N911KW

On July 7, 2021, about 1450 Pacific daylight time, a Bell 212 helicopter, N911KW, was substantially damaged when it was involved in an accident near Weed, California. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 133 external load flight.

The pilot reported that he was supporting a US Forest Service contract involving water drop operations and that the accident flight was on approach to a lake to retrieve water using a 100-ft long line and bucket. When the helicopter was about 400 ft above ground level (agl), the pilot performed an instrument scan and noted no anomalies. The pilot added that the radar altimeter was set to 200 ft agl at the time. When the helicopter crossed the shoreline, the pilot observed that the long line and bucket were “not yet in view and still out behind [him].” Shortly afterward, he felt a “sudden onset of a significant vibration,” which he also described as once-per-minute rotation. (An inspector with the US Forest Service explained that the term “indicates an imbalance in main rotor head or main rotor blades.) The pilot scanned the instrument panel and saw no annunciator lights or indications that the engines had stopped producing power. He then glanced outside and realized that the helicopter had descended.

The pilot aborted the water retrieval, made a forward cyclic input, and raised the collective, but the helicopter continued to descend. The pilot determined that he would be unable to stop the descent, and he did not want the engines or transmission to overspeed, so he performed a forced landing on water. After he leveled the helicopter and the skids touched down on the water, the pilot made a last attempt to take off, which was unsuccessful. The pilot reduced both throttle settings to idle in preparation to ditch the helicopter and applied right cyclic as the main rotors slowed. The pilot stated that the helicopter controls responded normally and that the helicopter did not yaw significantly. The helicopter rolled right, causing the main rotor blades to strike the water, and the helicopter subsequently sank into the lake.

Automatic dependent surveillance-broadcast data showed that the bucket was about 35 ft above the water when the last data point was recorded. At that time, the helicopter's descent rate was about 768 ft per minute. The pilot did not jettison the long line and bucket because he thought that he would be able to fly away. The helicopter sustained substantial damage to the fuselage, main and tail rotor blades, and drive systems.

Training records showed that the pilot completed six recurrent long-line training flights from May 3 to 29, 2021. On May 31, 2021, the pilot completed a helicopter pilot evaluation checkride with a US Forest Service pilot inspector and was cleared for solo external-load water bucket operations.

The 240-gallon water bucket was configured to gather 90% of its total capacity, which would add an additional weight of about 1,966 pounds to the helicopter and increase its 8,150-pound gross weight to about 10,116 pounds. The bucket was recovered attached to the helicopter by a cargo hook and the long line. Postaccident examination of the bucket and engine powerpack revealed no preimpact mechanical malfunctions or failures that would have precluded normal operations.

The pylon corner mounts and pylon dampers were hydraulically tested with dynamic loads and exhibited normal operational performance. The rod end bearings for the pylon dampers had tolerances that were more than the Bell 212 Maintenance Repair and Overhaul Manual tolerance of 0.004-inch radial play: the right aft damper bearing play was 0.006 inch, and the left aft damper bearing play was 0.016 inch. According to Bell Textron, the excessive radial clearance of rod end bearings reduces the amount of damper stroking to counteract vibratory motion of the pylon. Also, a flight crew in a separate event experienced a vibration that was traced back to excessive radial wear of the pylon damper rod end bearings. The helicopter’s hover out of ground effect (OGE) performance was computed using the helicopter operator’s Rotorcraft External Load Manual, which included a performance chart from Bell Helicopter’s BHT-212-MD-1 manual. According to the performance chart, at an air temperature of 35°C, heater OFF, engine rpm at 100%, skid height of 50 ft, a 15-knot headwind, and generator values of 150 amperes, the helicopter had an OGE takeoff power gross weight limitation of about 10,250 pounds. According to the Bell 212 Rotorcraft Flight Manual: Supplement Cargo Hook, at a helicopter gross weight of 8,150 pounds, an air temperature of 35°C, heater OFF, engine rpm at 100%, skid height of 60 ft, and generator values of 150 amperes, the helicopter had an OGE takeoff power limitation at a pressure altitude of about 8,000 ft and a maximum continuous power setting limitation at a pressure altitude of about 6,600 ft. The Bell Helicopters manual also showed that, at a gross weight of about 10, 116 pounds, the helicopter had an OGE takeoff power limitation at a pressure altitude of about 3,700 ft and a maximum continuous power setting limitation of 9,300 pounds.