History of Flight
On July 11, 2020, at 0732 mountain daylight time, an Aerospatiale AS355F1 helicopter, registration N708AC, was destroyed in an accident near Rifle, Colorado. The pilot and passenger sustained serious injuries. The helicopter was operating under 14 Code of Federal Regulations Part 91 as an aerial observation flight.
According to the operator, the company's flight tracking program showed the helicopter departed Rifle Garfield County Airport (RIL) and flew about 100 feet above ground level toward a substation for low-altitude observation of transmission and power lines. The flight track ended near the accident location after a 3- to 4-minute flight roughly one mile southwest of RIL. Witnesses on a distant roadway reported seeing the helicopter "take a nose-dive and start to spin out of control. It did several spins…and hit the ground."
The helicopter passenger, employed by Chesapeake Bay Helicopters and experienced as a camera operator for about two years, reported that the pilot began a hover over the electrical substation while discussing the flight plan. While in a low-altitude hover, the helicopter began swaying left and right, then entered a counterclockwise (left) spin. Shortly after, the helicopter struck the terrain, and a postcrash fire consumed most of the helicopter. The operator stated the pilot had no recollection of the accident.
Personnel Information
The accident passenger stated he had flown with the pilot on previous flights and had no concerns or issues, and never felt unsafe. Another Chesapeake Bay Helicopters employee, who served as an aerial observation passenger on recent flights with the same pilot, reported experiencing incidents that made her feel unsafe. After those incidents, she requested to be removed from the project with that pilot or to have the pilot replaced. She elected to stop flying on the project and was replaced by the passenger involved in the accident.
Aircraft Information
According to the operator, the helicopter's gross weight at the time of the accident was 5,040 pounds.
Meteorological Information
The calculated density altitude at the accident location was 6,566 feet mean sea level (msl).
Wreckage and Impact Information
The helicopter came to rest in rocky terrain at an elevation of about 5,500 feet msl. The vertical stabilizer and tail rotor assembly were found adjacent to the main wreckage and showed no thermal damage. One tail rotor blade was separated near the hub, and another blade was fractured and splintered. There was no evidence that the helicopter contacted any power or transmission lines before impact.
Postcrash fire consumed most of the fuselage and cockpit. Continuity of the flight control system and main rotor drive system could not be determined due to fire damage. Two of the three main rotor blades were separated at the mast head, and one blade was partially separated; the separation fractures exhibited fraying and broomstraw signatures consistent with high rotational energy.
Examination of both engines revealed no evidence of failures that would have precluded normal operation. Both engines contained dirt and debris throughout the gas path, consistent with operating after ground impact.
Additional Information
The Federal Aviation Administration (FAA) Advisory Circular 90-95 describes loss of tail rotor effectiveness (LTE) as a critical, low-speed aerodynamic flight characteristic that can result in an uncommanded rapid yaw rate, which does not subside on its own and, if uncorrected, can lead to loss of aircraft control. The FAA Helicopter Flying Handbook (FAA-H-8083-21B) defines LTE as occurring when airflow through the tail rotor is altered in angle or speed. It lists contributing factors such as main rotor downwash, blade vortices, turbulence, high power settings, slow forward airspeed, and specific relative wind directions.
Eurocopter service letter no. 1673-67-04, published February 4, 2005, noted several instances of loss of yaw axis control where pilot pedal input was insufficient to stop rotation. It stated that in those cases, given altitude and weight conditions, tail rotors were far from their maximum performance limits. This service letter was superseded by Airbus Helicopters Safety Information Notice 3297-S-00 (July 3, 2019) and Information Notice 3539-I-00 (September 4, 2020), which discuss detection and recommended response to unanticipated yaw, emphasizing prompt and large opposite pedal input.