Encino, TX, USA
N3948A
AMERICAN EUROCOPTER CORP AS350B3
The pilot reported that he descended the helicopter from 800 to 150 feet above ground level and then hovered on a southerly heading. The helicopter began to drift to the southeast, so the pilot repositioned the helicopter by making a left climbing turn. The wind was from the southeast with gusts estimated by the flight crewmembers to be 15 to 20 knots. As the helicopter turned left, it experienced a change from a right quartering tailwind to a left quartering tailwind, a flight environment associated with the onset of loss of tail rotor effectiveness. According to the pilot, the helicopter shuddered and spun rapidly left. The pilot applied full right pedal and attempted to accelerate because he believed that he had regained control. However, the helicopter then continued to spin left and subsequently impacted trees and terrain. The helicopter sustained substantial damage to the fuselage, main rotor blades, and tail rotor. The pilot stated that he heard a sound similar to the low rotor rpm horn before impact. The pilot reported no preimpact anomalies with the helicopter. A postaccident examination of the helicopter and engine revealed no mechanical malfunctions or failures that would have precluded normal operation. The vehicle engine monitoring display (VEMD) recorded an overtorque event and a free turbine overspeed event consistent with the main rotor blades striking the trees and ground and the main gear box coupling separating on impact. The VEMD also recorded a gas generator overspeed event consistent with the engine producing power at the time of impact. The VEMD did not record any preimpact events that would have precluded normal engine operation. On the basis of the evidence, it is likely that the pilot failed to maintain directional control while maneuvering, which resulted in a loss of tail rotor effectiveness.
HISTORY OF FLIGHTOn January 10, 2014, about 0815 central standard time, a Eurocopter AS350B3 helicopter, N3948A, impacted trees and terrain after a loss of control near Encino, Texas. The pilot sustained serious injuries and the two crew members sustained minor injuries. The helicopter sustained substantial damage. The helicopter was registered and operated by the United States Customs and Border Protection (CBP), Washington, DC, under the provisions of 14 Code of Federal Regulations Part 91 as a public use flight. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The local flight originated from the McAllen International Airport (KMFE), McAllen, Texas about 0610. According to the pilot, the crew was conducting a law enforcement patrol flight to assist other CBP agents on the ground near Encino, Texas. He flew the helicopter in a left orbit at 800 feet above ground level (AGL). Eventually he descended to 150 feet AGL and hovered on a southerly heading. The helicopter began to drift to the southeast so the pilot repositioned by making a left climbing turn. During the turn, he "felt the wind catch the tail and a slight shimmy" before the helicopter spun rapidly to the left. He applied full right pedal and attempted to accelerate as he believed that he regained control. The helicopter continued the left spin and impacted trees and terrain. Prior to impact with the trees and ground the pilot heard a sound which he recognized as the "low rotor RPM" horn. He reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation. According to the two crew members, the wind was strong and gusting during the flight, and was significantly stronger as their altitude increased. The pilot expressed that he was concerned about the wind while maneuvering the helicopter. While the pilot was maneuvering the helicopter in the left-turn pattern about 50-125 feet AGL and felt the helicopter shudder. The helicopter made a quick left rotation and then descended to the ground. They reported the helicopter seemed to be operating normally at the time of the accident and no anomalies were noted. One crew member stated that after impact he secured the fuel cutoff, activated the rotor brake and turned off the electrical master switch. He exited the aircraft and noticed smoke coming from the engine compartment. A witness reported observing the helicopter 50-100 feet above the ground and about 25 feet above the tree tops. The helicopter made an abrupt maneuver and descended into the trees. The helicopter impacted tress and terrain and came to rest on its right side. PERSONNEL INFORMATIONThe pilot, age 28, held a commercial pilot's license for airplane single engine land, airplane multiengine land, and helicopter. He held an instrument rating for airplane and helicopter and was a certified flight instructor for airplane single engine land and airplane multiengine land. He was issued a first class medical certificate on October 23, 2013, with no limitations. As of the accident date, the pilot had accumulated 111 hours in the helicopter make and model; 100 hours were as pilot in command. Further review of the pilots training records revealed that he had not received additional formal mission training from the local branch. He received informal on-the-job training and completed 15 hours of enforcement mission time in a helicopter prior to assignment. Prior to the accident flight the pilot completed two scheduled days off. The pilot reported that his prior training in loss of tail rotor effectiveness (LTE) was helpful during the accident, but more emphasis could have been placed on LTE during training. The second flight crew member, a supplemental air crew member, was seated in the left front seat and was not manipulating the flight controls. The third flight crew member, an air crew rifle operator, was seated in the left rear seat. AIRCRAFT INFORMATIONThe six seat, single-engine helicopter, serial number 4948, was manufactured in 2010. It was powered by a 747-shaft horsepower Turbomeca Arriel 2B1 turboshaft engine. A review of the airframe and engine maintenance logbooks revealed that the airframe and engine both had a total time of 1,397 hours at the time of the accident. The most recent 100-hour inspection was completed on December 13, 2013 at a total time of 1,378.8 hours. The most recent 600-hour inspection was completed on October 8, 2013 at a total time of 1,180 hours. METEOROLOGICAL INFORMATIONAt 0815, the automated weather report for Brooks County Airport (KBKS), which was 10 miles north of the accident reported at: wind from 160 degrees at 10 knots gusting to 14 knots, visibility 10 miles, scattered clouds at 3,600 feet, temperature 19 degrees Celsius (C), dew point 17 degrees C, and altimeter setting 29.89 inches of mercury. The flight crew members reported that the wind in the area of the accident was strong and gusting during the flight. They both reported that the wind was significantly stronger as the helicopter's altitude increased and estimated the wind to be about 15 knots and gusting to 20 knots. AIRPORT INFORMATIONThe six seat, single-engine helicopter, serial number 4948, was manufactured in 2010. It was powered by a 747-shaft horsepower Turbomeca Arriel 2B1 turboshaft engine. A review of the airframe and engine maintenance logbooks revealed that the airframe and engine both had a total time of 1,397 hours at the time of the accident. The most recent 100-hour inspection was completed on December 13, 2013 at a total time of 1,378.8 hours. The most recent 600-hour inspection was completed on October 8, 2013 at a total time of 1,180 hours. WRECKAGE AND IMPACT INFORMATIONThe helicopter wreckage was located on a ranch about 50 miles north of KMFE. The fuselage came to rest on its right side and oriented on a magnetic heading of approximately 290 degrees. Wreckage debris from the helicopter was mostly localized in close proximity of the main wreckage. There was evidence of a postimpact fire which was extinguished by first responders. The helicopter's cabin structure was mostly intact; however, the forward windows were broken and the left rear door was separated. The forward nose, underside fuselage, and right skids were driven into the loose sand and dirt. The tail boom was bent towards the ground at the aft flex-coupling of the forward steel drive shaft. The transmission and main rotor system remained attached to each other and the transmission deck. All the transmission supports arms were broken. The 'Starflex' remained in the center of the rotor hub with two of the arms broken mid-span (red and yellow); one arm was separated from its thrust bearing. Damage to all three main rotor blades was consistent with leading edge ground impact which was perpendicular to the flight path. Two of the blades came to rest in a position that held the helicopter off of the ground. The tail-rotor gear box remained attached to the airframe. One of the tail rotor blades was damaged and separated mid-span with wood debris identified in the leading edge. The other blade was less damaged and both blades remained attached to the hub. The tail-rotor rotated freely when the blades were moved by hand. The tail rotor chip plug was inspected and appeared clean and the oil appeared fresh. The fuel tank remained undamaged and secured within the fuselage; no fuel was observed leaking from the helicopter nor smelled at the accident site. The transmission mounts were broken consistent with impact damage. The fuel cut-off, and rotor brake handles in the cockpit were both extended to the "OFF" position. The right horizontal stabilizer exhibited impact signatures and was separated mid-span. However, the stabilizer structure remained attached by the trailing edge skin. The right stabilizer also exhibited impact signatures consistent with a tree strike. Flight control continuity was established from the pilot's cyclic and collective controls to the top of the transmission deck where the push-pull rods connect to the rotor system. The anti-torque pedals were in the full right forward position and could not be moved in either direction due to impact damage. The flex cable from the pedals to the tail servo area was jammed at the forward pedals. The collective was found up, and the throttle was in the idle position. All the circuit breakers were found in. The instrument panel was relatively intact. Thermal damage was evident on the left and top sides of the engine cowling and the inlet plenum. The thermal damage patterns on the structure were similar to a postimpact fire in the final resting position. The oil, fuel, and air lines remained secured; however, the external accessories of the engine were damaged from the postimpact fire. The transmission shaft coupling ruptured. The gas generator could not be rotated by hand. The free turbine blades were shed with some blades located on the ground directly below the helicopter. Continuity was established through the reduction gearbox to the powershaft. The free-wheel unit operated normally. The reduction gearbox was removed and the alignment marks on the output pinion nut were found to be misaligned in the tightening direction approximately 1 ½ mm, which is consistent with power delivery during main rotor blade ground strikes. Data from the Digital Engine Control Unit (DECU) and the Vehicle Engine Monitoring Display (VEMD) were reviewed. The data revealed that all of the failures and over limits occurred within a few seconds and corresponded with the impact sequence. The VEMD showed an over torque event, and a free turbine over speed event. Also, there was a gas generator over speed event recorded. The VEMD did not indicate any preimpact failures that would have precluded a normal flight. ADDITIONAL INFORMATIONFAA Advisory Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters, dated February 26, 1995 states that the loss of tail rotor effectiveness (LTE) is a critical, low-speed aerodynamic flight characteristic which could result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, could result in the loss of aircraft control. It also states, "LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots." Paragraph 6 of the AC covered conditions under which LTE may occur. It states: "Any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur." Paragraph 9 of the AC states: "When maneuvering between hover and 30 knots: (1) Avoid tailwinds. If loss of translational lift occurs, it will result in an increased high power demand and an additional anti-torque requirement. (2) Avoid out of ground effect (OGE) hover and high power demand situations, such as low-speed downwind turns. (3) Be especially aware of wind direction and velocity when hovering in winds of about 8-12 knots (especially OGE). There are no strong indicators to the pilot of a reduction of translation lift. (4) Be aware that if considerable amount of left pedal is being maintained a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw. (5) Be alert to changing aircraft flight and wind conditions which may be experienced when flying along ridge lines and around buildings. (6) Stay vigilant to power and wind conditions."
The pilot’s failure to maintain directional control while maneuvering, which resulted in a loss of tail rotor effectiveness. Contributing to the accident were the pilot’s limited experience in the operating environment and lack of formal mission training.
Source: NTSB Aviation Accident Database
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