Accident Narrative
On August 25, 2018, about 1545 Pacific daylight time, a Bell 212 helicopter, registration N512TA, was substantially damaged in an accident near Dardanelle, California. The pilot sustained serious injuries. The helicopter was operated by Trans Aero LTD under a Title 14 Code of Federal Regulations Part 133 external load flight, contracted to support the US Forest Service.
The pilot reported that he had flown the helicopter for about 13.5 hours in the prior two days without incident. On the accident day, he first conducted support flights and then configured the helicopter for water bucket operations. After departing for the Boulder Lake dip site, about 35-40 miles from the helibase, he heard a “one per revolution” whispering sound during water bucket operations. He released the water load and departed, feeling that everything was normal. He contacted the Air Attack controller and requested to return to the helibase. The controller, a passenger in another helicopter, cleared the return and elected to escort the accident helicopter.
Shortly after the return flight began, at about 60 knots, the pilot felt a one-per-revolution vibration and shudder. The vibration increased in intensity. Upon noticing the change from noise to tactile sensation, he decided to land as soon as possible. Just before reaching a landing area, the vibration stopped momentarily, then a loud bang occurred followed by an extremely aggressive vertical vibration and a substantial nose-down pitch attitude. The pilot described the feeling as a “one-foot hop that felt like he was being hammered.” He still had engine power but felt no control. The helicopter struck the terrain, rolled once, and came to rest on its left side. The pilot shut down both engines, pulled the fire T-handles, switched off fuel shut-off valves and boost pumps, and exited unassisted.
The controller, in an A-Star helicopter, reported that after clearing the accident helicopter at 9,500 feet mean sea level, the pilot requested a low-level return following a road. The controller offered to escort visually, which the pilot accepted. The controller heard the pilot report good gauges but a “hop.” The controller suggested a closer landing site, H1, which the pilot accepted. While overflying H1, the controller heard the pilot say “it smoothed out,” then the A-Star pilot said “he lost it.” The controller observed the helicopter about 500 feet above ground level, nose-down about 20°-30°, and heard the pilot grunting. The nose came up as the helicopter made a slow left 360° turn while losing altitude. The controller saw the helicopter hit the terrain left-side low, roll once, and come to rest on its left side.
Examination Findings
The helicopter came to rest on steep, previously burned mountainous terrain. At least one engine remained running; a US Forest Service officer reported it running for about 1.5 hours. The longline with the water bucket remained attached, looping over the fuselage and running uphill to the bucket.
Examination by the FAA, US Forest Service, NTSB, and the manufacturer revealed the swashplate had separated from its support with significant damage and metal transfer at the swashplate support to gimbal assembly mounts. The main rotor mast rotated only one to two revolutions, stopping quickly after the first rotor blade contacted trees and rocks; one blade was destroyed, the other nearly intact. Fuselage examination showed no preaccident mechanical anomalies. Engines were not examined due to the pilot’s report of continued power.
The helicopter had a semi-rigid main rotor system with two original equipment manufacturer blades (serial numbers A-7749 and A-7774), both installed at 6,217.9 hours. Blade A-7774 exhibited an overload fracture along skin doubler edges and the aft face of the spar, with features consistent with multiple contact events between the spar and afterbody. Blade A-7749 showed spanwise bending and wrinkling, with a hole near the tip and a fractured honeycomb afterbody section.
Examination of the main rotor control system revealed that swashplate drive link (serial number HE-017) was fractured at the clevis (lower lug) attaching it to a trunnion bearing on the rotating swashplate. The lower lug and inner race of the trunnion bearing were not recovered; the outer sleeve remained attached inside the rotating swashplate. The other drive link remained attached. The swashplate assembly separated from its support at two fractured clevises, with four liberated clevis tangs remaining attached to the gimbal ring inside the swashplate. The collective sleeve showed damage from multiple contacts with the gimbal ring. Both stationary swashplate arms had contact marks with shiny metal; one had multiple gouges consistent with rotating object contact.
The NTSB Materials Laboratory examined the swashplate support and clevises, finding necking and tearing adjacent to fracture faces, consistent with local plastic deformation. Some clevises had battering and smearing from post-fracture impact damage. The trunnion bearing outer sleeve showed evidence of brinelling from an axial impact or multiple impacts from the inner side. The drive link lower lug fractured due to overload.