La Junta, CO, USA
N20Z
BELL HELICOPTER TEXTRON 206L-1
The purpose of the flight was to videotape a pipeline corridor. The commercial pilot reported that, while flying over mainly flat rural areas, the helicopter entered an uncommanded left yaw, returned to straight flight for about 1 to 2 seconds, and then entered an uncommanded right yaw that continued until the helicopter impacted the ground. GPS data from the camera used to videotape the pipeline corridor indicated that the helicopter's groundspeed was about 37 knots when it entered the uncommanded left yaw. Weather information from an airport about 5 miles away from the accident site indicated that the helicopter was flying with a tailwind of 11 knots gusting to 16 knots, which resulted in a calculated airspeed of between 22 and 26 knots. A postaccident examination of the airframe, engine, and control system revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. The helicopter was operating in a high-power, low-airspeed condition that required the pilot to be especially aware of wind direction to avoid tailwinds, which can result in a loss of control due to a loss of tail rotor effectiveness. Based on the available information, it is likely that the pilot's operation of the helicopter at a low airspeed with a gusting tailwind resulted in the loss of tail rotor effectiveness.
On May 7, 2013, about 1149 mountain daylight time, a Bell model 206L-1 helicopter, N20Z, sustained substantial damage when it impacted the ground near La Junta, Colorado, following an in-flight loss of tail rotor control. The pilot and two passengers were not injured. The helicopter's tail boom was separated and it sustained additional damage to the fuselage and main rotor system. The aircraft was registered to Helicopters Inc. and operated by Corporate Aircraft Services under the provisions of 14 Code of Federal Regulations Part 91 as an aerial observation flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from the La Junta Municipal Airport (LHX), La Junta, Colorado at 1135.The pilot reported that the purpose of the flight was videotaping of a pipeline corridor. He stated that the flight was conducted maintaining altitude between 50 and 100 feet above ground level at 45 to 55 kts ground speed. At no point was a hover commanded, and the flight plan called for forward flight at all times. The helicopter completed one flight and then landed at the LHX and re-fueled. In order to maintain weight and balance, 50 gallons of fuel was added to the helicopter. The helicopter departed LHX to continue the videotaping work until the next re-fueling point. The flight path was generally southwest over mainly flat rural areas with low vegetation. About 1200, the helicopter made an uncommanded yaw to the left for about 1-2 seconds and then returned to straight flight. The pilot reported that a passenger noticed the engine torque spike from 55-percent to about 115-percent. He stated that there was a distinct change in engine pitch as well. Following the uncommanded left yaw, the helicopter returned to straight flight for about 1-2 seconds and then began an uncommanded yaw to the right. The right yaw continued until the helicopter struck the ground. The pilot stated that when the right yaw began he treated it as a loss of tail rotor effectiveness (LTE) and lowered the collective, reduced throttle, and kept the helicopter as level as possible while providing cushion for the landing with collective. After the airplane touched down, it rolled onto its left side. The wreckage was moved to a maintenance facility where a postaccident examination of the helicopter was conducted. The fuselage of the helicopter was predominately intact with crushing damage from impact evident. The engine was intact and the rotating components could be spun freely with no evidence of an internal failure. The main rotor gearbox remained attached to its supporting structure atop the fuselage. The main rotor mast was bent where it exited the gearbox. Due to the bending of the main rotor mast, the swashplate control connections were broken. The breaks exhibited signatures consistent with postimpact overload failure. The remainder of the control system was examined and no preimpact defects were found. There were pilot controls installed in the right seat position, but no controls installed on the left. The tail boom was separated about 3 feet forward of the tail rotor gearbox. Another separation point was located about 4 feet further forward. The separation points were consistent with main rotor blade impact. The tail rotor driveshaft was separated at several locations, and the breaks were consistent with post-impact damage. The tail rotor gearbox rotated freely and no defects were noted. The tail rotor control system was examined and the pushrod exhibited breaks consistent with postimpact damage due to the main rotor blade striking the tail boom. Except for the identified breaks, the tail rotor control system functioned normally with no preimpact defects noted. The postaccident examination of the airframe, engine, and control system revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. At 1153, the weather conditions recorded at LHX were: Wind 320 degrees at 11 kts, gusting to 16 kts; visibility 10 statute miles; clear skies; temperature 24 degrees Celsius; dew point 0 degrees Celsius; altimeter setting 29.91 inches of mercury. The pilot provided a copy of the video that was being made when the accident occurred. The handheld video camera was reportedly mounted on a monopod and the camera extended out of a small window on the left side of the helicopter during video recording operations. The NTSB Recorder's Laboratory examined the video recording and found that the recording contained embedded GPS information and extracted it from the video. The recorded data started about 1135, when the helicopter was near Cheraw, Colorado, about 4 nm north of LHX, and ended about 1149, when the helicopter was about 6nm south-southwest of LHX. The data depicted a flight that progressed from the initial recorded point southward for about 8 nm before turning to a heading of about 153 degrees. The helicopter proceeded on this heading for about 1 nm before turning further east to a heading of about 126 degrees. Examination of the GPS information revealed that about one minute prior to the end of the recording, the helicopter was travelling at a ground speed of 37 kts, at 4,265 feet above mean sea level (msl), and on a true heading of 126 degrees. This location was about 5 nm and 160 degrees from LHX. Based on the recorded ground speed and the reported wind at LHX (320 degrees at 11 kts gusting to 16 kts), the helicopter's airspeed was calculated to be between 22 and 26 knots. About 30 seconds before the end of the recording, the helicopter was travelling at a ground speed of 31 kts, at 4,296 feet above mean sea level (msl), and on a true heading of 128 degrees. The calculated airspeed at this point was between 16 and 20 knots. Review of the video portion of the recording revealed that for the majority of the video, the helicopter conducted a series of ground observations through various terrains. The flight appeared uneventful until 1148:28, when audible noise consistent with wind was recorded and the camera appeared to jerk slightly at the onset of the noise. The sound of wind intensified as 1148:33, and continued intermittently until 1148:49, when the helicopter began an increasing yaw to the right. At 1148:54, the helicopter's yaw continued to increase and the camera operator retracted the camera into the cockpit. About a second later, the camera captured a view that showed the helicopter inverted. The helicopter continued to descend and impacted the ground at 1148:57. According to FAA-H-8083-21A, "Helicopter Flying Handbook", LTE, or unanticipated yaw is defined as an uncommanded, rapid yaw towards the advancing blade which does not subside of its own accord. The handbook also states: "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." The handbook went on to describe weathercock stability, which is affected by tailwinds from 120 degrees to 240 degrees clockwise from the nose of the helicopter. "In this region, the helicopter attempts to weathervane, or weathercock, its nose into the relative wind. Unless a resisting pedal input is made, the helicopter starts a slow, uncommanded turn either to the right or left, depending upon the wind direction. If the pilot allows a right yaw rate to develop and the tail of the helicopter moves into this region, the yaw rate can accelerate rapidly. In order to avoid the onset of LTE in this downwind condition, it is imperative to maintain positive control of the yaw rate and devote full attention to flying the helicopter." The handbook stated the following to reduce the onset of LTE: 1. Maintain maximum power-on rotor rpm. If the main rotor rpm is allowed to decrease, the antitorque thrust available is decreased proportionally. 2. Avoid tailwinds below airspeeds of 30 knots. If loss of translational lift occurs, it results in an increased power demand and additional antitorque pressures. 3. Avoid out of ground effect (OGE) operations and high power demand situations below airspeeds of 30 knots at low altitudes. 4. Be especially aware of wind direction and velocity when hovering in winds of about 8–12 knots. A loss of translational lift results in an unexpected high power demand and an increased antitorque requirement. 5. Be aware that if a considerable amount of left pedal is being maintained, a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw. 6. Be alert to changing wind conditions, which may be experienced when flying along ridge lines and around buildings. 7. Execute slow turns to the right which would limit the effects of rotating inertia, and the loading on the tailrotor to control yawing would be decreased.
The pilot's inadequate compensation for wind during low-altitude operations, which resulted in a loss of tail rotor effectiveness.
Source: NTSB Aviation Accident Database
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