Del Valle, TX, USA
N7779M
ROBINSON R22 BETA
The helicopter was flying across a river when witnesses heard a loud "pop" or "bang" and saw something fall from the helicopter into the river. They heard the helicopter crash, saw smoke, and ran to the crash site. One main rotor blade was found approximately 780 feet from the wreckage. The other main rotor blade was found approximately 975 feet from the wreckage in the opposite direction. Both blades had separated from the main rotor hub at their coning hinges. All components of the helicopter were accounted for with the exception of portions of both coning bolts. The investigation was unable to locate the object observed falling from the helicopter or determine what it was. Examination of the rotor head components, including the recovered portions of the coning bolts, revealed fracture features consistent with overload failure and mechanical damage indicative of mast bumping (contact between the main rotor blades and the mast). The main rotor blade pitch links and attaching hardware displayed compression buckling and bending overload features consistent with an out-of-control main rotor that had diverged from its normal plane of rotation. The helicopter manufacturer reported that the damage signatures were consistent with both low-G mast bumping and low rotor rpm rotor stall. Low-G mast bumping is a phenomenon specific to two-bladed teetering rotor systems in which the rotor disc becomes unloaded, typically due to a cyclic pushover. Low rotor rpm rotor stall is a different phenomenon in which the main rotor blades experience an aerodynamic stall due to low rotor rpm caused either by the pilot raising the collective too much or by a loss of engine power followed by the pilot lowering the collective too slowly. Rotor blade stall can lead to mast bumping. The pilot’s control inputs leading up to the event are unknown, and, therefore, the reason for the main rotor divergence could not be determined.
HISTORY OF FLIGHT On June 27, 2011, at 1223 central daylight time, a Robinson R22 BETA helicopter, N7779M, struck trees and impacted terrain near Del Valle, Texas. The pilot, the sole occupant on board, was fatally injured. The helicopter was substantially damaged and a post-impact fire ensued. The helicopter was registered to and operated by the operator under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The cross-country flight originated from Taylor (T74), Texas, about 1205, and was en route to Luling, Texas. According to the owner, the pilot was en route to Luling to give a friend his first helicopter ride. There were two witnesses standing on the south side of the Colorado River. They said the helicopter was flying north to south just above the trees. As it crossed the Colorado River, they heard a loud "pop" or "bang" and saw something separate from the helicopter and fall into the river. Asked if the object floated or sank, the witnesses said they did not pay any attention because when they heard the helicopter crash and saw the smoke, they ran to the crash site. PERSONNEL (CREW) INFORMATION The pilot, age 60, held a private pilot certificate with airplane single/multiengine land, instrument-airplane, and rotorcraft-helicopter ratings. His second class airman medical certificate, dated October 2, 2009, contained the restriction: “Must wear corrective lenses and possess glasses for near and intermediate vision.” According to the operator, the pilot had accrued approximately 220 helicopter flight hours, all of which were in the Robinson R22. In the past 90 and 30 days, he had logged 15 hours and 3 hours, respectively. His last flight review was dated August 27, 2010. AIRCRAFT INFORMATION N7779M (serial number 1229), a model R22 BETA, was manufactured by the Robinson Helicopter Corporation in 1989. It was powered by a Lycoming O-320-B2C engine (serial number L-15943-39A), rated at 131 horsepower. The helicopter had been operated in Japan, and was purchased by the owner in 1999. According to the maintenance records, the engine was overhauled on October 14, 2010. In addition, the helicopter underwent a 2,200-hour overhaul on December 30, 2010. Both overhauls occurred at an airframe and tachometer total time of 2,294.8 hours. During the airframe overhaul, the following components were replaced with new or factory-overhauled components: main rotor gearbox, swashplate, gearset, hub, driveshaft, clutch shaft, bearing assembly, and sprag clutch; tail rotor blades, hub, gearbox, pitch control, actuator assembly, and drive shaft; tailcone assembly, horizontal stabilizer, and welded frame. At the conclusion of the airframe and engine overhauls, an annual inspection was accomplished at which time the main rotor blades were replaced as suggested by the manufacturer’s service bulletin. METEOROLOGICAL INFORMATION The following weather observation was recorded at Austin-Bergstrom International Airport (KAUS), Austin, Texas, at 1153: Wind, 190 degrees at 12 knots, gusting to 19 knots; visibility, 10 statute miles; sky condition, 3,600 feet scattered; temperature, 34 degrees C.; dew point, 22 degrees C.; altimeter, 29.93 inches of Mercury. WRECKAGE AND IMPACT INFORMATION The helicopter impacted trees and terrain on the south side of the Colorado River. The burnt wreckage was at the base of a large tree. A piece of tail boom skin was in the tree. One main rotor blade was found approximately 780 feet from the wreckage. The other main rotor blade was found approximately 975 feet from the wreckage in the opposite direction. Both blades were intact with no leading edge damage. Because the witnesses said they had seen something fall from the helicopter, the river was searched and dragged by the Texas Department of Public Safety. Nothing was found or recovered. The head of one bolt that attached one of the main rotor blades to the hub, and the nut and shank portion of the other bolt that attached the second main rotor blade were recovered. Missing was the head of the second bolt and the nut and shank portion of the first bolt. Effectively, only parts from two different bolts were recovered. Examination of the remainder of the wreckage revealed no other missing components. MEDICAL AND PATHOLOGICAL INFORMATION Autopsy and toxicology protocols were conducted by the Travis County Medical Examiner’s Office (MEO). According to the autopsy report, death was attributed to “blunt force injuries.” No drugs or ethanol were detected The Federal Aviation Administration’s (FAA) Civil Aerospace Medical Institute (CAMI) also conducted a toxicology screen. Carbon monoxide and cyanide tests were not performed. CAMI’s report corroborated the findings contained in the Travis County Medical Examiner’s report. TESTS AND RESEARCH The inboard portions of both rotor blades, the rotor hub, rotor grips, pitch linkages, swash plate, and transmission were sent to NTSB’s metallurgical laboratory in Washington, DC, for examination. The main rotor blades were later tested for blood and bird residue with negative results. The following is based on the Materials Laboratory Factual Report: THE HUB. The hub bore similar fractures on both sides. The fracture faces revealed a coarse grainy surface and surface lines consistent with overload. Mechanical damaged was consistent with being impacted by the arced upper edge of the tusk. Both lower edges adjacent to the fracture faces were curved outward. Examination of the droop stops revealed indentations from the edges of the tusk and displaced material. The teeter stops revealed a lateral fracture and a rounded indentation below the fracture. The tusk was bent inwardly and there was a contact mark made when the spindle contacted the teeter stop. The fracture face, which was contained within the hole in the spindle, displayed a uniform texture with surface lines. There was a crescent shaped gap between the coning bolt and the hole in the spindle, consistent with a shear overload. The fracture face on the shank was also consistent with shear overload. Both pitch arm 1 and 2 displayed fracture faces that displayed a grainy surface, consistent with overload. MAIN ROTOR BLADES, CYCLIC YOKE, BELLCRANK AND SHAFT. The cyclic yoke and bellcrank assembly was mechanically and fire damaged, and four of the five rod ends had portions of their push-pull tube assemblies still attached. The two yoke bolts were bent with the forward bolt restricting the movement of the attached bellcrank. Fire damage and mechanical damage had obliterated most of the finer fracture features on the tube portions remaining on four of the five rod ends, but the remaining larger features displayed shear lips consistent with overload. One rod end displayed a fractured thread with a distinct bend. A portion of the drive displayed a cut end, a fractured end, and circumferential scratches that, in some areas, had removed the paint to expose the metal. The fractured end was compressed with local inward deformations, and the fracture face displayed shear lips consistent with overload. A portion of push-pull tube displayed a cut end and a fractured end. The fractured end was bent and deformed to form a rough oval, with the fracture face displaying shear lips consistent with a bending overload event. MAIM MAST ASSEMBLY AND JACKSHAFT. One piece of the main mast assembly consisted of the main rotor gearbox with a major portion of the mast tube, and the other piece consisted of a minor portion of the mast tube with the swashplate assembly. The fractured end of a pitch link that originally mated to the rod end was still attached to the swashplate. Examination of the fracture face revealed a grainy surface consistent with an overload event. The collective jackshaft was corroded and portions of two control rods displayed white powdery surfaces, consistent with being exposed to fire. The bottom (or sump) of the main gearbox was cracked. The bolts attaching the fractured portions of the main rotor gearbox sump to the case appeared bent. Examination of the fracture faces revealed a uniform grainy surface, consistent with overload. The lower support bearing outer race bore distinct contact marks from the bearing’s rollers. Examination of the fracture face revealed a grainy surface consistent with an overload event. The arced outer edge of deformed material was centered on the fractured bolt, consistent with it being produced by the rod end originally at that location. PITCH LINK. The pitch link bore multiple bends and fractured bolts in the rod ends at both ends. The pitch link was suitably wire-locked to the fitting, and the threaded portions displayed jam nuts and palnuts. MAIN ROTOR BLADES. One main rotor blade was bent downward bend and there was skin wrinkling. The other blade displayed a smooth rearward bend and skin wrinkling. The outer tip of the main rotor blade was approximately 11 inches long, and the outer 8 inches of the black coating on the leading edge was eroded. There was mechanical damage to the tip cover. The outer surface of the tip cover had been sheared inboard. The metallurgical report noted that the main rotor hub rotates counterclockwise about the teeter bolt when viewed from above. The main rotor spindles rotate independently about the coning bolts, and the main rotor blade hubs rotate about the spindles controlled by the pitch change links. As the main rotor slows down during engine shut-off, the main rotor blades droop until the tusk contacts the droop stop. The report concluded that the Robinson R22 helicopter, like other helicopters with a semi-rigid rotor system, is susceptible to mast bumping. According to Robinson Helicopters, “mast bumping” refers to teetering beyond the allowable teeter clearance in the hub, resulting in contact (indirect in this case) between the hub and the main rotor shaft. “Low-G mast bumping” is the phenomenon specific to 2 bladed teetering rotor systems in which the rotor disc becomes unloaded, normally due to a cyclic pushover or something similar, and it frequently leads to separation of the main rotor shaft. “Mast bumping” can occur during many situations. “Low rotor RPM rotor stall” is a different phenomenon, but “mast bumping” frequently occurs within the rotor head as part of the sequence. In this accident, there are damage signatures in the controls, main rotor head and main rotor blades that are consistent with both “Low-G mast bumping” and “low rotor RPM rotor stall.” ADDITIONAL INFORMATION Radar data in the form of NOP (National Offload Program) was obtained from KAUS’s approach control. The data depicted N7779M departing Taylor, Texas, about 1705:03 UTC (1205 CDT), and proceeding towards Luling. Altitude varied from 1,200 to 1,400 feet msl (above mean sea level) for most of the flight. Shortly after the accident had occurred, the helicopter operator flew the same course flown by the pilot of N7779M, and requested that KAUS approach control track him on radar. He wanted to see how low radar would detect him. Radar tracked him to the top of the tree at the impact site. The operator calculated that it would take less than the five seconds to descend from 1100 feet msl (the last known altitude of the helicopter) to ground impact (less than 400 feet msl). After he calculated ground speed and altitude, he computed the rate of decent to be over 7,000 feet per minute. The operator stated autorotations do not produce more than a 2,000 feet per minute rate of decent.
The divergence of the main rotor from its normal plane of rotation for reasons that cannot be determined because the pilot’s control inputs leading up to the event are unknown, resulting in mast bumping and main rotor blade separation.
Source: NTSB Aviation Accident Database
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