Parlier, CA, USA
N313DA
ROBINSON HELICOPTER R22 BETA
The helicopter pilot was on the first leg of a ferry flight. The helicopter was 3.2 hours out of its 2,200-hour overhaul. The pilot reported that he was in straight-and-level flight and he initiated a right turn. The helicopter immediately rolled to the right and entered a low-G condition. The pilot responded appropriately and righted the helicopter. He then conducted a power-on landing. As the main rotor system came to a stop, one of the blades impacted the tailboom. Post landing examination of the rotorhead revealed that the pitch change link on the red main rotor blade had fractured, the spindle tusk had sheared off, and damage to both elastomeric teeter stops was evident. Metallurgical examination of the spindle tusk determined that it failed in a downward direction relative to the rotor blade due to shear overstress. Examination of the pitch change link determined that it failed due to overstress characterized by reverse bending numerous times to failure. Witness marks on the pitch link and pitch change arm indicated that the angle of the blade at the time of the pitch link failure was 45-degrees downward, an angle that could be achieved if the spindle tusk had already been fractured. According to Robinson Helicopter Company, this angle could also be obtained during extreme teeter of the hub/blade during a mast bumping event. A Robinson Helicopter Company publication notes, "Pushing the cyclic forward following a pull-up or rapid climb, or even from level flight, produces a low-G (weightless) flight condition. If the helicopter is still pitching forward when the pilot applies aft cyclic to reload the rotor, the rotor disk may tilt aft relative to the fuselage before it is reloaded. The main rotor torque reaction will then combine with tail rotor thrust to produce a powerful right rolling moment to the fuselage. With no lift from the rotor, there is no lateral control to stop the rapid right roll and mast bumping can occur."
HISTORY OF FLIGHT On February 17, 2010, at 1450 Pacific standard time, N313DA, a Robinson R22 Beta, force-landed in a field near Parlier, California. The commercial pilot and one passenger were not injured. The helicopter sustained substantial damage to the main rotor assembly. The pilot operated the helicopter under the provisions of Title 14 Code of Federal Regulations Part 91 and visual meteorological conditions prevailed. The flight departed from Fresno Yosemite International Airport, Fresno, California, at 1430, and was on the first leg of a trip to North Las Vegas Airport, Las Vegas, Nevada. The first planned stop was Meadows Field Airport, Bakersfield, California. According to the pilot, he was established in straight-and-level flight when he initiated a 10- to 20-degree banked turn to the right. The helicopter immediately rolled inverted and the pilot applied aft cyclic and lowered the collective. The pilot was able to regain control of the helicopter and perform a powered landing to a field. Damage to the helicopter included a fractured rotor blade spindle tusk, fractured pitch link, crushed elastomeric teeter stops, and a shallow, diagonal crease in the tail boom just forward of the tail rotor. Pilot Statement The pilot had just entered into a lease/purchase agreement on the helicopter. The pilot reported that he and a passenger were ferrying the helicopter from Fresno to Las Vegas and their initial stop was going to be Bakersfield. As he was approaching Reedley, California, at 1,100 feet mean sea level (800 feet above ground level (AGL)) and 85 knots, he initiated a “shallow right turn while slightly increasing collective.” The helicopter “abruptly rolled to the right at a rate well in excess of 100 degrees per second. By the time I recognized a possible low G situation, we were inverted.” The pilot immediately applied aft cyclic to reload the rotor disc and lowered the collective, which resulted in a split-S maneuver. He then entered a precautionary autorotation. At approximately 100 feet AGL and 70 knots, he arrested the descent and continued the autorotation to a field. As he approached the landing spot, he initiated a power recovery and entered a flare. He stated that he executed a normal set down, rolled off the power, and disengaged the clutch. As the main rotor slowed, it became evident to the pilot that the droop stops had broken and the blades began to sag. He applied the rotor brake and the retreating blade impacted the tail boom at a slow speed. Passenger Statement The passenger submitted a written statement. She reported that she was seated in the left seat of the helicopter and her boss was flying the helicopter. They were ferrying the helicopter from Fresno to Las Vegas. To her understanding, the helicopter was just purchased out of overhaul. When they got into the helicopter, the pilot did a preflight before they left the airport and began their flight to Las Vegas. The passenger was enjoying the flight and taking photographs when she noted to the pilot that the cyclic appeared shaky. The passenger’s only other flight in a helicopter had been on an introductory flight, and she did not experience the cyclic vibrating during that flight. She indicated that because she is not a pilot, she did not know if the vibration was normal or not. She was looking out of the window when the helicopter tilted to the right, front side, and then became inverted. The passenger could see the ground coming up quickly. She screamed and then closed her eyes thinking that the helicopter was going to collide with the ground. In just a few seconds, the helicopter was upright and they were descending slowly to land. According to the passenger, the landing was a very smooth and gentle. PERSONNEL INFORMATION The pilot, age 35, held a commercial pilot certificate for airplane single and multi-engine (limited to center thrust) instrument land, and helicopters. He held a first-class medical certificate that was issued on October 20, 2009. It did not have any limitations or waivers. The pilot reported 1,702 hours of total flight time, with 1,580 in airplanes and 64 in helicopters. The pilot had 36 hours of pilot-in-command time in helicopters. AIRCRAFT INFORMATION The helicopter had undergone a 2,200-hour overhaul and had operated 3.2 hours at the time of the accident. Following the maintenance performed on the helicopter, it was flight tested. According to the test pilot that flight tested the helicopter, the helicopter performed normally. The test pilot had flown the helicopter prior to the overhaul and following the overhaul. According to the pilot, the helicopter performance and handling characteristics both before and after the overhaul were similar. METEOROLOGICAL INFORMATION A Safety Board meteorologist reviewed the meteorological conditions along the helicopter's route of flight. Light wind conditions were reported just above the surface and light chop was reported near the coast at higher altitudes. There were no indications of turbulent activity at the helicopter's altitude or along its route of flight. Additionally, no mountain wave activity was reported. TESTS AND RESEARCH A fractured pitch change link and the root of the red blade with the spindle were sent to the NTSB Materials Laboratory for examination. Optical examinations of the fracture region of the spindle tusk revealed fracture features and smearing indicative of a shearing overstress fracture. The indicated shearing direction was as if the inboard end of the tusk moved downward relative to the main rotor blade. The pitch change link upper rod end fitting was fractured through the thread nearest to the head. Magnified optical examinations of the rod end fracture surfaces displayed fracture topography with opposed crescents and other features indicative of cyclic overstress where the rod was reverse bent a few times to failure. The orientation of the indicated axis of bending was measured to be about 81 degrees relative to the axis of the fitting head. During examinations of the rod end fracture, mating damage patterns were noted on the fitting and on adjacent areas of the pitch change arm. The mating damage included a dent in the hat spacer that matched the edge of the monoball outer race, a dent in the vertical face of the pitch change arm and a corresponding paint chipping in the rod end paint and a angular dent in the lower face of the pitch change arm matching the edge configuration and deformation on one corner of the upper pal nut. Matching the fracture surfaces and damage patterns of the pitch change link to the pitch change arm, positioned the link at nearly 45 degrees to the arm. According to the metallurgist, this angle would occur with the main rotor blade at a large droop angle that could be attained if the tusk on the spindle were fractured. A representative from Robinson Helicopter Company indicated that this angle could also occur during extreme teetering of the main rotor hub during a mast bumping event. ADDITIONAL INFORMATION According to section 4 of the pilot's operating handbook for the R22, "Mast bumping may occur with a teetering rotor system when excessive main rotor flapping results from low "G" (load factor below 1.0) or abrupt control input. A low "G" flight condition can result from an abrupt cyclic pushover in forward flight. High forward airspeed, turbulence, and excessive sideslip can accentuate the adverse effects of these control movements. The excessive flapping results in the main rotor hub assembly striking the main rotor mast with subsequent main rotor separation from the helicopter." According to Robinson Helicopter Company Safety Notice SN-11, Low-G Pushovers Extremely Dangerous, "Pushing the cyclic forward following a pull-up or rapid climb, or even from level flight, produces a low-G (weightless) flight condition. If the helicopter is still pitching forward when the pilot applies aft cyclic to reload the rotor, the rotor disk may tilt aft relative to the fuselage before it is reloaded. The main rotor torque reaction will then combine with tail rotor thrust to produce a powerful right rolling moment to the fuselage. With no lift from the rotor, there is no lateral control to stop the rapid right roll and mast bumping can occur. Severe in-flight mast bumping usually results in main rotor shaft separation and/or rotor blade contact with the fuselage." According to RHC Safety Notice SN-29, Airplane Pilots High Risk When Flying Helicopters, "To stay alive in the helicopter, the experienced airplane pilot must devote considerable time and effort to developing safe helicopter reactions. The helicopter reactions must be strong and take precedence over the pilot's airplane reactions because everything happens faster in a helicopter…To develop safe helicopter reactions, the airplane pilot must practice each procedure over and over again with a competent instructor until his hands and feet will always make the right move without requiring conscious thought."
The pilot inadvertently entered a low-G condition during cruise flight, which resulted in a momentary loss of control of the helicopter and mast bumping.
Source: NTSB Aviation Accident Database
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