Fair Oaks, CA, USA
N266SD
Eurocopter France EC120B
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 metering needle, which moves within an orifice. The needle, controlled by the working piston, is moved by a spring, which will change its position accordingly. The fuel pressure difference from both the upstream and downstream flows within the metering valve is maintained by the constant delta P valve. The purpose of the valve is twofold: keeping the pressure difference constant between the metering needle upstream and down stream pressures; and returning excess fuel to the pump inlet. The valve ensures that changes in controlled fuel flow are directly proportional to the position of the metering needle (obtaining fuel flow as a function of the metering needle position) resulting in improved response time and overall stability. The delta P valve contains a diaphragm (the delta P diaphragm) that is subjected to the upstream fuel pressure on the outside. On the inside, the diaphragm is subjected to the downstream fuel pressure as well as the force of the spring. In operation (transient conditions), the system is designed so when the metering unit needle moves, the pressure difference created by the fuel flow is sensed by the delta P valve. This in turn alters the fuel flow to return to the low-pressure system in an effort to maintain the metering valve flow under constant pressure difference. The delta P diaphragm is composed of an acrylonitrile butadiene rubber (NBR) and polyester fabric. The thimble or cup-shaped diaphragm is flexible with a slightly rigid rim around the perimeter of the open end, akin to the lip of a balloon. The malleable nature of the material enables the bottom of the diaphragm to be pushed through the lip, inversing the diaphragm. The diaphragm has a small hole centered in the bottom base. When correctly positioned, the inside walls of the diaphragm are lined with the fabric material, while the outer walls are the rubber composite. When inversed, the diaphragm appears to maintain an identical shape, with the visible distinctions being twofold: a lighter coloration and texture of the fabric wall as opposed to the dark smooth rubber wall, and a part number stamped on the fabric side. At the time of the accident, the accident helicopter's FCU part number 0319928730 (serial number 147B) had accumulated 2,391 total hours since new and 1,689 total hours since repair by Turbomeca France. The component log card indicates that on July 06, 2001, the FCU was removed from another Turbomeca Arrius 2F engine (serial number 34049) due to fuel leaking from the N1 and N2 drive drain. At that time the unit had accumulated a total time of 702 hours since new. The unit was subsequently repaired at Turbomeca France. After repair, the unit was installed another helicopter utilized by the operator, until reaching 802 hours of time in service, when it was removed and placed on the accident engine. Turbomeca recommended time between overhaul (TBO) for the FCU to be 2,800 hours. When the accident FCU was examined following the accident, the Turbomeca seals were still intact, which Turbomeca stated was consistent with it not having been tampered with or opened after leaving the manufacturer for repairs. 1.3.3 Weight and Balance The helicopter's empty weight was calculated to be 2,656.35 pounds, which corresponded to a useful load of 1,123.65 pounds. Investigators estimated the weight and balance of the helicopter immediately preceding the accident based on last known weights of the pilot and passengers, as well as the probable fuel on board. The total weight was calculated to be 3,538.35 pounds with a center of gravity (cg) arm of 153.63 inches. According to Eurocopter, the weight and balance was within the helicopter's operational limits, as the maximum gross weight is 3,780 pounds and the cg range for the helicopter (in its configuration just prior to the accident) is 152 to 162 inches. 1.3.4 Performance According to Eurocopter, there are too many variables to accurately determine the helicopter's rate of descent during the accident sequence. An estimated calculation was provided to a Safety Board investigator for an autorotation with the helicopter's estimated weight during the time of the accident. A main rotor revolutions per minute (rpm) of 410 (the estimation was calculated with a no/mild wind condition) was used. With the helicopter maintaining 65 knots, the descent would be about 1,550 feet per minute (0.7 nautical miles per 1, 000 feet). 1.3.5 Maintenance Manual The Turbomeca Component Maintenance Manual (0319928++0) listed instructions for the assembly of the FCU. In subtask 73-23-06-462-013, it addressed the assembly of the constant delta P assembly by referring to two figures that gave pictorials of the system. A note was listed immediately under that stated, "check the installation direction of the membrane (14-120), with the cloth side and marks against the membrane support (14-110)." 1.4 METEOROLOGICAL INFORMATION The closest official weather observation station was Sacramento Mather Airport, Mather, located 7 nautical miles (nm) southwest of the accident site. An aviation routine weather report (METAR) for Mather was issued at 1845. It stated: winds from 250 degrees at 10 knots; visibility 10 miles; skies clear; temperature 39 degrees Celsius; dew point 14 degrees Celsius; altimeter 29.78 inches of mercury (inHg). 1.5 WRECKAGE AND IMPACT INFORMATION The accident site was located on the north side of Lake Natoma adjacent to Nimbus Dam, which stretches across the narrow lake in a north-south orientation. On the immediate east side of the dam, the lake averages about 1,200 feet across. Power lines bordered the north side of the lake, running perpendicular to the dam. The wreckage was inside a ravine that was also oriented in a north-south direction. The debris was all contained within an 80-foot radius and located at the base of a hill. The hill, comprised of dry grass and dirt, peaked about 115 feet high from the level terrain where the helicopter came to rest. The slope of the hill averaged about 60 percent, with the terrain occasionally dropping off vertical. Tall trees were sporadically located on the hillside with a majority outcropped around the peak. The valley floor of the ravine was populated with medium brush. The terrain around the wreckage and up the hill sustained fire damage. A ground disturbance was located about 50 feet from the main wreckage on a bearing of 016 degrees. The disturbance was oriented downslope on the side of the hill (midway down). It consisted of a thin, elongated impression within the surrounding burnt grass, consistent in size and shape to that of a main rotor blade. The fuselage came to rest on the right side, parallel to the base of a hillside. The top of the fuselage was facing the hillside. The front end of the fuselage (nose of the helicopter) was oriented toward the northwest at 289 degrees. The debris field extended from the base of the hillside to the main wreckage and covered a small, well-contained area. Two of the three main rotor blades were located with the main wreckage; the third blade was located above the initial impact scar. The location and distribution of the wreckage was plotted on USGS survey maps by the CHP personnel; pertinent maps are contained in the public docket for this accident. The approximate global positioning system (GPS) coordinates of the primary wreckage were 38 degrees 38 minutes 21 seconds north latitude and 121 degrees 13 minutes 13 seconds west longitude, at an elevation of about 200 feet. 1.6 MEDICAL AND PATHOLOGICAL INFORMATION The Sacramento County Coroner completed an autopsy on the pilot. The FAA Office of Aerospace Medicine, Civil Aerospace Medical Institute, Oklahoma City, Oklahoma, performed toxicological testing of specimens obtained from the pilot. Analysis of the specimens contained no findings for carbon monoxide, cyanide, or volatiles. The analysis was positive for an unquantified amount of METOPROLOL, which was detected in the blood and present in the urine samples. 1.7 TESTS AND RESEARCH Representatives from the Safety Board, FAA, American Eurocopter, Sacramento Country Sheriff's Department, and Turbomeca USA examined the wreckage on scene and later at Plain Parts, Pleasant Grove, California, on July 15
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.
Source: NTSB Aviation Accident Database
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