Dardanelle, CA, USA
N512TA
Bell 212
The pilot was conducting water bucket operations when he heard a one-per-revolution whispering sound emanating from the helicopter. He elected to return to the helibase and accepted the Air Attack controller’s offer to escort him back. While en route to the helibase, at an airspeed about 60 knots, the whispering sound evolved into a vibration that progressively worsened. The pilot then elected to land as soon as possible at a nearby location. Upon arrival to the precautionary landing site, the vibration momentarily stopped and was followed by a loud bang, an aggressive vertical vibration, and a loss of control. The helicopter impacted steep mountainous terrain and rolled once onto its left side. The occupants of the escort helicopter observed the accident helicopter nose over about 20° to 30°, followed by a 360° flat turn before it impacted the terrain. Examination of the wreckage revealed that multiple components of the main rotor control system had failed along with a span-wise failure of one of the main rotor blades. The drive link associated with the failed rotor blade had failed in overload at its lower lug and swashplate trunnion bearing. The outer race of the trunnion bearing exhibited brinelling consistent with axial movement of the inner section of the bearing. The inner section of the trunnion bearing separated from the rotating swashplate along with the lower lug of the drive link. Gouges and impact marks found on the arms of the non-rotating swashplate were likely made from the lower lug of the drive link as the inner section of the trunnion bearing began to pull out of the outer race. Contact of the lower lug with one of the arms of the non-rotating swashplate likely resulted in the overload failure of the drive link. These liberated components were not recovered. The in-flight failure of the swashplate drive link resulted in the loss of control of the associated main rotor blade. The uncontrolled blade, free to move out of track with the other controlled blade, resulted in at least a partial loss of control of the main rotor system. The associated vibrations from the out-of-track rotors likely resulted in the failure of the swashplate support and the total loss of control of the helicopter.
On August 25, 2018, about 1545 Pacific daylight time, a Bell 212 helicopter, N512TA, was substantially damaged when it was involved in an accident near Dardanelle, California. The pilot was seriously injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 133 external load flight. The helicopter was operated by Trans Aero LTD and was under contract to support the US Forest Service. The pilot reported that he had flown the helicopter for about 13.5 hours in the previous 2 days without incident. On the day of the accident, he first conducted support flights, then configured the helicopter for water bucket operations. He departed for the Boulder Lake dip site which was about 35-40 miles away from the helibase. While conducting water bucket operations at the dip site, he heard a “one per revolution” whispering sound that he had not heard before. He released the water load and departed the dip site and recalled that everything felt correct at that time. He contacted the Air Attack controller (controller) and requested to return to the helibase. The controller, who was a passenger in another helicopter, cleared the pilot to return to the helibase and elected to escort the accident helicopter. Shortly after the return flight began, at an airspeed about 60 knots, the pilot began to feel a one-per-revolution vibration, and the helicopter began to shudder. When he noticed that it had changed from a noise to something that he could feel, he elected to land as soon as possible. The vibration steadily increased in intensity as the flight continued. Just prior to arriving at the landing area, the vibration stopped momentarily. He then heard a loud bang followed by an extremely aggressive vertical vibration, along with a substantial nose-down pitch attitude. He described the feeling as a “one-foot hop that felt like he was being hammered.” He stated that he still had [engine] power but it did not feel like he had control of the helicopter. The helicopter struck the terrain, rolled once, and came to rest on its left side. He then tried to shut down the engines, pulled both fire T-handles, and switched the fuel shut-off valves and boost pumps off. He then exited the helicopter unassisted. The controller reported that he was in the left front seat of an A-Star helicopter when he received a radio call from the pilot of the accident helicopter, requesting to go back to the helibase. He cleared the helicopter back at an altitude of 9,500 feet mean sea level (msl). The pilot then requested to maintain low level and wanted to follow the road back. The controller found that odd and offered to visually follow him back to the helibase. The pilot accepted. The A-Star joined with the helicopter and the pilot reported that all the gauges were good, but he felt a hop. The controller suggested a closer landing site called H1, which the pilot accepted. The controller then passed the helicopter on the left, overflew H1 and began a climb to the right to observe the helicopter land. The controller heard the pilot state that “it smoothed out.” Shortly after, the A-Star pilot stated, “he lost it.” The controller saw the helicopter about 500 ft above ground level, in a nose-down attitude of about 20°-30°. The controller could hear the pilot over the radio “grunting as if he were fighting to control the helicopter.” The controller saw the helicopter’s nose come back up as the helicopter made a slow left 360° turn while losing altitude. He saw the helicopter hit the terrain in a left-side low attitude, roll once, and come to rest on its left side. The helicopter came to rest on steep mountainous terrain that had previously burned from a wildfire. At least one of the two engines remained running. All major components were found in a debris area of about 100 ft by 100 ft. The longline holding the water bucket remained attached to the helicopter cargo hook. The line looped over the top of the fuselage and ran uphill to the water bucket. According to a US Forest Service law enforcement officer who arrive shortly after the accident, the engine continued to run for about 1.5 hours. According to a Federal Aviation Administration (FAA) representative and an aviation safety inspector for the US Forest Service who examined the helicopter at the scene, they observed the swashplate had separated from the swashplate support and significant damage and metal transfer/deformation to the area where the swashplate support to gimble assembly mounts were previously attached. Evidence at the scene indicated the main rotor mast only rotated one to two revolutions, coming to a quick stop after the first main rotor contacted trees and rocks, as one blade was destroyed and the other blade was nearly intact. Members of the National Transportation Safety Board (NTSB), FAA, and the helicopter manufacturer examined the helicopter after it was taken to a recovery site. Examination of the fuselage revealed no preaccident mechanical anomalies or failures that would have precluded normal operation. The engines were not examined due to the pilot’s report of the helicopter still making power. The helicopter was equipped with a semi-rigid main rotor system with two original equipment manufacturer main rotor blades, serial numbers A-7749 and A-7774. Both blades were installed on the helicopter at 6,217.9 hours. Examination of the main rotor blades revealed blade A-7774 exhibited an overload fracture along the edges of skin doublers and along or adjacent to the aft face of the spar. Some of the main rotor blade fracture surface features were consistent with multiple contact events between the spar and after body. Blade A-7749 exhibited spanwise bending and wrinkling along the entire span. There was a hole created near the tip of the blade and a section of the honeycomb afterbody was fractured near the blade tip. Figure 1. Photo of an exemplar main rotor control system like the accident helicopter (Source: the internet). Each of the main rotor blades had an associated drive link attached to the rotating swash plate by a trunnion bearing, as shown in figure 1. The swash plate assembly was attached to the swash plate support via the gimbal ring and two clevises. Figure 2. Image of the drive links and the rotating swashplate (Source: the FAA). Examination of the main rotor control system revealed that a swashplate drive link (serial number HE-017) was fractured at the clevis (lower lug) used to attach the drive link to a trunnion bearing mounted to the rotating swashplate. The lower lug and the inner race of the trunnion bearing separated from the rotating swashplate and were not recovered. The trunnion bearing’s outer sleeve remained attached inside the rotating swashplate. The outer drive link (serial number RR19-1373) remained attached to the rotating swashplate at its trunnion bearing (see figure 2). Figure 3. Image showing the swashplate support and the underside of the non-rotating swashplate (Source: the FAA). The swashplate assembly separated from the swashplate support at the two fractured clevises. The four liberated clevis tangs remained attached to the gimble ring inside the swashplate. The collective sleeve exhibited damage consistent with multiple contacts from the gimble ring (see figure 3). Figure 4. Image showing the swashplate assembly. Both arms of the stationary swashplate exhibited contact marks with shiny metal exposed. One of the arms exhibited multiple gouges on the top of the arm, consistent with contact from a rotating object (see figure 4). The swashplate assembly and the swashplate support were examined by the NTSB’s Materials Laboratory. Examination of the swashplate support and the four liberated clevises revealed that there were indications of necking and tearing adjacent to the fracture faces, consistent with local plastic deformation. The fracture faces on some of the clevises also exhibited battering and smearing. These features were consistent with post-fracture impact damage between the fragments and the support. Figure 5. Image of the accident trunnion bearing outer race and an exemplar trunnion bearing outer race (Source: Bell Helicopters). Examination of the swashplate drive links and the trunnion bearing outer sleeve revealed that the drive link HE-017 lower lug had fractured because of overload. The trunnion bearing outer sleeve was sectioned and examined and revealed evidence of brinelling which resulted from an axial impact or multiple impacts from the inner side (see figure 5).
An in-flight failure of the trunnion bearing and subsequent failure of the drive link and swashplate support, which resulted in the loss of helicopter control.
Source: NTSB Aviation Accident Database
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