Woodsboro, TX, USA
N49746
BELL 206B
The pilot was conducting an aerial pipeline observation flight when the helicopter suddenly yawed to the right and he received a low rotor rpm warning. The pilot applied left pedal to correct the yaw; however, the helicopter continued its right turn, impacted the ground, and rolled onto its side, resulting in substantial damage. Examination of the wreckage and testing of components did not reveal any preimapct anomalies that would have precluded normal helicopter operation. Nonvolatile memory obtained from the helicopter indicated that its groundspeed varied during the last 10 seconds of recorded data to a low of about 9 knots, increasing to about 25 knots, then decreasing to about 12 knots. During this time period, the helicopter's altitude decreased to about 50 feet above ground level (agl). The last recorded point showed the helicopter about 30 feet agl at a groundspeed of about 15 knots on a northwesterly heading. Weather observations from a nearby airport indicated variable wind from the south at 10 knots with gusts to 21 knots about the time of the accident. The helicopter's orientation to the prevailing wind and its slow airspeed made the helicopter susceptible to a loss of tail rotor effectiveness (LTE). Additionally, the helicopter's low altitude at the time of the LTE encounter was inadequate to allow for recovery before its impact with terrain.
HISTORY OF FLIGHTOn October 2, 2014, about 1255 central daylight time, a Bell 206B helicopter, N49746, had a hard landing following a loss of tail rotor effectiveness near Woodsboro, Texas. The airline transport rated pilot and two passengers received serious injuries. The helicopter sustained substantial damage during a subsequent roll over. The helicopter was registered to and operated by an individual doing business as Heartland Helicopters under the provisions of Code of Federal Regulations Part 91 as an aerial observation flight. Day visual flight rules conditions prevailed for the flight, which did not operate on a flight plan. The flight originated from the Alfred C 'Bubba' Thomas Airport (T69), near Sinton, Texas, about 1200, and was destined for the Beeville Municipal Airport, near Beeville, Texas. According to the pilot's accident report, the purpose of the flight was a laser examination of a pipeline to detect methane gas. He indicated that he was flying the helicopter between 35 and 40 meters above ground level. The pilot reported that the helicopter suddenly yawed to the right without warning. He indicated that he did not recall "when a warning horn went off, but there was a warning horn." He recalled it as a low rotor rpm warning. The pilot stated that there were wires to his right side and building structures on his left. The pilot reported the helicopter yawed, he indicated he was sure he applied pressure to the left pedal, and a loss of tail rotor effectiveness occurred. The pilot said that the helicopter hesitated turning right for a "short" period of time and then continued the turn to the right. He intended to guide the helicopter away from the wires and structure. However, the helicopter continued one full revolution to the right. The pilot, in part, stated: It was at this point I had two thoughts. I need to reduce collective in order to maintain sufficient rotor speed to cushion the landing. I also did not roll off the throttle because I did not want to shut down a good engine if I had one. I did my best to land the aircraft in a level and stable manner. The aircraft impacted the ground. I do not remember bouncing, but we impacted and rolled to the right and ended up with the aircraft positioned on the right side. According to a passenger on the pipeline inspection flight, he heard another passenger guide the pilot to make a right turn as the helicopter approached a set of power lines. He felt the helicopter change "position" while making the turn. The passenger then "heard a change, decrease, in the engine RPM, followed almost immediately by a warning horn" and he "heard the pilot say that he had a low tail rotor warning." The helicopter started to spin and the he braced for impact. When asked to describe the sounds he heard, the passenger reported that the engine sounded "similar to that of a lawnmower, running at a normal speed and how it bogs down in tall weeds or grass. The engine bogging was immediately followed by the master warning horn." PERSONNEL INFORMATIONThe pilot, age 47, held a Federal Aviation Administration (FAA) airline transport pilot certificate with an airplane multi-engine land rating. He held commercial pilot airplane single-engine land, rotorcraft helicopter, and instrument helicopter privileges. He held a flight instructor certificate with airplane single-engine and rotorcraft helicopter ratings. He also held a flight engineer certificate with a turbojet powered airplane rating. He reported that he held a FAA second-class medical certificate dated August 15, 2014, with a limitation that he must wear corrective lenses. His most recent flight review was completed on May 20, 2014. The pilot's report indicated that he accumulated a total flight time of 13,916 hours, of which 2,376 hours were in rotorcraft and 35 hours were in the same make and model as the accident helicopter. AIRCRAFT INFORMATIONN49746 was a 1976 model Bell 206B helicopter with serial number 1928. A 420-shaft horsepower Rolls Royce 250-C20B engine with serial number CAE-834605 powered the helicopter. It was a five-place, single main rotor helicopter with a tail mounted anti-torque rotor. According to the pilot's accident report, the helicopter was maintained under a manufacturer's inspection program and its last inspection was a 100-hour inspection dated June 5, 2014. The helicopter accumulated 18,323.2 hours of total flight time at the time of the inspection. According to National Transportation Safety Board (NTSB) report FTW81DRA33, the helicopter was involved in a prior accident on May 19, 1981, when it sustained substantial damage during a takeoff accident in the Gulf of Mexico. The helicopter was fitted with a Pergam Aerial Laser Methane Assessment (ALMA) device. The Pergam website indicated the system is a laser based natural gas detection system. ALMA consists of an on-board laptop that manages the data acquisition and data processing, an on-board electronic laser-control system, a GPS receiver, and a helicopter belly-mounted optical unit. The optical unit is a refraction-based system, which consists of a laser, a reference channel, and three digital video recording cameras. The cameras are integrated into the ALMA system and indicate the location that is scanned by the system's laser beam. According to the website, it is simultaneously used to help the pilot position the helicopter over the object that needs inspection. A copy of a fuel receipt indicated the helicopter was serviced on October 2, 2014, with 60.1 gallons of Jet A fuel at T69. METEOROLOGICAL INFORMATIONAt 1353, the recorded weather about 17 miles and 122 degrees from the accident site at the Aransas County Airport, near Rockport, Texas, was: Wind 170 degrees at 10 knots, gusting to 21 knots with winds varying from 130 degrees to 190 degrees; visibility 10 statute miles; sky condition few clouds at 1,600 feet, scattered clouds at 2,400 feet, broken clouds at 2,900 feet; temperature 32 degrees C; dew point 26 degrees C; altimeter 29.86 inches of mercury. The field elevation at the airport was about 24 feet above mean sea level (MSL). AIRPORT INFORMATIONN49746 was a 1976 model Bell 206B helicopter with serial number 1928. A 420-shaft horsepower Rolls Royce 250-C20B engine with serial number CAE-834605 powered the helicopter. It was a five-place, single main rotor helicopter with a tail mounted anti-torque rotor. According to the pilot's accident report, the helicopter was maintained under a manufacturer's inspection program and its last inspection was a 100-hour inspection dated June 5, 2014. The helicopter accumulated 18,323.2 hours of total flight time at the time of the inspection. According to National Transportation Safety Board (NTSB) report FTW81DRA33, the helicopter was involved in a prior accident on May 19, 1981, when it sustained substantial damage during a takeoff accident in the Gulf of Mexico. The helicopter was fitted with a Pergam Aerial Laser Methane Assessment (ALMA) device. The Pergam website indicated the system is a laser based natural gas detection system. ALMA consists of an on-board laptop that manages the data acquisition and data processing, an on-board electronic laser-control system, a GPS receiver, and a helicopter belly-mounted optical unit. The optical unit is a refraction-based system, which consists of a laser, a reference channel, and three digital video recording cameras. The cameras are integrated into the ALMA system and indicate the location that is scanned by the system's laser beam. According to the website, it is simultaneously used to help the pilot position the helicopter over the object that needs inspection. A copy of a fuel receipt indicated the helicopter was serviced on October 2, 2014, with 60.1 gallons of Jet A fuel at T69. WRECKAGE AND IMPACT INFORMATIONThe helicopter impacted a field near Woodsboro, Texas, and was relocated to a local salvage yard. The NTSB investigator in charge (IIC) examined the wreckage at the salvage yard with the engine manufacturer. The observed damage and deformation was consistent with the pilot's report of the helicopter coming to rest on its right side. The helicopter's mast exhibited a separation, which was consistent with overload. The main rotor blades exhibited damage consistent with impact with terrain. The helicopter's transmission rotated when manipulated by hand. The tailboom was separated from the helicopter fuselage and the tail rotor driveshaft inside exhibited a separation consistent with being cut by recovery personnel. The tailboom separation exhibited deformation consistent with its contact with terrain during a helicopter rollover to the right. The tail rotor blades were intact on the aft portion of the tailboom. The skids exhibited deformation and separations consistent with overload. There was a tear observed in the lower portion of the fuel cell. Both of the cell's fuel boost pumps were intact and they pumped fuel when electrical power was applied to them. Airframe filters were opened and no anomalies were observed within them. The tail rotor and main rotor systems moved when the turbine blades inside the exhaust were rotated by hand. No airframe pre-impact anomalies were observed. The engine did not exhibit any exterior damages and it was separated from the airframe for a teardown examination. The engine was examined under supervision of the IIC at the engine manufacturer. Disassembly did not reveal any anomalies that would have precluded engine operation. The engine's fuel control and governor were hand carried by the IIC for testing at their manufacturer. ADDITIONAL INFORMATIONFAA Advisory Circular 90-95 - Unanticipated Right Yaw in Helicopters and the Helicopter Flying Handbook describe the phenomenon of loss of tail rotor effectiveness (LTE). The handbook, in part, stated: LTE or an unanticipated yaw is defined as an uncommanded, rapid yaw towards the advancing blade which does not subside of its own accord. It can result in the loss of the aircraft if left unchecked. It is very important for pilots to understand that LTE is caused by an aerodynamic interaction between the main rotor and tail rotor and not caused from a mechanical failure. Some helicopter types are more likely to encounter LTE due to the normal certification thrust produced by having a tail rotor that, although meeting certification standards, is not always able to produce the additional thrust demanded by the pilot. ... LTE is an aerodynamic condition and is the result of a control margin deficiency in the tail rotor. It can affect all single rotor helicopters that utilize a tail rotor of some design. The design of main and tail rotor blades and the tail boom assembly can affect the characteristics and susceptibility of LTE but will not nullify the phenomenon entirely. Translational lift is obtained by any amount of clean air through the main rotor system. Chapter 3 discusses translational lift with respect to the main rotor blade, explaining that the more clean air there is going through the rotor system, the more efficient it becomes. The same holds true for the tail rotor. As the tail rotor works in less turbulent air, it reaches a point of translational thrust. At this point, the tail rotor becomes aerodynamically efficient and the improved efficiency produces more antitorque thrust. The pilot can determine when the tail rotor has reached translational thrust. As more antitorque thrust is produced, the nose of the helicopter yaws to the left (opposite direction of the tail rotor thrust), forcing the pilot to correct with right pedal application (actually decreasing the left pedal). This, in turn, decreases the [angle of attack] AOA in the tail rotor blades. Pilots should be aware of the characteristics of the helicopter they fly and be particularly aware of the amount of tail rotor pedal typically required for different flight conditions. LTE is a condition that occurs when the flow of air through a tail rotor is altered in some way, either by altering the angle or speed at which the air passes through the rotating blades of the tail rotor system. An effective tail rotor relies on a stable and relatively undisturbed airflow in order to provide a steady and constant antitorque reaction as discussed in the previous paragraph. The pitch and angle of attack of the individual blades will determine the thrust output of the tail rotor. A change to any of these alters the amount of thrust generated. A pilot's yaw pedal input affects a thrust reaction from the tail rotor. Altering the amount of thrust delivered for the same yaw input creates an imbalance. Taking this imbalance to the extreme will result in the loss of effective control in the yawing plane, and LTE will occur. This alteration of tail rotor thrust can be affected by numerous external factors. The main factors contributing to LTE are: 1. Airflow and downdraft generated by the main rotor blades interfering with the airflow entering the tail rotor assembly. 2. Main blade vortices developed at the main blade tips entering the tail rotor. 3. Turbulence and other natural phenomena affecting the airflow surrounding the tail rotor. 4. A high power setting, hence large main rotor pitch angle, induces considerable main rotor blade downwash and hence more turbulence than when the helicopter is in a low power condition. 5. A slow forward airspeed, typically at speeds where translational lift and translational thrust are in the process of change and airflow around the tail rotor will vary in direction and speed. 6. The airflow relative to the helicopter; a. Worst case—relative wind within ±15 degrees of the 10 o'clock position, generating vortices that can blow directly into the tail rotor. This is dictated by the characteristics of the helicopters aerodynamics of tailboom position, tailrotor size and position relative to the main rotor and vertical stabilizer, size and shape. b. Weathercock stability—tailwinds from 120 degrees to 240 degrees, such as left crosswinds, causing high pilot workload. c. Tail rotor vortex ring state (210 degrees to 330 degrees). Winds within this region will result in the development of the vortex ring state of the tail rotor. 7. Combinations (a, b, c) of these factors in a particular situation can easily require more anti-torque than the helicopter can generate and in a particular environment LTE can be the result. Certain flight activities lend themselves to being more at high risk to LTE than others. For example, power line and pipeline patrol sectors, low speed aerial filming/photography as well as in the Police and Helicopter Emergency Medical Services (EMS) environments can find themselves in low and slow situations over geographical areas where the exact wind speed and direction are hard to determine. Unfortunately, the aerodynamic conditions that a helicopter is susceptible to are not explainable in black and white terms. LTE is no exception. There are a number of contributing factors but what is more important to understanding LTE are taking the contributing factors and couple them with situations that should be avoided. Whenever possible, pilots should learn to avoid the following combinations: 1. Low and slow flight outside of ground effect. 2. Winds from ±15 degrees of the 10 o'clock position and probably on around to 5 o'clock position 3. Tailwinds that may alter the onset of translational lift and translational thrust hence induce high power demands and demand more anti-torque (left pedal) than the tail rotor can produce. 4. Low speed downwind turns. 5. Large changes of power at low airspeeds. 6. Low speed flight in the proximity of physical obstructions that may alter a smooth airflow to both the main rotor and tail rotor. Pilots who put themselves in situations where the combinations above occur should know that they are likely to encounter LTE. The key is to not put the helicopter in a compromising condition but if it does happen being educated enough to recognize the onset of LTE and be prepared to quickly react to it before the helicopter cannot be controlled. Early detection of LTE followed by the immediate flight control application of corrective action; applying forward cycli
The pilot’s failure to recognize and correct for flight conditions conducive to a loss of tail rotor effectiveness, which resulted in a rapid, uncommanded right yaw and subsequent hard landing.
Source: NTSB Aviation Accident Database
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