ERA21FA195

The impact of the main rotor blades with the tail boom during low-altitude maneuvers, which severed the tail rotor drive shaft and resulted in a loss of control and impact with terrain.

The helicopter departed and flew cross-country to begin a powerline patrol, which included flying adjacent to powerlines at an altitude of about 100 ft above ground level and with the helicopter gaining altitude during turns and when flying between line inspections. During the final minutes of flight, the helicopter was traveling at an airspeed between 50 and 60 kts along powerlines at an altitude between 700 and 775 ft above mean sea level (msl). As the helicopter flew south, near the end of the north-south powerline’s right of way, the helicopter began a climbing right turn towards the west. A utility lineman in the helicopter’s front left seat recalled that the pilot was reversing course and the helicopter was in a banking turn to the right when he heard a very loud noise, “almost cannon-like, very deep, within a second or two we were heading into the trees.” Onboard data recovered from the helicopter showed rotor speed suddenly increased to 107% about 1.5 seconds before the end of recorded data. Examination of the wreckage showed evidence the main rotor blades impacted the tail rotor drive system. Therefore, it is likely the loud noise heard by the onboard lineman was caused by the main rotor blades impacting the tail rotor drive system, resulting in a severance of the tail rotor drive shaft and subsequently a loss of directional control. Successful recovery of the helicopter after the loss directional control at a low altitude was unlikely and resulted in impact with terrain. Based on analysis of flight data and a scan of the local terrain, it is unlikely the helicopter impacted powerlines or nearby trees at the time the main rotor blades impacted the tail rotor drive system. The loss of clearance between the main rotor and the tail of the helicopter can be caused by sudden aft cyclic control inputs, usually in conjunction with a reduction in collective pitch that would reduce main rotor thrust and increase downward flapping of the blade. The lack of flight control parametric data precluded analysis of flight control inputs at the time the main rotor blade contacted the tail boom. However, it is likely the pilot was not maneuvering to avoid trees based on the increasing distance between the accident helicopter and treetops near the end of the accident flight. Examinations of the helicopter wreckage found no evidence of any preimpact anomalies or malfunctions that would have precluded normal operation of the helicopter.

On April 28, 2021, about 1324 eastern standard time, a Bell 429 helicopter, N53DE, was destroyed when it was involved in an accident near Eden, North Carolina. The commercial pilot was fatally injured, and two passengers were seriously injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight. The helicopter was equipped with a GPMS Foresight MX Health and Usage Monitoring System (HUMS), which captured data for the accident flight. The HUMS device recorded position, attitude, acceleration, rotor speed (Nr), and engine data. The HUMS data showed the helicopter departed Danville, Virginia, about 1233 and flew cross-country south-west into North Carolina. By 1245, the helicopter was flying a powerline patrol around Eden, North Carolina. The powerline inspection was accomplished at an altitude of about 100 ft above the terrain with the helicopter gaining altitude during turns and when flying between line inspections. During the final minutes of flight, the helicopter was traveling at an airspeed between 50 and 60 kts parallel to powerlines at an altitude between 700 and 775 ft msl (Figure 1). Nr was near 100%, torque varied between 15% and 25%, and the engine gas generator (N1) speeds were between 84 and 88%. Figure 1- Combined map of flight track with corresponding flight parameters. A utility lineman seated in the front left seat recalled that the pilot was reversing course and the helicopter was in a banking turn to the right when he heard a very loud noise, “almost cannon-like, very deep, [and] within a second or two we were heading into the trees.” Several ground witnesses nearby observed the accident flight. One witness driving westbound observed the helicopter cross the road heading southbound, flying low over the trees along the powerlines. The helicopter made a right turn before it disappeared behind trees. Two other witnesses observed the helicopter flying from east to west over the trees before making a steep “left turn” as seen from their vantage point; however, as the helicopter pointed toward the witnesses, it was in a right turn. The witnesses stated they could see the underside of the helicopter and skids before it “slid at an angle downward and disappeared into the woods.” The Bell 429 is a light, twin-engine helicopter. It was maintained by the operator under the manufacturer’s recommended inspection program. Log sheet 1071, which was found at the accident site and, although undated, was presumably from the day of the accident, showed that at the start of the day the helicopter had an airframe total time of 283.5 hours and both engines had an engine total time of 283.5 hours. There were abnormalities in the recovered HUMS data that led to the attitude and acceleration data being unreliable for use in this investigation. The accident helicopter was not equipped with, nor was it required to be equipped with a cockpit voice/flight data recorder (CV/FDR), which is an available option offered by Bell. The flight was also recorded by Automatic Dependent Surveillance-Broadcast (ADS-B) data provided by the Federal Aviation Administration (FAA). ADS-B broadcasts an aircraft’s Global Positioning System (GPS) position and other data to the ground where it is recorded. The helicopter impacted wooded terrain at an elevation of 570 ft and came to rest on a heading of about 251° magnetic and about 393 ft from the powerlines the crew was observing. All major components of the helicopter were accounted for at the accident site. A post-crash fire occurred and damaged the majority of the fuselage. The overall concentrated debris path was about 200 ft long on a 245° heading with several pieces scattered in multiple directions outside the main debris path. Remnant carbon fiber layup was present in the area of the upper cowlings, fuselage skin, and doors. The cockpit, cabin floor, and the transmission and engine deck were present but sustained heavy thermal damage. The avionics and wiring were strewn outside the nose section, with pieces of wood branches embedded within the wiring. The main rotor hub remained attached to the main rotor mast. The two yoke assemblies remained installed with the mast nut intact. The main rotor blades remained installed to their respective grips via blade pins. All blade attachment hardware was present and secured. All four blade tips exhibited impact damage and their spars exhibited a “broom straw” appearance. Three separated leading edge pieces near the tip end, including the tip cap lap joint, were found in the debris field (surrounding the main wreckage). All four main rotor blade pitch horns remained intact, and all four pitch change links were connected to their respective pitch horns and the rotating swashplate. The main rotor transmission was partially separated from the airframe. The tail boom had fractured into multiple pieces, the majority of which were found adjacent to and behind the fuselage. The tail rotor was hanging to one side of the gearbox due to fracturing of the output shaft and bending of the pitch control rod. The two tail rotor yokes remained installed on the tail rotor output shaft and were whole. All four tail rotor blades remained installed on the yokes. None of the tail rotor drive shaft covers remained installed on the tail boom. The horizontal stabilizer remained attached to a separated portion of the tail boom. The left horizontal stabilizer was generally whole with a puncture near the inboard side of its lower surface as well as a puncture near the outboard side of its upper surface. The left finlet remained attached with the lower portion intact and the upper portion fractured and partially separated. The right horizontal stabilizer was generally whole on its inboard end but was fractured at its outboard end. The lower portion of the right finlet was fractured and partially separated, remaining attached only by wiring. The upper portion of the right finlet was fractured and completely separated. The leading edge slats were present on the inboard portions of both left and right horizontal stabilizers but were fractured and separated on their outboard sections. Pieces of blue-colored composite fairing consistent with the tail rotor drive shaft cover were found in the flight path leading up to the main wreckage. The leading edge of the upper vertical fin showed evidence of impact deformation and separation above its attachment point. The four main rotor blades were reconstructed in their respective positions and laid out by their designated colors, Green, Red, Orange, and Blue (Figure 2). Three separated leading edge pieces near the tip end, including the tip cap lap joint, were found in the debris field surrounding the main wreckage. Two swept tip ends, which were fractured and separated, were reconstructed by the investigation team to the ‘blue’ and ‘orange’ main rotor blades. Two additional swept tip ends, also fractured and separated, exhibited impact damage with a width consistent with contact with the tail rotor drive system aft snubber and belonged to the ‘red’ and ‘green’ main rotor blades (Figure 3). All four swept tip ends were found in the debris field leading up to and around the main wreckage. Figure 2- A reconstruction of the four main rotor blades; From top to bottom, Green, Red, Orange, and Blue. Figure 3- An approximate reconstruction of rotor blade impact damage on the blade tip that is consistent with the dimensions of the snubber housing. The yaw hydraulic actuator remained attached to both the airframe and its control bell crank within the tail boom. The lower portion of the right finlet was fractured and partially separated, remaining attached only by wiring. The upper portion of the right finlet was fractured and completely separated. The leading edge slats were present on the inboard portions of both left and right horizontal stabilizers but were fract