Albany, KY, USA
N108AE
BELL 206L1
The pilot flew the helicopter at a steep approach angle to clear obstacles. At 30 to 35 feet above the ground, in a slow forward hover, the helicopter crossed over the approach end of the landing zone. It then started an uncommanded turn to the right that the pilot could not arrest with left pedal input. The pilot attempted to go around, but the helicopter kept rotating to the right, and the pilot's ability to reduce power was limited due to obstacles. The pilot also attempted to "weather vane" the helicopter; however, it would not align with the wind, and the rate of rotation increased. With the helicopter continuing to spin to the right, the pilot maneuvered it over the landing zone, closed the throttle, arrested the spin, and completed an autorotation. A tail strike occurred during the landing flare. According to the pilot's diagram of the accident scene, winds were about 50 degrees left of helicopter centerline during the approach. Winds reported at area airports were approximately 90-100 degrees left of helicopter centerline during the approach, at 11 to 15 knots, gusting to 18 knots. Unanticipated yaw can occur in the direction of main rotor torque reaction. Contributors to unanticipated yaw include: relative wind blowing towards the tail rotor thrust outflow, the main rotor tip vortex being directed into the tail rotor, and additional torque demand produced by power increases when airspeed drops below translational lift. The most effective way to avoid unanticipated yaw is to avoid the relative wind contributors.
On November 8, 2002, about 1345 eastern standard time, a Bell 206L1, N108AE, operating as an Air Evac Lifeteam flight, was substantially damaged during an emergency landing at an unimproved site near Albany, Kentucky. The certificated commercial pilot was not injured. The helicopter departed Air Evac Base #14, Albany, Kentucky, at 1325. Visual meteorological conditions prevailed, and no flight plan was filed for the medical evacuation flight, conducted under 14 CFR Part 91. The pilot provided a written statement, and a diagram that depicted the landing zone and flight path. According to the pilot's statement, he flew over the landing zone, then entered a right-hand traffic pattern for landing. He approached the landing zone on a heading of 340 degrees magnetic, and flew at a steep approach angle to clear obstacles. The helicopter was 30 to 35 feet above the ground, in a slow forward hover, when it crossed over the approach end of the landing zone. It then began an uncommanded turn to the right that could not be arrested with left pedal input, so the pilot attempted to go around. The helicopter continued to rotate to the right as the pilot initiated the go-around, and he was unable to reduce power due to obstacles. Through two complete rotations to the right, the pilot attempted to "weather vane" the helicopter. However, it would not align with the wind, and the rate of rotation only increased. With the helicopter spinning to the right, the pilot maneuvered it over the landing zone, closed the throttle, arrested the spin, and completed an autorotation. During the landing flare, the tail rotor boom struck the ground. A paramedic at the scene stated that when he first noticed the helicopter, it was in a "right-turning hover." The helicopter started "turning faster," then banked, and spun towards the ground. The paramedic also stated that during the spin, the engine was "revving" and the tail rotor was turning "slower than normal." According to the pilot's diagram of the accident sequence, the helicopter approached the landing zone on a heading of about 290 degrees, then initiated a right-turning go-around on a heading of about 340 degrees. Winds were from about 240 degrees, or 310 degrees relative to the helicopter during the approach. Beyond the landing field, there were trees, a barn and a house. A review of the Bell 206 flight manual confirmed that, viewed from above, the helicopter's rotor blades rotated in a counter-clockwise direction. According to Federal Aviation Administration (FAA) Advisory Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters, unanticipated yaw, or loss of tail rotor effectiveness (LTE) is a "critical, low speed aerodynamic flight characteristic which can result in an uncommanded yaw rate which does not subside on its own and, if not corrected, can result in the loss of aircraft control." The Circular also noted that on U.S.-manufactured helicopters, the main rotor blades rotated in a counter-clockwise direction, and the torque produced would cause the fuselage to rotate in the opposite direction, or nose right. The Circular reported that, as a result of extensive flight and wind tunnel tests, four relative wind azimuth regions and certain helicopter flight characteristics were identified that could singularly or in combination create an LTE-conducive environment. The characteristics would be present only at airspeeds less than 30 knots, and applied to all single rotor helicopters: 1. Main rotor disc vortex interference (285 degrees to 315 degrees), in which wind velocities of about 10 - 30 knots cause the main rotor vortex to be blown into the tail rotor. During a right turn, a tail rotor would experience a reduction of thrust, as it encounters the main rotor disc vortex. A reduction in tail rotor angle of attack, and thus a reduction in thrust would occur, and a right yaw would begin. The acceleration could be surprising to the pilot, since he/she was previously adding right pedal to maintain a right turn. 2. Weathercock stability (120 degrees to 240 degrees), in which the relative wind would attempt to "weathervane" the nose of the helicopter due to the fuselage and vertical fin. The helicopter would make a slow uncommanded turn to the right or left depending on the wind direction. If a yaw rate became established, it could accelerate rapidly within the azimuth region. 3. Tail rotor vortex wind state (210 degrees to 330 degrees), in which tail rotor thrust would be limited by relative wind blowing toward the tail rotor thrust outflow. A left cross wind would result in a tail rotor vortex ring state to form, and cause a non-uniform, unsteady flow into the tail rotor. The net effect would be an oscillation of tail rotor thrust, which would require rapid and continuous pedal movements when hovering in a left crosswind. The characteristic is well known, and presents no significant problem unless action is delayed. If a right rate is allowed to build, the helicopter can rotate into the wind azimuth region where weathercock stability will then accelerate the turn. 4. Loss of translational lift (all azimuths), in which increased power demand and additional anti-torque are required. As airspeed is reduced, a helicopter could experience an accelerated right yaw rate as power demand is increased and the helicopter develops a higher sink rate. "Insufficient pilot attention to wind direction and velocity can [also] lead to an unexpected loss of translational lift." The Circular further stated that at airspeeds below translational lift, "the tail rotor is required to produce nearly 100 percent of the directional control. If the required amount of tail rotor thrust is not available for any reason, the aircraft will yaw to the right." In addition, the onset of LTE can be reduced, between 30 knots and a hover by avoiding tailwinds, avoiding out of ground effect hovers and high power demands, and "being especially aware of wind direction and velocity when hovering in winds of about 8-12 knots (especially OGE). Recommended Circular recovery techniques included the application of full left pedal and simultaneously moving the cyclic forward to increase airspeed. If altitude permitted, reduce power. Any large, rapid increase in collective to prevent ground or obstacle contact may further increase yaw rate. If the rotation cannot be stopped and ground contact is imminent, an autorotation may be the best course of action. "Avoiding LTE may best be accomplished by pilots being knowledgeable and avoiding conditions which are conducive to LTE. Appropriate and timely response is essential and critical." The accident was initially reported as an incident. However, the company director of maintenance examined the helicopter and discovered substantial damage to the tail boom due the tail strike. He also confirmed control continuity from the flight controls to the main and tail rotor systems, and noted that the helicopter was "over-torqued" during the accident sequence. The pilot reported 3,450 hours of flight experience, 270 hours of which were in make and model. The pilot also reported 45 hours of flight experience in the 90 days prior to the accident. The pilot held a commercial pilot's certificate with ratings for rotorcraft helicopter and instrument helicopter. His most recent Federal Aviation Administration second-class medical certificate was issued October 22, 2002. At 1353, winds reported at Crossville, Tennessee, 40 miles south of Albany, were from 200 degrees true at 11, gusting to 15 knots. At Bowling Green, Kentucky, 60 miles west of Albany, winds were from 190 degrees true at 15, gusting to 18 knots. At London, Kentucky, 50 miles east of Albany, winds were from 200 degrees true at 8 knots.
The pilot's inadequate in-flight planning, which resulted in an angle of approach in relative wind conditions conducive to a loss of helicopter tail rotor effectiveness. A factor was the obstacles surrounding the landing field.
Source: NTSB Aviation Accident Database
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