The pilot of the helicopter was performing firefighting operations in mountainous terrain. He performed a series of water drops and crew relocation activities in the accident helicopter earlier in the day. Those flights were uneventful, and the helicopter’s performance was nominal. The helicopter was refueled at a local airport and returned to the water dip location, which was a lake about 2 miles south and at a lower elevation than the fire.By the time of the next series of water drops, a cold front had passed through the area and the winds had shifted to the north and increased in velocity. As a result, the pilot adjusted the approach path and water dip location accordingly. He then flew a series of uneventful dip and drop patterns between the lake and the fire.On the fifth dip cycle, the pilot initiated a hover and began filling the water bucket, and as he was pulling it out of the water, the helicopter began to exhibit heavy airframe vibrations accompanied by a descent and yaw to the left. He stated that the vibrations were so extreme that he could not focus on the gauges. He lowered the collective control and was presented with a series of caution lights and a low-rotor RPM horn. He maneuvered the helicopter to the shore and attempted to release the water bucket. As the shoreline approached, the helicopter began to rapidly descend, and the pilot initiated a flare and applied collective to cushion the landing. The helicopter landed in the water, where the main rotor blades struck the airframe, and the tailboom detached. The pilot shut off the fuel and electrical power and egressed as the helicopter rolled over in the water.The pilot determined that the event was initiated by a loss engine power, and his actions under such a condition were appropriate. However, no anomalies were found in the airframe, drive Page 2 of 15 WPR22LA339system, fuel delivery system, engine or its ancillary components that would have precluded normal operation. Additionally, internal engine damage signatures indicated that it was producing power at impact, when it ingested a section of main rotor blade and fragments of the main driveshaft coupling that separated as the airframe struck the water. The helicopter was equipped with a measured gas temperature (MGT) gauge,   which was capable of continuously recording MGT to non-volatile memory (NVM). The unit recorded both the accident and the previous flight flown by the pilot. The gauge clearly showed nominal engine performance for the water dip and drop cycles for the previous flight along with an orderly decrease in MGT as the engine was shut down. The data for the accident flight closely mirrored the cycles observed on the previous flight. For the final cycle the MGT climb appeared nominal and showed an increase in temperature consistent with the helicopter maneuvering during the bucket dip and pickup sequence. However, the MGT then almost instantaneously dropped to shutdown temperature, at a rate much faster than the previous engine shutdown sequence. This drop was likely a result of impact.The accident site was surrounded by mountainous terrain, particularly to the north where the fire was located a short distance away. Wind analysis indicated that downdrafts would have been present at the time of the accident, which were likely unpredictable and exacerbated by the proximity of the fire. The pilot had stated that the area was notorious for downslope katabatic wind, and that there was unusual turbulence in the area. Furthermore, review of the MGT data indicated that for the three cycles leading up to the accident, less engine power was required to transition to the water dip hover, which indicated wind strength had likely increased during the short time leading up to the accident.Rather than losing engine power, the helicopter likely encountered strong downdrafts that caused a rapid descent. The pilot attempted to release the bucket during the accident sequence, and while he was ultimately able to do so, up until release it was being dragged through the lakebed, and thus hindering the helicopter’s maneuverability.Water dip operations required that the single-engine helicopter be flown at a low height and speed such that it was operating within the “AVOID” range of its Height-Velocity Envelope. In this range, a safe landing could not be guaranteed following an engine failure. The pilot was able to demonstrate the presence of mind and helicopter control to maneuver towards the shore with limited altitude and time, and therefore avoid a more significant impact.