PENDLETON, OR, USA
N4734F
Bell UH-1H
The pilot and helicopter were under contract for firefighting. The helicopter was equipped with a Bambi waterbucket on a cargo hook. The pilot's mission was to fill the water bucket from a pond, then drop the water on a designated part of a nearby fireline. He was en route to the fire drop location, when he experienced a loss of hydraulic control. He noted that the master caution and one caution segment light illuminated. The pilot made a turn to initiate an autorotation to a clear-cut area, but due to low rotor rpm, the helicopter landed hard with substantial downward velocity and some forward speed. The landing skids collapsed and the helicopter rolled to the right. The main rotor blades contacted the ground and the engine remained running at idle power until the fuel lines were exhausted of fuel. After the accident, the flight controls, hydraulic system, electrical system, fuel system, tailrotor system, and transmission were checked. Also, the hydraulic servo actuators, the hydraulic pump, fuel control, and power turbine governor were checked and bench tested. During this postcrash examination, no preimpact mechanical failure or malfunction was found.
On August 19, 1996, about 1025 Pacific daylight time, a Bell UH-1H, N4734F, registered to Walters Helicopters and operated by the United States Forest Service as a public use fire fighting flight, landed hard and rolled over near Pendleton, Oregon, after the pilot reported a loss of hydraulic control. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was substantially damaged and the commercial pilot was not injured. The flight originated from a temporary helibase near the fire area about 39 minutes prior to the accident. U.S. Forest Service (USFS) personnel reported that the pilot's mission was to "hover-fill" the Bambi waterbucket on the cargo hook at a water pond, then drop the water on a designated part of the fireline. It was reported that the pilot had completed five or six round trips between the water pond and the fireline without incident. Prior to the flight operation, the pilot had prepared a load calculation which indicated that the helicopter was capable of picking up 1,800 pounds of water at the calculated density altitude, and 800 pounds of fuel starting out. At the time of the accident, the pilot had picked up a load of water from the pond and was returning to the fireline for the water drop in the company of another helicopter on the same mission. It was reported that the helicopters were about 300-500 feet above the tree line when the pilot reported that the master caution and one caution segment light illuminated. The pilot reported that there was a loss of all hydraulic control and he made a turn to initiate an autorotation to a clear-cut area. The pilot reported that he heard the low rotor rpm horn sound. Prior to touch down, the pilot dumped the water from the bucket, but did not jettison the bucket. The helicopter landed hard in the almost level clearcut area among small trees, downed logs and stumps. The landing skids collapsed and the helicopter rolled over 90 degrees to the right. At the time of the accident, approximately 200 pounds of fuel remained on board. Documentation of the wreckage at the accident site revealed that the helicopter impacted the nearly level terrain with substantial downward velocity and some forward speed. The impact collapsed the landing skids upward and aft. A tree stump, that the landing skids were straddling, punctured the fuselage belly to the left of the cargo hook. The helicopter rolled to the right and the main rotor blades struck the ground and small trees. One main rotor blade struck and severed a tail rotor blade. It appeared that the engine ran at idle power after impact and rollover until the fuel lines were exhausted. Position of the cockpit switches and flight controls revealed that the fuel was in the off position. The pilot confirmed that he shut off the fuel before he exited the helicopter. The hydraulic switch was in the on position. The throttle was in the flight-idle position. The collective was full up and the cyclic and pedals were centered. Examination of the fuel system revealed that the boost pump, check valves, low level switch, shut-off valve, drain valve, pressure indicating system and quantity indicating system all functioned properly. There was no visible damage to the fuel tank and plumbing. The fuel filter was clean, and there was one-half ounce of fuel present in the fuel bowl which was clean with no visible contamination noted. The hydraulic system revealed that the hydraulic filters were clean of contamination. There was no visible damage to the hydraulic reservoir, pump, lines, cylinder pilot valve, and boost cylinder. Examination of the fuselage and flight controls indicated that continuity was established throughout the flight controls and rigging. Excessive wear in the control bearings was noted in the forward/aft cyclic, lateral cyclic, collective, tail rotor and elevator. The servo extension control tubes were bent in each system. The tail rotor cable pulleys were torn in the vertical fin by the tail rotor blade strike. Excessive wear was noted in the throttle control lines and bearings. The transmission case sump was fractured at the attachment flange and the support case was fractured in several places by impact overloading. Transmission mounts displayed torsional overloading, twisting and bending. The free wheeling clutch unit was free to rotate with sudden stoppage damage noted. The mast was found to rotate, however, it displayed bending damage. Deep gouges were noted to the static stop and the splines were twisted. The lower swashplate bent in sudden stoppage, and the swashplate support was bent at the base. The upper swashplate bent and the driving bearing mounts broke off. The collective stops were found fractured and the collective levers bent. The pitch sleeve fractured at the base. One of the scissors was found broken. One control rod was broken and the other was bent. All of the pitch change links were bent. The main rotor hub and blade assemblies were found torsionally bent, twisted and broken. One of the static stops was fractured from the yoke while the other was bent from contact with the mast. The blade grips yielded from ground impact of the main rotor blades. Both main rotor blades were destroyed from ground impact and impact with tree stumps. Both blades displayed evidence of low rotor rpm contact and were coned upward. The main rotor drive shaft Kflex drive shaft was destroyed when the transmission moved during the ground impact. In the tail rotor shafting, the number 6 shaft in the vertical fin was cut. The 42 degree and 90 degree gear boxes displayed sudden stoppage damage. The tail rotor system displayed sudden stoppage damage. One tail rotor blade was cut twice by the main rotor blade. The second blade was damaged by contact with a hard object. Inspection of the electrical system found the fuel system, oil system, force trim, hydraulic system solenoids, caution lights, and pressure instruments were functional. The three hydraulic servo actuators and the hydraulic pump were removed from the helicopter and sent to Bell Helicopter Textron, Fort Worth, Texas, for testing. During the tests, it was found that the actuators and pump operated within normal operating parameters. The main fuel control and the power turbine governor were sent to Chandler Evans Control Systems Division, West Hartford, CT, for testing. During the bench test it was noted that both the fuel control and power turbine governor operated satisfactorily and all components were functioning normally. During the inspection of the fuel control, it was noted that the control flow schedules were "slightly higher (5% or less) than service limits." The N1 governor was set to cut in at "4205 rpm instead of 4536 (approximately 7.7 % lower than service limits.)" The power turbine governor high speed and low speed stops were "offset approximately 25 degrees in the high speed direction, resulting in approximately a 7% higher N2 governor cut in speed." It was also noted that the power turbine stop screws were backed out and there was no safety wire installed. One of the four fuel control mount nuts was found loose and the fuel control to the engine mounting gasket was deformed. (See Fuel Control Diagram).
undetermined malfunction/failure of a flight control or hydraulic system. Rough terrain in the emergency landing area was a related factor.
Source: NTSB Aviation Accident Database
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