WPR12GA106

The pilot’s failure to maintain yaw control while hovering at high density altitude, which resulted in a loss of tail rotor effectiveness.

The single-main-rotor helicopter was dispatched on a search and rescue (SAR) mission in response to a snowmobile accident in remote, mountainous terrain. The helicopter was on lease to the local sheriff's office, and was being flown by the chief pilot of the company that owned it. Two SAR personnel were passengers. After an initial but fruitless search to locate the snowmobile victim, the helicopter located members of the snowmobile party. After a landing and brief discussion with the snowmobilers, they agreed to lead the helicopter to the accident site on their snowmobiles. The helicopter lifted off to follow the snowmobilers, flying about 100 to 200 feet above the trees. Because the helicopter was faster than the snowmobiles, the pilot stopped several times in a hover to allow the snowmobiles to catch up. During one hover, the pilot experienced a slight left yaw, which he believed he corrected. The helicopter then began to spin rapidly and descended into the trees. The sheriff's office dispatchers used a commercial flight following system to track the helicopter, but system difficulties prevented them from detecting the accident via that system. The injured pilot climbed a nearby hill, and notified the dispatchers of the accident via radio. The recovered wreckage was examined, and no evidence of any preimpact deficiencies or failures that would have prevented normal operation and continued flight was discovered. Data recovered from the engine control unit indicated that the engine operated normally until the accident, when parameter exceedances and torque spikes, consistent with main rotor blade strikes, were recorded. The directional control actuator (DCA), which was the hydro-mechanical unit used to control the tail rotor, was removed and sent to the helicopter manufacturer for testing and examination. The DCA passed the functional checks, and no evidence of any anomalies that would have prevented normal operation was detected. A damaged tree near a trail sign in the search area suggested the possibility that the tail rotor might have struck the tree and been damaged just before the accident. However, examination of the damage to the tree and the tail rotor did not support that hypothesis. Single-main-rotor helicopters are susceptible to a phenomenon known as "loss of tail rotor effectiveness" (LTE), which can occur at low airspeeds, and is a function of relative wind direction. The loss of control occurred in a hover, but the relative wind direction could not be determined. LTE is more likely at high density altitude and/or gross weight. The accident occurred at a density altitude of about 9,000 feet, and at a weight about 800 pounds below the maximum certificated operating weight. In the accident helicopter, LTE would result in a nose-right spin direction. Both the pilot and surviving passenger recalled that the spin direction was nose left, which is contrary to LTE. However, neither was certain of the spin direction, and a ground witness reported that the spin direction was nose right, consistent with LTE. Examination of the wreckage did not provide conclusive evidence as to the direction of the spin, but did indicate that both the main and tail rotor were rotating under power at the time of impact. Based on the witness observation and the uncertainty of the helicopter occupants about the direction of spin, the high density altitude, and the lack of indications of a tail rotor mechanical failure, it is likely that the loss of control was due to LTE.

HISTORY OF FLIGHT On February 15, 2012, about 1301 mountain standard time, a Bell Helicopter model 407, N407HL, was substantially damaged when it impacted trees and terrain in the Bridger Teton National Forest near Moran Junction, Wyoming, during a search and rescue (SAR) mission. The commercial pilot and one SAR crewmember received serious injuries, and the other SAR crewmember received fatal injuries. The public-use flight was operated by the Teton County Sheriff's Office (TCSO). Visual meteorological conditions prevailed, and no Federal Aviation Administration (FAA) flight plan was filed for the flight. The mission was in response to a reported snowmobile injury accident, in order to transport medical personnel to that scene. Initial attempts to locate that victim, based on trail network and geographic coordinate information, were unsuccessful. The helicopter began a search, and the crew observed two snowmobilers signaling to them in a meadow. The helicopter landed in the meadow, and a SAR member exited and spoke with the snowmobilers. They were from the victim's party, and they agreed to lead the helicopter on their snowmobiles to the accident site. The SAR member re-boarded the helicopter, the helicopter lifted off to follow the snowmobilers, flying about 100 to 200 feet above the trees. Since the helicopter was faster than the snowmobiles, the pilot stopped two or three times in a hover to allow the snowmobiles to catch up. On either the second or third hover, the pilot experienced a slight left yaw, which he believed he corrected. The helicopter then started "spinning rapidly" and descended into the trees. The pilot and the surviving SAR member both believed that the spin direction was nose left, but they were not certain. The SAR member in the rear seat remained conscious throughout the event, and extricated himself, despite an injured leg. He assisted the other SAR member and the pilot out of the helicopter after they became verbally responsive. The pilot carried a radio to the top of a nearby ridge and requested assistance. Reports varied as to when the first ground personnel who were originally responding to the snowmobile accident arrived at the helicopter, but the first TCSO personnel reached the helicopter about 1622. PERSONNEL INFORMATION Federal Aviation Administration (FAA) information indicated that the pilot held multiple certificates and ratings, including a commercial pilot certificate with a rotorcraft-helicopter rating. The pilot reported a total flight experience of about 22,250 hours, including about 4,500 hours in the accident helicopter make and model. His most recent FAA second-class medical certificate was issued in May 2011. He was the chief pilot for Hillsboro Aviation, Hillsboro, Oregon. AIRCRAFT INFORMATION According to FAA information, the helicopter was manufactured in 2008 and was equipped with a Rolls Royce 250-C47 series turboshaft engine. The helicopter was registered to Hillsboro Aviation, leased to TCSO, and operated by Hillsboro flight personnel. At the time of the accident, the helicopter had accumulated a total time in service of 828.6 hours. The operator cited the maximum allowable gross weight of the helicopter as 5,250 pounds, which agreed with the FAA type certificate value when the helicopter was equipped with kit 407-706-020. METEOROLOGICAL INFORMATION The 1251 automated weather observation for the departure location, Jackson Hole Airport (JAC), Jackson, Wyoming, located about 25 miles southwest of the accident site, included winds from 090 degrees at 4 knots, visibility 10 miles, few clouds at 4,500 feet, temperature -6 degrees C, dew point -9 degrees C, and an altimeter setting of 29.96 inches of mercury. Additional relevant meteorological information obtained from a variety of sources, indicated that the entire region, including the accident locale, was dominated by a high pressure region. This resulted in visual meteorological conditions (VMC) conditions, with no or high ceilings, and generally easterly winds of 10 knots or less. The density altitude was calculated to be between 8,500 and 9,300 feet. WRECKAGE AND IMPACT INFORMATION Site Information The accident site was in a partially forested area, with coordinates of 43.726 degrees N, 110.217 degrees W. Site elevation was approximately 9,550 feet above mean sea level (msl). The ground in the immediate vicinity of the wreckage was relatively flat but sloped, the terrain in the general locale of the accident site was uneven, and the overall region was mountainous wilderness. The entire region was snow-covered, and the ground in the immediate vicinity of the wreckage was under deep snow. Wreckage Overview The helicopter came to rest on its right side in a stand of evergreen trees approximately 80 -100 feet high. One of those trees bore a diagonal cut, which separated its top, about 50 feet above the ground. The tree had a diameter of approximately 1 foot at that location. Review of on-scene photographs revealed the following information. The fuselage (cockpit and cabin) was essentially intact, with minimal intrusions into, or compromises of, the occupiable volume. All cabin doors remained attached and operable, but damaged. Of the ten cabin transparencies, only the one for the aft left door was intact. All four main rotor blades (MRB) were fracture-separated from the rotor mast/hub. The tail boom was fracture-separated into several sections, but damage precluded determination of the spin direction. The empennage, which consisted of the aft vertical fin, tail rotor gearbox, and tail rotor, was fracture-separated from the tail boom. The two tail rotor blades remained attached to the tail rotor hub, which remained attached to the gearbox. The engine remained intact and attached to the airframe. There was no fire. Wreckage Examination The wreckage was recovered from the accident site by helicopter on February 24, 2012. Some minor sectioning and/or disassembly of the wreckage was necessary for recovery purposes. The wreckage was examined by investigators and manufacturers' representatives about 1 week after that. Several components or component elements were determined not to have been recovered with the wreckage. These included most afterbody portions of the MRBs, a short tip section of one MRB, and an approximate 30-inch segment of the tail boom (just forward of the tail rotor assembly). Inspection of the engine exterior did not reveal any obvious signs of damage from impact forces or internal engine failure. The engine mounting structure was intact and properly secured, and no evidence of oil or fuel leaks was observed in the engine bay or surrounding area. The N1 and N2 rotor systems rotated smoothly and quietly by hand, and N1 and N2 gear train continuity was confirmed. There was no impact damage, metal splatter or evidence of rubbing around the power turbine air shrouds. Inspection of the inlet plenum chamber did not reveal any evidence of foreign material or missing hardware. Inspection of the compressor inlet revealed no visible damage or signs of foreign object ingestion. Drive continuity from the engine to the main rotor and tail rotor drive was verified. The Engine Control Unit (ECU) was visually undamaged. Extraction of the ECU Incident Recorder (IR) data revealed a total of 15 data-points, recorded at 1.2 second intervals. In addition to IR data, eight additional "Snapshot" data lines were recorded in a separate file. Snapshot data is recorded immediately following an engine exceedance. All snapshot data lines were recorded within the final 3 seconds of the accident flight. Examination of ECU IR data recorded during the accident sequence revealed the engine responding normally to control inputs. Physical examination of the available wreckage did not reveal evidence of any pre-impact deficiencies or failures that would have prevented normal operation and cont