WPR14FA324

The pilot's loss of helicopter control due to settling with power while maneuvering for takeoff.

The helicopter was being used to transport passengers and equipment for an off-road automobile event in a remote mountainous area. The airline transport pilot picked up two passengers, and the subsequent 3-minute-long inbound flight and landing into the wind at the helispot were uneventful. The pilot reported that, after a third passenger boarded, he initiated a vertical climb to clear trees. He reported that he expected and felt a "bump" as the helicopter encountered a headwind above the trees, and he then began to initiate a left turn for a downwind departure. As the turn progressed, the helicopter rapidly descended back to the ground, and, unable to maintain altitude, the pilot attempted to ditch the helicopter into trees and down-sloping terrain. The helicopter struck a rock slab and rolled onto its side, during which time both the pilot and a passenger sustained serious injuries. Postaccident examination did not reveal any anomalies with the airframe or engine that would have precluded normal operation, and both the pilot and operator reported that there were no mechanical malfunctions or failures. The pilot attributed the loss of lift to a wind gust; however, the closest weather reporting station, about 14 miles away, reported light wind. Further, a weather simulation was performed to estimate wind and turbulence conditions in the area of the accident. The results revealed a low potential for turbulence with wind, which closely matched the conditions reported by the weather station, at the time and location of the accident. The fuel load could not be definitely established; however, given the helicopter's estimated gross weight, it was likely operating very close to or slightly above its hovering ceiling. Therefore, it is likely that the pilot attempted to hover the helicopter out of ground effect at an altitude above its hovering ceiling, which resulted in it settling with power.

The helicopter was recovered from the accident site, and examined by the IIC along with representatives from Turbomeca and Airbus Helicopters. Examination did not reveal any anomalies with the airframe or engine that would have precluded normal operation, and both the pilot and operator reported that there were no mechanical malfunctions or failures. The engine's axial compressor blades exhibited leading edge damage consistent with the engine producing power at the time of impact and ingesting debris as the helicopter came to rest on its side. Rotational damage was noted to components within the main and tailrotor transmission system, consistent with operation of the transmission components at the time of impact. A complete examination report is included in the public docket for this accident. Performance The pilot reported that he had serviced the helicopter with 40 gallons of fuel from the fuel truck while at Loon Lake, just prior to the accident flight. He estimated that this addition would have provided him with about 1.5 hours flight time, based on the helicopter consuming about 45 gallons per hour. Examination of the fueling records revealed that no entry had been made that day; however, the truck's fuel delivery meter indicated a metered amount of 81.1 gallons. The helicopter came to rest on its side following the accident, resulting in fuel leaking from the filler neck. Therefore and accurate assessment of the total amount of fuel onboard at the time of the accident could not be made. According to the helicopters most recent weight and balance chart from May 2010, the basic empty weight was 2,686 pounds. Utilizing the weight of the occupants, along with baggage and both 40 and 81 gallons of fuel, the helicopters gross weight at the time of the accident would have been about 3,850 or 4,096 pounds respectively. The limitations section of the helicopter's flight manual indicated a maximum gross weight of 4,300 pounds. Utilizing the flight manual located onboard the helicopter, the hover in ground effect (HIGE) and hover out of ground effect (HOGE) altitudes were calculated for both weights utilizing a temperature of 28 C. HIGE at a gross weight of 3,850 pounds would have been possible up to 8,500 ft pressure altitude, and up to 6,700 ft at a weight of 4,096 pounds. Likewise, HOGE at 3,850 pounds would have been possible up to 6,750 ft, and up to 4,800 ft at a weight of 4,096 pounds. Fuel Gauge An entry in the helicopters maintenance logbook indicated that 10 days prior to the accident the fuel gauge was replaced. The entry stated that the gauge was still inoperative following its replacement, and no other entry was located indicating the gauge was subsequently repaired. No inoperative placard was found in the helicopter following the accident, and the operator's technical representative stated that he could not confirm if the gauge was working at the time of the accident. The five subsequent entries in the logbook were signed by the accident pilot, and affirmed compliance with the pre-flight visual inspections of the main and tail rotor drive systems. The pilot stated that the gauge was operating at the time of the accident (it had been sticking prior to the repair), and that since its replacement he also confirmed fuel load between flights utilizing the sight gauge in the aft of the helicopter as a precaution. Vortex Ring State According to the FAA Rotorcraft Flying Handbook, "vortex ring state" (or "settling with power") describes an aerodynamic condition where a helicopter may be in a vertical (with regard to the air mass) descent with up to maximum engine power applied, and little or no cyclic authority. The term "settling with power" comes from the fact that a helicopter keeps settling, even though full engine power is applied. However, when the helicopter begins to descend vertically, it settles into its own downwash, which greatly enlarges the main rotor blade tip vortices. In this vortex ring state, most of the power developed by the engine is wasted in accelerating the air in a doughnut pattern around the rotor. A vortex ring state may be entered during any maneuver that places the main rotor in a condition of high upflow and low forward airspeed, including near-vertical descents of at least 300 feet per minute, and a horizontal velocity slower than that for effective translational lift. Vortex ring state susceptibility increases with increases in gross weight and density altitude. The handbook also noted that "when recovering from a settling with power condition, the tendency on the part of the pilot is to first try to stop the descent by increasing collective pitch. However, this only results in increasing the stalled area of the rotor, thus increasing the rate of descent. Recovery is accomplished by increasing forward speed, and/or partially lowering collective pitch." With sufficient altitude, temporary entrance into an autorotation will also enable safe exit from the vortex ring state. The wreckage was examined at the accident site by the NTSB investigator-in-charge (IIC). The accident site was located on the northwest shore of the lake, at an elevation of 6,445 ft mean sea level (msl) within a geographic bowl inside the confines of the Eldorado National Forest, 8 miles southwest of Tahoma. The primary wreckage consisted of the fuselage and tailboom, and was located on the lakeshore adjacent to the intersection of two concrete dam walls. Fragments of the main rotor blades were strewn around the site and into the adjoining lake. The farthest located component was a main rotor blade tip, which came to rest about 350 feet northeast. The remaining two blade tips were located in the immediate vicinity of the fuselage. All primary airframe components were accounted for at the accident site. The takeoff point was on a flat rock slab in between the two dam walls. Thirty- to forty-feet-tall trees surrounded the area 60 ft to the north and northwest. The primary wreckage was located about 170 feet south, and 10 feet below the takeoff point. A series of cut trees were in line with the departure path, and all were cut at a height approximately level with the takeoff point. The main cabin came to rest on its right side on a heading of 090 degrees magnetic at the base of a north-south oriented slab of rock. The rock exhibited multiple blue and silver paint transfer marks and scratches oriented on a heading of 140 degrees. The main landing gear had become fragmented and pushed aft, with crush damage and longitudinal scrape marks present on the lower left side of the nosecone, and the lower forward right section of the belly. The windshield was shattered, with fragments of Plexiglas and window gasket material strewn around the immediate vicinity. The mast fairing exhibited rotational score marks, and the ground around the hub had been excavated, in a manner consistent with the hub and mast continuing to spin while the airframe was on its side. The tailboom had separated at the junction frame with the main cabin, and was oriented on a north-south heading, about 8 feet to the north. A 12-inch-thick tree, severed about 4 feet from the ground, was located between the tailboom and the cabin. The tailboom was positioned just below a set of three trees, which were severed at the 8-foot-level. Five more trees were severed at the same level, 15 feet to the north. The engine remained attached to the airframe, and encased within the cowling. The air inlet screen was free of debris and obstruction. The gravel below the exhaust pipe exhibited dark discoloration and sooting emanating in a fan-shaped plume about 5 feet beyond the outlet, consistent with continued engine operation after the accident. The closest aviation weather observation station was located at Lake Tahoe Airport, which was about 14 miles southeast of the accident site, at a similar elevation. An aviation routine weather report (META