WOODBRIDGE, NJ, USA
N4NJ
Bell 206L-3
The pilot took off into the wind to conduct a high orbit, and as the pilot 'gently and smoothly' started a right turn, an uncommanded right yaw, along with a 'high engine whine sound,' was experienced. At that time the pilot estimated the wind was at his 10 o'clock position. The pilot immediately applied left pedal to compensate for the right yaw, but it did not arrest the rotation. The collective was then lowered, and in an attempt to fly out of the ensuing condition, the helicopter was turned into the direction of rotation. After the helicopter made three complete 360-degree turns to the right, the PIC closed the throttle to idle, and performed an autorotation to a dry lakebed. FAA Advisory Circular 90-95 stated that '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.' The AC also advised of greater susceptibility for LTE in right turns and the phenomena may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots.
HISTORY OF FLIGHT On July 6, 1999, about 1645 Eastern Daylight Time, a Bell 206L-3, N4NJ, operated by the New Jersey State Police, was substantially damaged during a forced landing near Woodbridge, New Jersey. The certificated commercial pilot and the certificated private pilot received minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the aerial observation flight conducted under 14 CFR Part 91. The commercial pilot, who was pilot-in-command (PIC) for the flight, stated that the helicopter had been on a test flight for an engine replacement, prior to the accident flight. The pilot who flew the test flight reported no difficulties with the engine, and the helicopter was signed off as airworthy. About 1545, the PIC and private pilot were detailed to search for a missing police officer near the Raritan Industrial Center, along the Raritan River. The crew reported that the flight en route to the search area was uneventful. Once on scene, the crew conducted several orbits and hover work in an attempt to locate the missing officer. The missing officer, along with additional rescue personnel, was located in the middle of a swamp, concealed by high standing reeds, as the crew made a low slow pass over the area. The PIC continued to hover the helicopter over the scene, when he decided to take off into the wind and conduct a high orbit. During the takeoff, as the PIC "gently and smoothly" started a right turn, an uncommanded right yaw, along with a "high engine whine sound," was experienced. At that time the PIC estimated the wind was at his 10 o'clock position. The PIC immediately applied left pedal to compensate for the right yaw, but it did not arrest the rotation. The collective was then lowered, and in an attempt to fly out of the ensuing condition, the helicopter was turned into the direction of rotation. After the helicopter made three complete 360-degree turns to the right, the PIC closed the throttle to idle, and performed an autorotation to a dry lakebed. During the autorotation, the helicopter struck a tree and came to rest on the right side of the fuselage. A witness who was standing about 225 feet from the accident site stated that the helicopter was facing southwest and was about 75 feet in the air. The witness recalled the winds as being "light"; however, he could not recall the direction from which they originated. He noticed the helicopter's tail start to swing to the left and right, then proceed to drop straight down into a clump of bushes and small trees. The witness did not notice any smoke, flames, or unusual sounds. The accident occurred during the hours of daylight, approximately 40 degrees, 30 minutes north latitude, and 74 degrees, 18 minutes west longitude. PERSONNEL INFORMATION The PIC indicated that he had accumulated 2,244 total rotary wing flight hours, of which 2,050 hours were as PIC. The PIC also reported 500 hours in the Bell 206, of which, all were as the PIC. According to the Bell Helicopter Customer Training Academy records, on February 9, 1998, the PIC had successfully demonstrated simulated tail rotor failure procedures, during recurrency training that was completed on February 10, 1998. There was no record of any unanticipated right yaw training on the form provided by Bell, however, personnel at Bell Helicopter Training Academy stated that unanticipated right yaw is discussed during training and all pilots are encouraged to view videos pertaining to the matter. The private pilot indicated that he had accumulated 286 total rotary wing flight hours, of which 122 hours were as PIC. The private pilot also reported 223 hours of experience in the Bell 206, of which 100 hours were as PIC. According to the Bell Helicopter Customer Training Academy records, the private pilot had completed initial training May 14, 1999. On May 13, 1999, the private pilot successfully demonstrated tail rotor malfunction recovery procedures. There was no record of any unanticipated right yaw training. WRECKAGE AND IMPACT INFORMATION Examination of the accident site revealed cut tree branches, about 15 feet from the base of a tree. The helicopter was found next to the tree, on a heading of about 340 degrees magnetic. To the right of the helicopter, were reeds that extended from the ground to a height of about 12 feet. A circular swath, which extended from the fuselage downward at a 45-degree angle, was cut into the reeds. When the helicopter rotor blades were pulled downward and through, they matched the swath that was cut into the reeds. The tailboom was bent downward about 10 degrees, about 15 inches from the main fuselage. There was no evidence of either a main rotor or tail rotor impact with the tailboom, and the 90-degree gearbox was intact. The left and right horizontal stabilizers were not damaged. The low skid landing gear was attached to the airframe, and exhibited crushing, upward and outward. The fuel system was not ruptured. Further examination of the helicopter revealed no evidence of a pre-impact mechanical failure or malfunction. All major components were accounted for, and continuity of the flight control system was established. Both main rotor blades exhibited chordwise scoring along their entire lengths. One blade had an approximately 5-inch dent on the bottom trailing edge of the blade, about 4 feet from the blade attachment point. On the bottom of the other blade, about 4 feet from the blade attachment point, there was a deep dent with tree bark imbedded in it. METEOROLOGICAL INFORMATION The weather reported at an airport located 10 miles northeast of the accident site, at 1651 was, winds from 290 degrees at 15 knots, gusts to 24 knots, 10 statute miles of visibility, and a temperature of 100 degrees Fahrenheit. MEDICAL AND PATHOLOGICAL INFORMATION Toxicology specimens were not requested or performed. ADDITIONAL INFORMATION Advisory Circular 90-95 states, "LTE is a critical, low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control. 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. LTE is not necessarily the result of a control margin deficiency.... 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.... There is greater susceptibility for LTE in right turns.... The aircraft characteristics and relative wind azimuth regions are... Main rotor disc vortex interference (285 degrees to 315 degrees). (See figure 1.) Winds at velocities of about 10 to 30 knots from the left front will cause the main rotor vortex to be blown into the tail rotor by the relative wind. The effect of this main rotor disc vortex is to cause the tail rotor to operate in an extremely turbulent environment. During a right turn, the tail rotor will experience a reduction of thrust as it comes into the area of the main rotor disc vortex. The reduction in tail rotor thrust comes from the air flow changes experienced at the tail rotor as the main rotor disc vortex moves across the tail rotor disc. The effect of this main rotor disc vortex is to increase the angle of attack of the tail rotor blades (increase thrust). The increase in the angle of attack requires the pilot to add right pedal (reduce thrust) to maintain the same rate of turn. As the main rotor vortex passes the tail rotor, the tail rotor angle of attack is reduced. The reduction in the angle of attack causes a reduction in thrust and a right yaw acceleration begins. This acceleration can be surprising, since the pilot was previously adding right pedal to maintain the right turn rate. This thrust reduction will occur suddenly and, if uncorrected, will develop into an uncontrollable rapid rotation about the mast. When operating within this region, the pilot must be aware that the reduction in tail rotor thrust can happen quite suddenly and the pilot must be prepared to react quickly and counter that reduction with additional left pedal input. Review of the Bell Long Ranger III, model 206L-3 flight manual revealed no written procedures pertaining to loss of tail rotor effectiveness or unanticipated right yaw phenomenon, nor were they required. The Safety Board issued a recommendation to the Federal Aviation Administration (FAA) on September 26, 1994. Number A-94-140 recommended that the FAA: Strongly encourage the manufacturers of single main rotor/anti-torque rotor helicopters to include in the operator's handbook, and flight manual, discussions of the characteristics of, and recovery techniques from the phenomenon know as loss of tail rotor effectiveness (LTE). On April 4, 1995, the FAA sent a letter to all U.S. helicopter manufacturers & European aviation authorities asking them to include in the operator's handbook, and flight manual, a discussion of the characteristics of the phenomenon known as LTE, and appropriate recovery techniques. The wreckage was released to a representative of the New Jersey State Police on July 7, 1999.
The pilot's improper takeoff procedure, which resulted in a loss of tailrotor effectiveness.
Source: NTSB Aviation Accident Database
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