WALNUT CREEK, CA, USA
N581BG
SIKORSKY S-58T
THE HELICOPTER WAS OBSERVED FLYING LOW IN A GRADUAL DESCENT OVER A RESIDENTIAL AREA HEADING TOWARD AN OPEN FIELD. WITNESSES SAID THE HELICOPTER NOISE LEVEL WAS QUIET, BUT NOT SOUNDLESS. AT 200 FEET ABOVE THE GROUND, THE HELICOPTER ROLLED LEFT TO AN INVERTED FLIGHT ATTITUDE AND THEN DESCENDED IN A NOSE-DOWN ATTITUDE. WRECKAGE EXAMINATION DID NOT FIND ANY PREIMPACT MECHANICAL FAILURES WITH THE HELICOPTER'S FLIGHT CONTROL OR DRIVE SYSTEMS. SIGNATURES ON THE INTERNAL ROTATING COMPONENTS OF BOTH ENGINES, THE DRIVE SYSTEM, MAIN ROTOR BLADES, AND TAIL ROTOR BLADES WERE INDICATIVE OF LOW RPM. EXAMINATION OF THE ENGINE FUEL SYSTEM REVEALED EVIDENCE OF FOREIGN PARTICLES IN THE FLOW DIVIDER AND FUEL NOZZLES. POSTACCIDENT FUEL SAMPLES FROM THE REFUELING TRUCK WERE TESTED IN A LABORATORY. ACCORDING TO THE LAB REPORT, THE FUEL HAD A TENDENCY TO ABSORB AND RETAIN WATER, AND A PRESENCE OF MICROBIAL CONTAMINATION WAS NOTED. IN ADDITION TO THE LAB TESTS, FUEL SAMPLES FROM THE OPERATORS REFUELING TRUCKS WERE INDEPENDENTLY TESTED WITH WATER DETECTION PASTE AND TESTED POSITIVE FOR WATER.
HISTORY OF FLIGHT On February 21, 1994, at 1015 hours Pacific standard time, a Sikorsky S-58T, N581BG, lost control and collided with the ground near Walnut Creek, California. The helicopter was being operated by Air One, Inc., San Jose, California, under 14 CFR Part 91. The helicopter was destroyed. The airline transport pilot and two other crew members were fatally injured. The helicopter departed a job site in Concord, California, about 1010 hours after completing an external load operation on the previous flight. The helicopter was positioning back to San Jose at the time of the accident. One witness watched the helicopter fly past his position in the back yard of his residence. The witness described the helicopter as "low for this area and slow (60 to 80 mph)." The witness described the sound as "quiet but not soundless" and the flight path as descending. The witness continued to watch the helicopter descend thinking it might land. The witness stated, "The aircraft appeared to mush. Then it did a (like a snap) roll to the inverted position with a fast descent." The witness estimated the altitude to be about 200 feet above ground level when the helicopter appeared to mush. Two witnesses reported hearing a "sputtering" noise, then observing an abrupt rolling maneuver. The helicopter disappeared behind hilly terrain in a nose-down attitude. WRECKAGE AND IMPACT INFORMATION The helicopter was found aligned on a magnetic heading of 343 degrees on the western slope of Shell Ridge. The terrain was open grassland and sloped 10 to 15 degrees. The elevation of the accident site was approximately 330 feet above sea level. The wreckage was spread in a southeasterly direction about 93 feet. The initial impact point was marked with a ground scar which matched the contour of the square nose of the engine air inlet and bulbous shape of the helicopter's engine cowling. The ground scar was oriented to the 343-degree magnetic heading of the wreckage path. There were four longitudinal gouges on the right side of the initial impact point. The longitudinal gouges extended from 17 feet to 22.3 feet and were about a foot deep. Debris from the helicopter's main rotor blades was found in and around the longitudinal gouges. The helicopter's engines, combining gearbox, and pieces of the engine cowling were found in the gouge at the initial impact point. The combining gearbox case, which is located forward of the engines, was fractured in many pieces exposing the internal gears. There was no evidence of rotational damage found on the gears in the combining gearbox. Both engines displayed compression deformation on their respective external cases. The accessory gearboxes were separated from the compressor inlet cases exposing the first stage compressor discs. Both first stage compressor discs displayed nicks and gouges to the blade leading edges. The power section of both engines exhibited circumferential rubbing from axial machining on the power turbine shroud and power turbine guide vane by the power turbine. Both accessory gearboxes were exposed to the postimpact fire. Testing of the accessories was precluded by the fire damage. Debris was found in both engine fuel flow dividers. The No. 1 engine flow divider had 4.4 milligrams of debris. Chemical analysis of the debris material revealed the composition to be silicious, metallic, and fibrous matter with some glass particles. The No. 2 engine flow divider had 1.3 milligrams of debris. Chemical analysis of the debris material revealed the composition to be silicious and metallic matter, and with calcium and lead present. The No. 11 fuel nozzle was removed from both engines and examined. Debris was found on the sheath and in the nozzle body. The debris appeared in be granular, about the size of medium coarse sand, and black in color. The center section of the fuselage was consumed by fire. The main transmission and angled gearbox were in the center of the burn area about 29 feet from the initial impact point. The drive shaft from the combining gearbox to the angled gearbox was fractured at the angled gearbox end. The fractured end exhibited evidence of bending and torsional overload. Rotational scoring was observed on the circumference of the shaft. The forward end remained attached to the coupling at the combining gearbox. A portion of the combining gearbox housing and an internal gear was also attached to the drive shaft. The drive shaft from the angled gearbox to the main gearbox was destroyed by fire. Examination of the Thomas couplings did not reveal any evidence of rotational damage. The main gearbox came to rest upright with the rotor head attached. The main gearbox was exposed to fire. A hole was melted in the main gearbox case at the bottom. Examination of the internal gears did not reveal any evidence of broken or damaged teeth. The main rotor head was also exposed to fire. Two of the three hydraulic flight control servos were destroyed by fire and impact forces, and were disconnected from the stationary star assembly. The geometry of the rotating star assembly and the pitch change horns was not aligned. The rotating star assembly was positioned aft, opposite the direction of rotation. Three of four pitch change control rods were angled. The fourth was burnt in half. The scissors assembly was missing and later found in debris in a hangar where the helicopter was recovered. The four main rotor blades remained attached to the hub and exhibited damage from striking the ground. About 6 feet of the yellow blade was broken off and was found about 130 yards from the initial impact point. The tail rotor gearbox was separated and found embedded in the ground adjacent to the vertical stabilizer. Three of the tail rotor blades were bent and one was separated. One blade was bent opposite the direction of rotation. Two blades were bent outward from the gearbox with the crease in the bend parallel to the chord. TESTS AND RESEARCH Fuel Samples Fuel samples were tested by the National Transportation Safety Board utilizing water detection paste. One sample was taken by the operator on the day of the accident and stored in a mason jar. Federal Aviation Administration (FAA) air safety inspectors from the Oakland Flight Standards District Office took custody of the sample and transported the sample to a hangar where the helicopter wreckage was being temporarily stored. The water detection paste indicated the presence of water in the sample. Fuel from two of the operator's fuel trucks (California License Numbers: 3D46592 and 1S37514) were also tested. The water detection paste indicated a trace of water in one truck (3D46592) and about 1/4 inch of water in the other truck (1S37514). Additional fuel samples from one of the trucks were sent by the operator to Analysts, Inc., Oakland, California, for laboratory analysis. According to the Laboratory Report Number 9096, dated February 28, 1994, the fuel sample did not meet the applied requirements for water reaction. The report indicated, "The fuel sample had a tendency to retain or absorb water, visible debris is present, and Micro-Organism presence is indicated." The report also stated, "Conditions noted may contribute to filter plugging and/or accelerated (excessive) deposit formation." Operator Fuel Quality Assurance According to the Federal Aviation Administration, the operator does not perform daily fuel quality control procedures except when performing flights under U.S. Forest Service contract. The U.S. Forest Service contract requires the operator to perform fuel quality assurance procedures daily on the fuel trucks. The procedures dictate that the trucks will be checked for water, and the water removed, every morning before the vehicle is used. The checks must also be performed after reloading of fuel, washing of the truck, and after heavy rain or snow. Fueling records maintained by the operator indicate the accident helicopter was last serviced on February 13, 1994, with the addition of 40 gallons of Jet A fuel from truck number 3D46592. The records reflect that a quantity of 293 gallons remained in the truck's 1,510-gallon-capacity tank after the fueling operation. Additionally, the operator does not completely fill the helicopter's fuel cells during servicing. According to the operator, the helicopter can sit for periods longer than a day partially fueled, allowing an airspace in the fuel cells. Information published by the Federal Aviation Administration The "Airframe and Powerplant Mechanics General Handbook" is published by the Federal Aviation Administration to provide basic information to mechanics, in part, on aircraft fuels and fuel systems. The manual delineates material concerning fuel system contamination and results from contamination. The manual indicates that water, foreign particles, and microbial growth can cause fuel system malfunctions and result in a loss of engine power. ADDITIONAL INFORMATION The wreckage was released to the representatives of the owner on October 19, 1994.
Inadequate fuel quality control procedures by the helicopter operator resulting in fuel contamination and a loss of engine power. The pilot's decision to extend the autorotative glide to avoid striking residential homes, which resulted in low main rotor rpm and a subsequent in-flight loss of control, is also causal in this accident. A factor in the accident was the unsuitable nature of the residential area over which the loss of power occurred for a successful forced landing.
Source: NTSB Aviation Accident Database
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