WOODRUFF, UT, USA
N5013P
Hughes 369D
As the helicopter was being maneuvered at low altitude, the pilot heard a series of compressor stalls, accompanied by a rapid decay of rotor rpm. The helicopter struck the ground, spun around and rolled over on its right side. The engine was functionally tested and was 20 horsepower below overhaul specifications. The compressor stalled repeatedly. Compressor disassembly disclosed case liner erosion, most noticeably at the 5th and 6th stages, and the rotating labyrinth seal was missing. After replacing the seal and installing a slave compressor module, the engine tested normal.
On January 4, 1999, approximately 1645 mountain standard time, a Hughes 369D, N5013P, registered to and operated by Hawkins & Powers Aviation, Inc., was substantially damaged during a hard landing following an engine power loss 7 miles south of Woodruff, Utah. The commercial pilot and two passengers were not injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the aerial observation flight being conducted under Title 14 CFR Part 91. The flight originated at a ranch near Woodruff, Utah, approximately 30 minutes before the accident. The flight had been chartered by Deseret Land and Cattle Company in support of coyote research being conducted by the Utah State University. The helicopter was being maneuvered in an attempt to capture a coyote with a handheld net gun. According to the pilot's accident report, he made an initial downwind pass over the coyote and was going downhill at an altitude of 8 to 10 feet agl (above ground level). The speed was 25 knots with power reduced to near minimum. The pilot said it was his intention to increase power and apply left pedal to bring the helicopter around into the wind and reduce the airspeed. As power was applied and the turn initiated, he "heard a series of compressor stalls and noticed the auto relight system ignite, accompanied with a rapid decay of rotor rpm." The pilot wanted to reduce power to stop the compressor stall, but he was too close to the ground. The helicopter settled and the tail stinger struck the ground, flexing and shearing the tail rotor drive shaft. The compressor stalls then stopped and full power was restored, causing the helicopter to spin. The pilot made a hovering autorotation to a landing, but the spinning motion caused it to roll over on its right side. The statement the pilot gave to the National Transportation Safety Board was similar to the one he gave to the Federal Aviation Administration (FAA). Statements provided by the two passengers corroborated the pilot's report. According to the report filed by an FAA aviation safety inspector, maintenance record keeping irregularities were detected but none of which were causal to the accident. According to the Boeing Company accident investigator's report, there was no evidence of preimpact airframe malfunction or failure. Damage to the airframe was due to overload failures. According to the Rolls-Royce Allison accident investigator's report, the engine was functionally tested on February 11, 13, and 24, 1999. During the first test, engine performance was 20 horsepower below overhaul specifications, and the compressor stalled repeatedly. Compressor disassembly disclosed case liner erosion, most noticeably at the 5th and 6th stages, and a missing rotating labyrinth seal. The seal was replaced. The second functional test showed a 3-shaft horsepower performance improvement, but compressor stalls continued to occur during power accelerations from ground idle to flight idle. For the third test, a slave compressor module was installed on the engine. There were no compressor stalls and the engine produced an additional 14 shaft horsepower.
A partial loss of engine power due to erosion in the compressor assembly module. Factors were a missing rotating labyrinth seal, the installation of which had not been performed by unknown maintenance personnel.
Source: NTSB Aviation Accident Database
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