the power loss occurred. The external load was jettisoned as the helicopter began to descend rapidly. Witnesses reported hearing a change in the helicopter's sound, and one witness observed the impact. The tail boom contacted the ground first and was severed by the main rotor system. The skids spread and separated from the fuselage, and the helicopter came to rest on its right side. Several witnesses noted that the engine continued to run after the impact.
Aircraft and Pilot Information
The helicopter was a two-seat, single-engine Hughes 369E, serial number 0188E, powered by a Rolls Royce M250-C20B 420-horsepower turboshaft engine. The total airframe time at the time of the accident was 2,757 hours, with the last inspection being a 100-hour inspection on June 21, 2012. The engine, serial number CAE835944, had a total time of 4,923 hours, time since overhaul of 286 hours, and time since last inspection of 47 hours.
The pilot, age 64, held a commercial pilot certificate with ratings for airplane single-engine land and rotorcraft helicopter, and a second-class medical certificate issued March 5, 2012, with a limitation requiring corrective lenses. The operator reported that the pilot had 27,900 total flight hours, including 27,650 hours in rotorcraft and 9,000 hours in the Hughes 369.
Examination and Testing
An on-scene examination by a Federal Aviation Administration inspector revealed that the main rotor blades were coned upward, the tail boom separated from the fuselage, and the landing skids were sheared off. Control continuity from the cyclic to the main rotor swash plate was established. No fuel leakage was observed from the fuel system. The fuel gauge indicated 125 pounds, and a visual measurement showed the fuel level at 9 inches from the lower lip of the fuel filler flange. The operator drained 117 pounds (17.2 gallons) of fuel from the helicopter.
Rolls-Royce conducted an engine functionality test on the M250-C20B engine on September 13, 2012, under National Transportation Safety Board supervision. Prior to the test, four components damaged during the impact—the outer combustion case, combustion liner, igniter, and igniter lead—were replaced with serviceable factory components. The engine started and operated per specification with noted performance deviations: predicted power at the Cruise-C setting (300 horsepower, 1261°F MGT) was 273 horsepower (9% below specification); at the Normal Cruise setting (370 horsepower, 1360°F MGT) it was 347 horsepower (6.2% below). Specific fuel consumption was above specification by 3.5% at Cruise-C and 1.5% at Normal Cruise. At the Maximum Take Off setting (420 horsepower, 1490°F MGT), predicted power was 453 horsepower (7.9% above specification). No flameouts occurred during acceleration/deceleration tests, and the power turbine governor functioned per specification. The testing revealed no anomalies that would contribute to a power loss.
Additional Information
The report references two previous investigations: Australian Transport Safety Bureau occurrence AO-2008-025 involving an MD369ER during power line stringing, which found that air could be introduced into the fuel system when fuel quantity was less than 85 liters (151 pounds) and the aircraft was subjected to a 20-degree nose-up and 20-degree right roll attitude. Additionally, National Transportation Safety Board investigation LAX91LA054 involving an MD368D during external long line operations found that fuel unporting occurred at fuel levels at or below 115 pounds when duplicating the helicopter's pitch attitude and fuel load.