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On March 14, 2000, at 1911 hours Pacific standard time, a Bell OH-58C, N911JN, collided with a trailer and the ground during an autorotation at Compton, California. The autorotation was precipitated by pilot perceived malfunctions in the electrical and hydraulic systems during cruise. The helicopter was owned and operated by the Compton Police Department as a public-use aircraft and was engaged in a routine law enforcement patrol mission. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was destroyed in the ground collision sequence. The private pilot and an observer, the sole occupants, sustained minor injuries. The local area patrol flight originated from the Compton Municipal Airport at 1730.
According to the statements of the pilot and observer, their shift began at 1600 and they conducted a full preflight inspection of the helicopter prior to the 1730 takeoff. The pilot stated that there were no maintenance items carried forward for this flight. A 1-hour routine patrol mission was flown and they landed at a remote refueling pad and conducted a hot refueling to maximum tank capacity, then took off again about 1840 to continue their patrol. They responded to a traffic accident call about 1900 and orbited the location in left turns. Following release from the call, the pilot exited the orbit. Shortly after that, the pilot and observer noticed that the low rotor rpm warning light flashed on, then back off, then on again. The pilot used the push to test switch and the light returned to normal operation. The pilot checked the tachometer and noted normal rotor and engine rpm. The helicopter was only 3 miles from their base at the Compton airport and the pilot decided to return to base and have maintenance examine the helicopter.
The low rotor warning light then began to flash on and off, with an increasing frequency. Shortly thereafter, the low rotor light came on steady. Suddenly all the cockpit warning and caution lights illuminated, both on the eyebrow panel and center pedestal panel. Concurrent with illumination of the warning and caution lights, the hydraulic system turned off and the pilot had to resort to manual force on the flight controls. The pilot checked the engine and rotor gages and noted that the rotor needle on the tachometer was registering off scale high; however, neither he nor the observer perceived any change in the background engine and rotor sounds. The observer reported that he believed they lost their radios and exterior lights at this time. In his interview, the observer reported that the engine and rotor tachometer needles were married together at 100 percent until the autorotation.
During this period, the pilot had been on an extended final approach to runway 25 left and control of the helicopter was becoming difficult. Suddenly, a fixed wing airplane appeared on base leg to the same runway and the pilot had to maneuver to avoid a collision. According to the pilot, with the control difficulties and uncertainty about what was happening to the helicopter, he decided to autorotate to a clear area in a schoolyard. Both the pilot and the observer stated that during the autorotation the night sun spotlight went out. The pilot cleared a building but could not avoid a tractor-trailer rig that he had not observed before. The helicopter collided with the trailer, then the ground, and rolled over on its back.
In his interview, the pilot stated that he was familiar with the hydraulic system and knew that the solenoid valve was electrically operated and it takes electrical power to close the valve and turn off the hydraulics; he stated that he was aware that the valve fails to the open position when electrical power is lost.
The pilot is a patrol officer with the Compton Police Department, assigned as a helicopter pilot with the department's air unit. Review of the Federal Aviation Administration (FAA) airman records database disclosed that he holds a private pilot certificate, with a rotorcraft helicopter rating, which was issued December 6, 1996. In addition, the pilot holds a second-class medical certificate that was issued without limitations on December 23, 1997. The pilot reported that he began flying in 1996 with assignment to the air unit and completed his primary training in the OH-58. The pilot estimated that he has accrued about 3,000 hours in helicopters, all of it in the OH-58, with about 1,500 hours flown in the accident helicopter. His total night experience is about 2,400 hours. The air unit instructor pilot conducted the pilot's most recent biennial flight review on December 13, 1998.
The observer is a patrol officer with the Southgate Police Department and is assigned as an observer with the Compton Police Department air unit. He does not have any aeronautical pilot licenses or ratings. He has been an observer with the Compton Air Unit for 4 months and has flown in the OH-58 about 300 hours.
The helicopter, a Bell OH-58C, serial number 68-16751, was obtained from military surplus by the Compton Police Department in June 1996 and certificated in the Restricted Category. During military service, the US Army operated the helicopter.
Mission specific equipment, including police band radios and a night sun spotlight, were installed by contractors to the Air Unit maintenance department in accordance with FAA Forms 337. Review of the documents and examination of the helicopter revealed that the wiring met the recommendations for gauge standard and circuit protection specified in FAA Advisory Circular 43.13-1A.
The maintenance program utilized by the Air Unit consisted of the inspections and procedures specified in the US Army OH-58C maintenance manual, TM 55-1520 228-23-1 and -2. The most recent 100-hour inspection was accomplished on March 3, 2000, 77 hours prior to the accident. Review of the maintenance records disclosed that all life-limited components were within the specified time limits.
The closest aviation meteorological observation station is the Hawthorne, California, airport, which is located 7 miles west of the accident site. At 1953, the station was reporting clear skies with visibilities 4 miles in haze. A Safety Board computer program calculated that the Nautical Twilight ended at 1855. At the time of the accident, the moon was 75 degrees above the horizon on a bearing of 134 degrees; 71 percent of the disk was illuminated.
WRECKAGE AND IMPACT
The accident site is in a side yard of a high school campus about 1/4-mile from the approach end of runways 25L and 25R at the Compton Municipal Airport. All wreckage components were located within the confines of the yard. The helicopter was laying on its right side. Markings consistent with the color and dimensional geometry of the skids were noted on the roof of the trailer.
The right frontal area of the helicopter nose was crushed rearward. The crush line was oriented about 40 degrees to the horizontal axis (nose down), and, 20 degrees to the lateral axis (left yaw). The tail boom remained attached to the fuselage; however, it was buckled downward at a point just aft of the horizontal stabilizer. The tail rotor blades were retained in their grips and undamaged. The upper and lower vertical stabilizers, including the stinger, were intact and undamaged.
The skids and the forward and rear cross tubes were spread and deformed upward. The right skid was separated from the rear cross tube at the skid attach point. The right skid forward cross tube was fractured and separated at the fuselage attach point. The left skid remained attached to the forward and rear cross tubes. The left forward cross tube was fractured and separated from the fuselage attach point. The left rear cross tube remained attached to the left skid and the fuselage.
The white main rotor blade remained attached to the hub grip. The outboard 3 feet of the blade was smoothly curved rearward, with span wise compression wrinkles noted along the trailing edge. The entire blade is bent downward about 15 degrees at a point about 42 inches outboard of the grip. Minor leading edge damage and cord wise scoring is noted on the outboard one third of the blade.
The red main rotor blade was fractured in two places, at 40 inches outboard of the grip pin and 31 inches inboard from the tip. Leading edge damage, chordwise scoring and trailing edge compression wrinkles were noted.
The main rotor hub, grips, and pitch horns were intact. The pitch change links were separated just below the clevis connection to the pitch horns. The links were deformed and the fracture faces angular and shinny. Main rotor drive shaft failed about 2 inches above the center of the droop stop with torsion and bend deformation evident.
The main and tail rotor transmissions rotated freely in both directions. The tail rotor transmission remained attached to the tail boom. The main rotor transmission lower isolation mounts were broken and the entire transmission had rocked forward and contacted the upper deck.
The hydraulic pump was broken from the front of the transmission. A dent matching the dimensional geometry of the pump was noted on the deck directly below the pump. The pumps attach lugs were fractured, with the fracture face angular and shiny. The pump drive coupling shear point was intact.
The engine to transmission drive shaft was connected at both ends; however, rotational scoring was noted where the shaft passes through the transverse bulkhead. The tail rotor drive shaft was separated between the Nos. 5 and 6 hangar bearings, with rotational scoring evident on both sides of the fracture point; the shaft was torsionally deformed at the fracture point.
TESTS AND RESEARCH
The helicopter was recovered from the accident site to the Compton Airport where a detailed examination was conducted.
Complete control system continuity was established from the cockpit to the main and tail rotor systems. The hydraulic lines were intact and the reservoir contained fluid. An electric drill was adapted to the hydraulic pump drive shaft and turned on; the pump turned and pressurized the system. The flight controls were actuated and the longitudinal, lateral, and collective hydraulic actuators moved in the correct direction when cockpit control inputs were applied.
The main rotor tachometer drive is on the hydraulic pump. During the test of the pump with the electric drill, the cockpit rotor tachometer needle was observed to move in the correct direction and was responsive to speed changes of the drill.
The airframe and engine fuel and pneumatic lines were traced and found to be intact. The engine controls from the cockpit to the fuel control and governor were intact and functional.
Drive train continuity was established throughout the helicopter. All gearboxes turned smoothly. The engine N2 turbine rotated freely and smoothly, and turned the main rotor drive shaft through the main rotor transmission gearbox. Minor fuzz was found on the chip plugs for the main rotor, tail rotor, and engine gearboxes.
Using the battery on the helicopter, the submerged tank fuel boost pump was energized to defuel the helicopter; about 60 gallons of Jet A was pumped from the tanks through the main fuel filter in the pressure line. Following the defueling operation, the battery charge was measured at 22.6 volts. No arcing evidence was noted on the battery terminals. The wires between the battery and the voltage regulator were examined and found to be unremarkable.
The wiring and terminals between the generator and the voltage regulator were examined and found to be unremarkable. No evidence of arcing or other unusual electrical activity was found in the Line Control Relay, Non-essential Bus Relay, or the Start Relay. The wiring from these source elements was traced to the distribution busses, with no unusual signature or condition noted. The electrical wires were then traced through the airframe to the bus distribution bars and the circuit breakers, with no evidence of arcing, cracked insulation, or other unusual condition noted.
No evidence of arcing was observed between the bus bars or across any terminal on the bars or circuit breaker terminals. The circuits identified from the warning panel, caution panel, and the hydraulic power valve were identified and traced from their circuit breaker to the ground point. No evidence of arcing, cracked insulation or other unusual condition was found. All cannon plug connectors for these circuits were examined and found to be unremarkable.
The wiring and circuit breakers for the mission specific equipment was traced, with no unusual conditions noted except as follows. The circuit breaker for the Night Sun spot light power control unit in the nose was found tripped, with the breaker shank bent.
The US Army Safety Center at Fort Rucker, Alabama, was contacted. The pilot described circumstances were provided to the Safety Officer for the OH-58 program. The Army records were checked back to 1981, with no recorded instance in their fleet where all of the warning and caution lights illuminated at one time. They did have several instances where all caution panel lights illuminated. According to the system reports, the problem was traced to a short in the caution panel impedance pad, which grounded all the caution circuits. In these instances, a rubber grommet, which electrically isolates the impedance pad from the airframe structure, either deteriorated or was not reinstalled during maintenance.
The impedance pad for the accident helicopter was identified and checked, and the rubber grommet was found intact. An ohmmeter was used and no electrical leakage was found to the aircraft structure.
Bell Helicopter was contacted and they reported no historical fleet record of the event as described happening in the OH-58 series, the 206B series, or the 206L series helicopters. They opined that an almost complete electrical system voltage spike/surge simultaneously in a number of circuits would be required to replicate the event as described by the pilots.
The FAA Aviation Data Systems Branch, AFS-620, was contacted for a Service Difficulty system data run on the Bell 206 series helicopters for electrical and warning/caution system reports. No record matched the event as described by the crew of N911JN. Only one report was contained in the data system pertaining to an over voltage condition; in that report, the over voltage relay tripped the generator off-line when the condition was detected. All other reports in the data system run concerned either specific circuit shorts or the loss of electrical power due to generator/voltage regulator failures.
The helicopter wreckage was released to the owner at the conclusion of the examination.