ERA19LA285

The pilot was operating the helicopter out of ground effect at an airspeed below effective translational lift while approaching the helipad for landing when the nose of the helicopter began a yaw to its right, which the pilot could not arrest with the application of left pedal. The yaw rate increased, and the helicopter rotated around the main rotor mast 2.5 times as it descended into terrain and vehicles. The pilot reported and a postaccident examination confirmed that there were no mechanical anomalies that would have precluded normal operation at the time of the accident. The helicopter's slow airspeed, its operation out of ground effect, the uncommanded right yaw, and the lack of mechanical anomalies are consistent with a loss of tail rotor effectiveness, which resulted in the pilot's loss of control.

On September 28, 2019, about 1930 eastern daylight time, an Enstrom F-28F helicopter, N380SH, was substantially damaged when it impacted terrain and vehicles during an approach to a helipad at the Bloomsburg Fair in Bloomsburg, Pennsylvania. The commercial pilot sustained serious injuries and the two passengers sustained minor injuries. The helicopter was operated by J&J Shop HeliAir LLC. under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91 as a local sightseeing flight. The pilot reported that the accident flight was the last helicopter "ride" the operator planned to fly, as this was the 10th and final day of fairgrounds flying for him. He reported that he completed a routine 3-minute flight around the fairgrounds and approached the paved helipad as he normally did. During the final approach, the helicopter "just yawed right" and he said he did "everything I could to recover, with left pedal," but the action "did not stop the yaw." He could not recall what the power settings were, or how high the helicopter was above ground when the loss of yaw control occurred. He also did not remember hearing or seeing an alarm in the cockpit. He said it was "just a standard approach" and "there was nothing unusual about it." The pilot reported there were no mechanical malfunctions or failures with the helicopter. Surveillance videos captured the final 19 seconds of the flight. The helicopter approached the helipad over several vehicles in the fairgrounds parking lot. The approach was shallow, and about the speed of a hover-taxi. The helicopter then momentarily climbed and simultaneously its forward momentum stopped and a right yaw (spin) began. The helicopter subsequently descended and completed two and a half rotations around the main rotor mast before impacting vehicles and terrain. The helicopter appeared about 20-25 ft above the terrain when the right yaw began. A windsock that was visible in the surveillance video was consistent with a calm wind. A post-accident examination and photographs provided by a Federal Aviation Administration (FAA) inspector revealed that the helicopter came to rest on its left side and the fuselage, tail boom, main and tail rotors sustained substantial damage. The passenger doors were not installed. Continuity for the cyclic and collective controls were confirmed. Examination of the engine did not reveal evidence of any catastrophic internal failures and no anomalies were observed with the spark plugs or magnetos. Weight and balance calculations based upon estimations of the pilot and passenger's weight, and 16 gallons of fuel, revealed the helicopter was likely within its center of gravity limits and about 220 lbs under its maximum allowable gross weight. The pilot's operating handbook stated the rotor diameter was 32 ft. Based upon nearby weather observations, the density altitude at the accident site was about 1,550 ft. The FAA issued Advisory Circular (AC) 90-95, Unanticipated Right Yaw inHelicopters, 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 8 of the AC stated in part: 8. OTHER FACTORS. The following factors can significantly influence the severity of the onset of LTE. (a) Gross Weight and Density Altitude. An increase in either of these factors will decrease the power margin between the maximum power available and the power required to hover. The pilot should conduct low-level, low-airspeed maneuvers with minimum weight. (b) 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. (c) Power Droop. A rapid power application may cause a transient power droop to occur. Any decrease in main rotor rpm will cause a corresponding decrease in tail rotor thrust. The pilot must anticipate this and apply additional left pedal to counter the main rotor torque. All power demands should be made as smoothly as possible to minimize the effect of the power droop. The FAA Helicopter Flying Handbook stated in part: In Ground Effect (IGE) Ground effect is the increased efficiency of the rotor disk caused by interference of the airflow when near the ground. The air pressure or density is increased, which acts to decrease the downward velocity of air. Ground effect permits relative wind to be more horizontal, lift vector to be more vertical, and induced drag to be reduced. These conditions allow the rotor disk to be more efficient. Maximum ground effect is achieved when hovering over smooth hard surfaces. When hovering over surfaces as tall grass, trees, bushes, rough terrain, and water, maximum ground effect is reduced. Rotor efficiency is increased by ground effect to a height of about one rotor diameter (measured from the ground to the rotor disk) for most helicopters.