N708ACAerospatiale AS 355F1 · 1982

On July 11, 2020, at 0732 mountain daylight time, an Aerospatiale AS355F1 helicopter, N708AC, was destroyed when it was involved in an accident near Rifle, Colorado. The pilot and passenger sustained serious injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight.

According to the operator, the company flight tracking program showed the helicopter departed the Rifle Garfield County Airport (RIL), Rifle, Colorado, and was en route about 100 ft above ground level to a substation to perform a low altitude aerial observation flight of transmission/power lines. The flight track ended near the accident location after a 3-4-minute flight about one mile southwest of RIL. Witnesses, located on a distant roadway, stated the helicopter “took a nose-dive and started to spin out of control. It did several spins…and hit the ground.”

The helicopter passenger, who was employed by Chesapeake Bay Helicopters and had performed aerial observation as a camera operator for about 2 years, reported that the pilot began a hover over the electrical substation while they discussed a plan for the flight. While in a low altitude hover, the helicopter began swaying left and right, then entered a spin in a counterclockwise (left) direction. Shortly thereafter, the helicopter impacted the terrain, and a postcrash fire ensued that consumed a majority of the helicopter. According to the operator, the pilot had no recollection of the accident.

The accident passenger stated that he had flown the with the accident pilot on past flights and had no concerns or issues, and he never felt unsafe.

Another Chesapeake Bay Helicopters employee, who was the aerial observation passenger on recent flights with the accident pilot, reported experiencing some incidents with the accident pilot that made her feel unsafe. After the incidents, the employee requested to a supervisor to be removed from the project with the accident pilot or would remain on the project if the accident pilot was replaced. The passenger elected to discontinue her future project flights and was replaced by the employee who was involved in the accident.

According to the operator, the gross weight of the helicopter at the time of the accident was 5,040 lbs.

The calculated density altitude was 6,566 ft mean sea level (msl).

According to the operator, the gross weight of the helicopter at the time of the accident was 5,040 lbs.

The helicopter came to rest in rocky terrain at an elevation of about 5,500 ft msl (See Figure 1. Main Wreckage). The vertical stabilizer and tail rotor assembly came to rest adjacent to the main wreckage and did not display any thermal damage. One tail rotor blade was separated near the hub, and one blade was fractured and splintered. There was no evidence the helicopter contacted any power or transmission lines prior to the ground impact.

Figure 1. Main Wreckage (photo provided by Beegles Aircraft Services)

Airframe Examination

Postcrash fire consumed a majority of the fuselage and cockpit. Continuity of the flight control system and main rotor drive system could not be determined due to postcrash fire damage. Two of the three main rotor blades were separated at the mast head, and one blade was partially separated. The separation fractures displayed fraying and broomstraw signatures consistent with high rotational energy.

Engine Examinations

Examination of the engines’ components revealed no evidence of failures which would have precluded normal engine operation. Both engines exhibited dirt and debris present throughout the gas path which was consistent with the engines operating after ground impact.

According to Federal Aviation Administration (FAA) Advisory Circular 90-95,

“LTE is 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 a loss of aircraft control.”

Information on this subject has been published by many organizations including the FAA and helicopter manufacturers.

The FAA Helicopter Flying Handbook (FAA-H-8083-21B) contains an in-depth discussion on LTE. Chapter 11, Helicopter Emergencies and Hazards, defines LTE as a condition that occurs when the flow of air through a tail rotor is altered in some way, by altering the angle or speed at which the air passes through the rotating blades of the tail rotor disk. It further states…

The main factors contributing to LTE are:

Airflow and downdraft generated by the main rotor blades interfering with the airflow entering the tail rotor assembly.

Main blade vortices developed at the main blade tips entering the tail rotor disk.

Turbulence and other natural phenomena affecting the airflow surrounding the tail rotor.

A high-power setting, hence large main rotor blade pitch angle, induces considerable main rotor downwash and hence more turbulence than when the helicopter is in a low power condition.

A slow forward airspeed, typically at speeds where translational lift and translational thrust are in the process of change and airflow around the tail rotor will vary in direction and speed.

The airflow relative to the helicopter;

Worst case – relative wind within ± 15° of the 10 o'clock position, generating vortices that can blow directly into the tail rotor. This is dictated by the characteristics of the helicopter's aerodynamics of tailboom position, tail rotor size and position relative to the main rotor and vertical stabilizer size and shape.

Weathercock stability – tailwinds from 120° to 240°, such as left crosswinds, causing high pilot workload.

Tail rotor vortex ring state (210° to 330°). Winds within this region will result in the development of the vortex ring state of the tail rotor.

Combinations of (a, b, c) of these factors in a particular situation can easily require more antitorque than the helicopter can generate and in a particular environment LTE can be the result.

…There are a number of contributing factors, but what is more important in preventing LTE is to note them, and then to associate them with situations that should be avoided. Whenever possible, pilots should learn to avoid the following combinations:

Low and slow out of ground effect.

Winds from ±15° of the 10 o'clock position and probably on around to 5 o'clock position

Tailwinds that may alter the onset of translational lift and translational thrust, and hence induce high power demands and demand more antitorque than the tail rotor can produce.

Low speed downwind turns.

Large changes of power at low airspeeds.

Low speed flight in the proximity of physical obstructions that may alter a smooth airflow to both the main rotor and tail rotor.

According to Eurocopter service letter no. 1673-67-04, published on February 4, 2005, several instances of a loss of yaw axis control occurred when “the [pilot’s] action applied to the [right] yaw pedal was not enough (amplitude/duration) to stop [left] rotation as quickly as the pilot wished.” In this situation, as the aircraft continues to rotate, the pilot may suspect a tail rotor failure and either climb or descend, which, respectively, can increase the leftward rotation, or cause the aircraft to tilt while rotating and subsequently contact the ground. In the cases mentioned, “given their altitude and weight conditions the tail rotors were far from their maximum performance limits.”

The Eurocopter service letter addresses helicopters with main rotors that rotate clockwise (FAA Handbook excerpts previous mentioned describe factors for counterclockwise rotating main rotor systems), though loss of yaw control can occur with any helicopter with a tail rotor. A loss of yaw axis control can also occur when the helicopter is in fact operated beyond its performance limits (due to loading or environmental conditions or extreme maneuvers), and in this case even the prompt application full right pedal might not be sufficient to counter a left rotation.

Eurocopter service letter no. 1673-67-04 was superseded by Airbus Helicopters Safety Information Notice 3297-S-00, issued on July 3, 2019, and Airbus Helicopters Information Notice 3539-I-00, issued on September 4, 2020, that discussed the detection and recommended response to unanticipated yaw, emphasizing a prompt reaction with large amplitude of opposite pedal input.