CEN13TA165

The pilot and two passengers were surveying a preplanned transect about 50 to 100 feet above ground level with a 5- to 10-knot left crosswind and an indicated groundspeed of 30 to 35 knots. As the terrain began to rise, the pilot added power to clear a ridge. The pilot reported that, when the helicopter was about 100 feet from the top of the ridge, he heard a " slight pop" and that the helicopter began to yaw to the right. He added power to clear the ridgeline, which greatly increased the right yawing motion, and the helicopter began to spin. He stated that the helicopter crossed over the ridgeline backward and continued to spin before it contacted the ground and rolled over onto its left side. A passenger reported that, although the wind was about 10 knots when they started the survey, the wind speed increased when the helicopter reached the top of the ridge, and the pilot had to correct for it twice before the helicopter began to spin to the right. A postaccident examination of the helicopter revealed no preimpact mechanical malfunctions or failures that would have precluded normal operation. The helicopter was operating in an environment conducive to the loss of tail rotor effectiveness (LTE), and the unanticipated right yaw and subsequent spinning of the helicopter are consistent with LTE.

On February 14, 2013, at 1542 central standard time, a Bell OH-58A helicopter, N337PW, sustained substantial damage when it collided with terrain near Pine Springs, Texas. The airline transport rated pilot was not injured and the two passengers sustained minor injuries. The helicopter was operated by and registered to the Texas Parks and Wildlife Department, Austin, Texas. The flight originated from a remote fueling site in Cornudas, Texas, at 1525 and was destined for a remote area north of Pine Springs, Texas. Visual meteorological conditions prevailed and no flight plan was filed for the Public Use flight conducted under 14 Code of Federal Regulations Part 91. The pilot, along with a biologist and a wildlife survey technician, were performing a survey of mule deer and were flying along a pre-planned transect when the accident occurred. The pilot stated they were progressing along the route at 50-100 feet above ground level (agl) on a 045 degree heading, with a 5-10 knot left crosswind. He said the terrain began to rise gradually so he added power to about 75 percent torque. The pilot said the airspeed was 35 knots, with a GPS indicated ground speed of 30 knots. When the helicopter was about 100 feet from the top of the ridge line, the pilot said he heard a slight "pop", and the nose of the helicopter began to yaw to the right. He added power to clear the ridge line, which accelerated the turn and he was unable to correct the yawing motion with the anti-torque pedals. The helicopter continued to spin and crossed over the ridge line backwards. The pilot said that when he pulled the collective to clear the ridge line, it greatly increased the yawing motion. He said, "I told my passengers to hold on and brace themselves because the yawing motion was becoming more and more violent. By this point, the helicopter had made 4 to 5, 360-degree circles. As I continued toward the flatter terrain, it became apparent that I could not arrest the yawing motion. The aircraft contacted the ground, left skid first, in a level attitude and rolled onto the left side." According to the wildlife survey technician, who was seated in the right rear seat, they started the transect route at 1530. At that time the wind speed was about 10 knots. As the helicopter was coming up a ravine to cross over a ridge line, they were about 30 to 35 feet agl with a ground speed of about 30 to 35 knots. The technician said that when they reached the top of the ridge line, the wind "hit" the left side of the helicopter, and the nose of the helicopter turned into the wind. The pilot corrected for it and got the helicopter back on the transect heading. Then the wind pushed the helicopter a second time and the nose of the helicopter again turned into the wind. The technician said that when the pilot corrected for the wind a second time, the helicopter began to spin to the right. The technician said he did not hear "a pop" but did recall hearing a pulsating alarm from the cockpit forabout 5 to 6 seconds before it "went away." The technician did not recall how many times the helicopter spun before it impacted the ground and rolled over. The technician said he exited the helicopter from an open window. After he got out of the helicopter, he noted that it had become very windy, about 25 knots gusting to 35 knots. According to the biologist, who was seated in the front left seat, she stated that after flying over the steep face of the western edge of a mountain range and other steep ridges, the helicopter's tail came around towards the front when she felt a "sharp crack, or snap." The biologist said the helicopter spun clockwise at least three or four times and seemed to slow down and almost straighten up when it spun again even faster. She said the pilot was working really hard to gain control before the helicopter impacted the ground. All three occupants were able to exit the airplane and used a cell phone to call for help. Examination of the helicopter by the National Transportation Safety Board (NTSB) Investigator-in-Charge and the Chief Pilot of the Texas Parks and Wildlife flight department found no pre-accident mechanical deficiencies that would have precluded normal operation. The Federal Aviation Administration (FAA) issued Advisory Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters, in February 1995. The AC stated that the loss of tail rotor effectiveness (LTE) was 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 stated, "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 stated: "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 8 of the AC stated: "OTHER FACTORS...Low Indicated Airspeed. 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." Paragraph 9 of the AC stated: "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... (6) Stay vigilant to power and wind conditions."