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On January 4, 1994, at 0920 mountain standard time, a Bell 206B, N771GM, operated by Southwest Helicopters, Inc., cruised into an unmarked power line approximately 3 miles west-southwest of the Coolidge Dam, Arizona. Visual meteorological conditions prevailed at the time of the aerial observation flight. The helicopter was destroyed by impact forces and postimpact ground fire. The commercial pilot and two of the three passengers sustained serious injuries; the third passenger was fatally injured. The local area flight originated from Kearny, Arizona, at 0754.
According to the State of Arizona Game and Fish Department (G & F), the accident occurred while the pilot was transporting three G & F employees on an authorized wildlife population survey flight over the specified survey area. At times during the mission, the helicopter pilot was required to fly within 200 feet of the ground.
The pilot reported to the National Transportation Safety Board that, the night before the accident flight he had reviewed the sectional aeronautical chart which covered the general area where the G & F personnel desired to be flown. He reviewed the airspace hazards. The pilot said that he was aware the flight would be performed, at times, within 200 feet above ground level.
Also, the pilot indicated that because of his previous flying experience, he had some familiarity with the general survey flight area.
Regarding the accident flight, the pilot stated in his accident report that during the 2-minute period which immediately preceded the crash, he was in cruise flight and had been maintaining an altitude of approximately 200 feet above the ground. He was looking for wires and obstructions and did not see any.
The pilot additionally reported to the Safety Board that although "the sun was very bright" it was not obstructing his forward vision. At the time of impact, the helicopter was in near-level flight and was not turning. He was flying in an easterly direction at 65 miles per hour (mph) and was proceeding along a direct route toward the next designated search area.
To aid in navigation, the pilot had use of a global positioning system (GPS) receiver which was equipped with a moving map display. The pilot reported that he was aware of his location.
Despite the fact that he was looking for obstructions, he did not observe the wires until a brief moment prior to the collision. He never observed the supporting towers. No avoidance maneuver was performed. The pilot further reported that the wires and towers were not conspicuous.
There were no ground-based eyewitnesses to the collision. The accident site was located in the vicinity of 110 degrees 35.46 minutes west longitude, by 33 degrees 9.20 minutes north latitude.
The helicopter was not equipped with a wire strike protection system. According to a review of the contract between the Arizona G & F Department and the operator of the helicopter, no requirement had been placed upon the operator to utilize a wire cutter-equipped helicopter during the flight.
On January 11, 1994, recovery personnel reported to the Safety Board that a smear of red paint was observed on the white-colored tips of the main rotor blades. The tail boom, which was found severed, was painted red. The main rotor blades were measured, and their lengths were compared with the point at which the tail boom was found severed. The blade length and the point at which the tail boom was found severed were approximately the same.
Wire-like abrasions (score marks) were observed circumferentially around the entire exposed portion of the tail rotor drive shaft. No evidence of wire marks was observed on the main rotor blades, control tubes, drive shaft, or blade grips. No evidence of wire marks were found on the right landing gear skid tube or on the right, forward, saddle portion of the attachment cross tube.
A wire-like laceration mark was found oriented diagonally across the top of the left landing gear skid tube. The mark extended in an aft direction to near the leading edge of the left, forward, cross tube saddle assembly. At this location, a laceration mark was observed approximately 1/2-inch above the top of the left landing gear skid tube, on the inside of the saddle. The mark was approximately the size of the accident wire.
Between 1 and 2 dozen wire-like abrasion marks were observed near the leading edge of the inside portion of the left, forward saddle (see photographs). The upper portion of the saddle, where the cross tube existed, was destroyed by fire.
No other wire-like marks were observed on the left landing gear assembly or the attachment cross tubes. (For additional information, see the wreckage diagram and photographs.)
TESTS AND RESEARCH
The manufacturer of a wire strike protection system evaluated the likelihood of whether a wire cutter would have been effective given the reported flight conditions. The manufacturer opined that its wire cutter could have been effective assuming contact had occurred above the calculated 2.41-inch vertical distance from the skid to the tip of the cutter. This region is referred to as being "unprotected" by the cutter because a wire which enters this area will be lower than the bottom of the cutter blade and therefore will not be severed. (See correspondence from Aeronautical Accessories and its related 5-degree nose-down flight profile drawing.)
The power line, which the helicopter impacted, was depicted on the Phoenix Sectional Aeronautical Chart. In general, the power line was oriented in a northeast to southwest direction, and spanned the area from the Coolidge Dam to near Kearny.
According to the power line owner (the San Carlos Irrigation Project, Bureau of Indian Affairs) only 1 power line was impacted. The impacted power line was within a group of 6 lines which were all supported between the same 2 towers.
The towers were located on adjacent hills, and they had different elevations. Tower number 65, located to the northeast of the crash site, had a base elevation of about 3,698 feet. Tower number 64, located to the southwest of the site, had a base elevation of about 3,877 feet. The horizontal distance between the towers was 1,680 feet. (See the Coolidge Dam profile and alignment drawings for additional information.)
The helicopter impacted the ground to the south of the southern-most power line. The elevation of the lines in the vicinity of the point of severance, between tower numbers 64 and 65, was estimated by U.S. Department of the Interior personnel at 3,730 to 3,735 feet mean sea level (msl). The elevation of the terrain below the point of severance varied between 3,560 and 3,600 feet msl.
The impacted power line was the southern-most of the two highest elevation lines. The line, which was not energized at the time of impact, had an estimated 7/16-inch diameter and was composed of 100 percent copper material in 7 wire strands.
Following the accident, the Safety Board received reports from several other helicopter pilots regarding the conspicuousness of the power lines in the accident site area. In summary, the pilots described their ability to see the obstructions using the following terms: "difficult," "extremely hard," or "virtually invisible."