GRAND CANYON, AZ, USA
N8533F
Bell 206B
The instructor pilot and new company pilot were practicing autorotations, which were terminating in a power-recovery. The flight instructor demonstrated a straight-in autorotation and said that the governor lagged behind but stayed in the green range. The instructor stated that this was typical of this particular helicopter and that he didn't consider the governor lag unusual. The instructor then had the dual student set up for a 180-degree autorotation. During the maneuver he told the student that he was ready to roll on the throttle at 200 feet agl. He stated that at 50 feet and 50 to 55 knots, the pilot flared the helicopter and at 15 feet he had the pilot pull in more collective. At this point, the instructor pulled in some collective himself. He stated that the torque gauge did not move and that it sounded like the engine sound was dragging down and the rpm's in the dual tach decayed. The flight instructor proceeded to take the controls and level the aircraft. The aircraft landed hard and came to a complete stop. After the aircraft was shut down, the tail boom was found still attached but hanging to the ground. A Bell Operations Safety Notice advises that tail boom failure can occur in autorotations where touchdown occurs at 70 percent main rotor speed due to a resonance response of the structure to the main rotor. A postaccident test of the governor function was performed. Power checks were done to duplicate an autorotation and the tests revealed that the engine was slow to accelerate and the governor surged with continual fluctuation of Ng readings.
On April 20, 1998, at 1615 hours mountain standard time, a Bell 206B helicopter, N8533F, landed hard following a practice autorotation 7 miles southeast of Grand Canyon National Park, Arizona. The aircraft, operated by Papillon Grand Canyon Helicopters, sustained substantial damage. The flight instructor and the pilot were not injured. The flight originated at Grand Canyon Airport at 1506 as a training flight for a new company pilot. The autorotations were going to terminate in a power recovery. According to the flight instructor, he was training new company pilots in preparation for their 14 CFR Part 135 check ride. He stated that he had the pilot perform a quickstop to introduce flaring and settling. He said that this maneuver was a good demonstration of the governor lag. He then proceeded to demonstrate a straight-in autorotation, and said the governor lagged behind but stayed in the green. He stated that "this was typical of Copter 3, and I did not find it unusual." The instructor told the pilot to set up for a 180-degree autorotation. His written statment notes that "at 200 feet agl, I announced ready to roll on throttle." After feeling he reached full throttle, the instructor verified with his collective throttle and announced full throttle. He said about 50 feet and 50 to 55 knots the pilot flared the helicopter. About 15 feet, he told the pilot to check in a little more collective and even pulled (the collective) slightly up himself. At this point, he said he watched the torque gauge, and it did not respond. Additionally, he said he noticed the engine sound was "dragging down and the rpm's on the dual tach decayed." The flight instructor took the controls at this point, and leveled the aircraft. He said as they settled the low rpm horn went off, and the dual tach continued to decay. He said the needles never split and they contacted the ground with the needles descending through 90 percent. The aircraft landed hard and dug in the dirt about 3 inches deep and then came to a complete stop. The flight instructor brought the aircraft to flight idle and got out and inspected the aircraft for damage. He found the tail boom still attached, but hanging to the ground. The engine was examined on April 23, 1998, at Dallas Airmotive, Inc., Scottsdale, Arizona, following the hard landing. The engine was installed in a test cell for a check of the governor function on acceleration. The test revealed that the governor Ng readouts were normal above 90 percent. Power checks were done to duplicate an autorotation and the tests revealed that the engine was slow to accelerate and surged. The governor was noted to rise to 104 percent, drop to 97 percent, rise again to 103 percent, and then drop to 97 percent with continued fluctuation noted throughout the rest of the test. Bell Helicopter Operations Safety Notice 206-84-10 notes that in autorotations where the main rotor slows to about 70 percent, tail boom failure can occur due to a resonance response of the structure to the main rotor.
The malfunction and erratic speed control of the engine's governor, and the flight instructor's delayed remedial action.
Source: NTSB Aviation Accident Database
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