LAX05GA231

The failure of the constant delta P diaphragm in the fuel control unit, which resulted in an increased fuel flow and subsequent catastrophic failure of the engine. The diaphragm's failure was the result of improper installation by the engine manufacturer. A factor in the accident was the unsuitable nature of the terrain for a successful autorotation.

The engine experienced an overspeed and catastrophic failure on a law enforcement patrol flight and the helicopter collided with hilly terrain during a subsequent autorotation attempt. Witnesses reported the helicopter began emitting smoke and subsequently descended, impacting terrain near the bottom of a 60-degree sloped hillside. The terrain around the accident site was either steeply sloped hillside with mature trees or bordered by power lines. A post accident examination of the engine and components revealed the constant delta P diaphragm (located in the fuel control unit) had ruptured, resulting in a high and uncontrolled increase of fuel flow to the engine. The power turbine exhibited evidence of a substantial overspeed. All of the turbine blades were separated at their respective shear points. The gas generator turbine exhibited evidence of extreme thermal erosion; the blades were eroded to about 50 percent of their normal height. The centrifugal compressor exhibited evidence of extreme rubbing. A detailed examination of the diaphragm revealed that it had been installed incorrectly (inside-out). The maintenance and engine manufacturer's build records revealed that the diaphragm was last replaced at the engine manufacturer's facilities in France. Recovered Vehicle and Engine Multifunctional Display (VEMD) information supported the evidence of an engine and main rotor revolutions per minute (rpm) exceedance.

1.1 HISTORY OF FLIGHT On July 13, 2005, about 1910 Pacific daylight time, a Eurocopter EC120B, N266SD, operating as Star 6, experienced a catastrophic engine failure and collided with terrain near Fair Oaks, California. The Sacramento County Sheriff's Department was operating the public-use helicopter under the provisions of 14 CFR Part 91. The commercial pilot and the front-seat observer sustained fatal injuries; the aft-seated observer trainee sustained serious injuries. The helicopter was destroyed during the accident sequence. The local area flight departed Mather, California, about 1815. Visual meteorological conditions prevailed, and a flight plan had not been filed. During interviews with investigators, witnesses stated that they observed the helicopter flying westbound, along the north shore of Lake Natoma. The helicopter continued in that flight path toward the Nimbus Dam over hilly terrain. Witnesses then reported hearing a "pop" and observed flames and smoke emitting from the exhaust of the helicopter. The helicopter then descended, impacting terrain near the bottom of a 60-degree sloped hillside. Upon impact, it bounced and rolled inverted, impacting the terrain again, and coming to rest near the base of the hillside. One witness, who was located on the south side of the lake, commented that the helicopter "seemed awfully low." Another witness stated that he observed and heard the helicopter flying "loud and low," just the north of his location. He witnessed a 2.5-foot yellow flame coming from the base of the engine, just below the main rotor blades. He noted that the main rotor blades appeared to be turning and the helicopter fuselage seemed stable with no visible roll or turning motion. Prior to the accident, a California Highway Patrol (CHP) airplane pilot was in radio contact with the pilot of Star 6 via the Sacramento County Sheriff's Department air-to-air frequency, at which time there was no indication that the helicopter was experiencing any anomalies. About 1 minute after his last radio contact with Star 6, the CHP pilot heard "a yelling voice and what . . . sounded like car doors slamming" transmitted over the frequency. The CHP pilot subsequently made numerous attempts to contact Star 6, to no avail. A review of the recorded transmissions revealed that the front-seat observer of Star 6 transmitted a distress call about 1910 via the 800-megahertz radio frequency communication system of the Sacramento County Sheriff's Department. Over an approximate 5-second duration he declared "Mayday, mayday, we're going down, north of Lake Natoma." 1.2 PERSONNEL INFORMATION 1.2.1 Pilot National Transportation Safety Board investigators reviewed the pilot's personal flight logbooks, the operator's records regarding the pilot's flight history, and the Federal Aviation Administration (FAA) Airman and Medical records for the pilot. On September 24, 2004, he was issued a commercial pilot certificate with a rating for rotorcraft-helicopter. The pilot's most recent second-class medical certificate was issued on May 17, 2005, with the limitation that the pilot must wear glasses for near and distant vision. A review of the pilot's personal logbook disclosed that he had amassed 742.6 hours of total flight experience. He had accumulated 109.6 hours in the last 90 days, and 32.7 hours in the last 30 days. He had an estimated 650 hours of flight time in the EC120B helicopter. He was issued a private pilot license on September 2003, at which time he reported a total time of 95 hours, of which 10 hours were as pilot-in-command and 53 hours were accumulated in the Schweitzer 269C-1 helicopter. The Sacramento County Sheriff's Department Air Operations Bureau maintained records of the pilot's proficiency checks. The most recent check was preformed on May 11, 2005, and consisted of mission procedures. The flight check encompassed 3.8 hours of flight instruction; there were no notations of autorotation practice. The last emergency procedure proficiency check was conducted in addition with mission procedures from November 15 though 17, 2004. As part of the check, the pilot accumulated 3.5 hours of ground instruction and 6.7 hours of total flight time. The instructor's comments indicted that the pilot satisfactorily completed numerous simulated engine failures with the helicopter in multiple configurations including, "hover, low altitude, airspeed below 50 knots, 180-degree turn, zero speed entry, etc." On February 4, 2005, the pilot was awarded a certificate of achievement from the American Eurocopter Training Center. The certificate acknowledged his successful completion of the Transition Ground School course and his receipt of 3.1 hours of flight training in the EC120B. As part of the training he demonstrated over 30 full touchdown autorotations, all of which were marked as completed satisfactorily. He accumulated about 210 hours after completing the factory training until the time of the accident. 1.2.2 Front-Seat Observer The front-seat observer was issued a student pilot certificate on December 09, 2004. The pilot's second-class medical was issued on the same date, with the limitation that he must wear corrective lenses. A review of the observer's training record revealed that he had not accumulated any flight training. The Sacramento County Sheriff's Department requires observers to obtain a second-class medical certificate and provides an initial 10 hours of flight training. Every month thereafter, they receive 1 hour of recurrent training. 1.3 AIRCRAFT INFORMATION 1.3.1 General Information The Eurocopter EC120B, serial number 1133, was manufactured in 2000, and had accrued a total time in service of 2,562.3 hours at the time of the accident. The last airframe inspection occurred on July 4, 2005, at 2,500 hours. A Turbomeca Arrius 2F, serial number 34144, was the original Eurocopter factory installed engine. The engine had accumulated a total of 2191.6 hours since new. The time disparity between the airframe and engine is a result of the engine being removed in April 2004 (at a total time of 1,691.5 hours) and sent to the manufacturer for a repair after sustaining foreign object damage (FOD). The engine was reinstalled after the repair. The EC120B is designed to operate with one pilot in either the right or left front seat. The accident helicopter was configured with five seats, and was approved to operate day or night in visual meteorological conditions. The helicopter was equipped with dual controls, although the operator required the pilot-in-command to be positioned in the right seat. The total fuel capacity was 107 gallons. According to the operator, the helicopter completed a 1.7-hour flight prior to the accident flight. At the end of that flight, the helicopter was refueled with about 38 gallons of JP-8 fuel, equating to a total of about 66 gallons (450 pounds) of fuel on board. The accident flight spanned about 0.9 hours before the mishap occurred, which the operator estimated would leave the helicopter with about 35 gallons of fuel remaining. The EC120B's standard equipment includes a Vehicle and Engine Multifunctional Display (VEMD), an instrument that displays the engine and vehicle parameters in the cockpit. It was designed to replace conventional indicators by informing the flight crew of engine information, fuel quantity, electrical power, and torque readings. When in maintenance mode, the VEMD can additionally display recorded in-flight anomalies and discrete helicopter performance monitoring data. 1.3.2 Fuel Control Unit The helicopter's fuel control unit (FCU) is comprised of a fuel pump and a metering unit. The metering unit encompasses a working piston, metering needle, and constant delta P valve; its function is to meter the fuel flow in response to the control system. The actual metering of fuel is accomplished by the