SEA97LA126

While returning to pick up another log, the pilot heard a high-speed shredding sound' accompanied by high-frequency vibrations. As the noise and vibration increased, the pilot turned downhill and looked for a location where he could make an emergency landing. He then heard a second loud noise, and the rotor RPM immediately began to decay. He stretched his glide over the 130 foot high trees and managed to maneuver the helicopter so that it 'fell into' a switchback on an old dirt logging road. A post accident examination of the K-FLEX drive shaft, which runs between the engine and the transmission, revealed that one of its flex band legs had experienced an inflight fatigue fracture. A review of the records revealed that the shaft had accumulated 459 hours while being used by the United States Army, and had accumulated an additional 432 hours during civilian logging operations.

On June 3, 1997, approximately 0630 Pacific daylight time, a Garlick UH-1H helicopter, N85NW, collided with the terrain during a forced landing near Addy, Washington. The commercial pilot, who was the sole occupant, was not injured, but the aircraft, which was being operated by Northwest Helicopters, Inc., sustained substantial damage. The 14 CFR Part 133 logging flight, which had been in the air about 30 minutes, was being conducted in visual meteorological conditions, and no flight plan had been filed.

According to the pilot, he was about 30 minutes into the first logging cycle of the day, and was heading back to the hooker to pick up another log when the accident sequence began. He said that suddenly he "...heard a high-speed shredding sound with accompanying high-frequency vibration." He immediately lowered the collective, released the 150 foot long-line (which was not attached to any load), and checked the RPM and all other instruments. He said that all the instruments looked normal, but because the sound and vibration were quickly getting worse, he headed downhill over the 130 foot high trees in hopes of finding a location where an emergency landing could be made. While heading downhill, he heard a second loud noise, which he suspected was the failure of some part of the main rotor drive shaft system. Immediately after hearing the second noise, the rotor RPM began to decay, and the pilot "...stretched the downhill glide over a last group of trees, and fell into a switchback road." As the aircraft fell onto the road it "...hit four small trees in the process of crash landing." Upon hitting the terrain, the aircraft pitched up on its nose, whereupon the main rotor severed the tail boom.

After the accident a number of the flex bands on the K-FLEX drive shaft, which is located between the engine and transmission, where found to have failed. The shaft and as many of the flex band sections as could be found at the accident site were sent to the NTSB metallurgical laboratory in Washington, D.C., in order for all fracture surfaces to be examined. That examination found that "One fracture on a rectangular flex leg section showed evidence of preexisting cracking in the form of a fatigue crack region." All the remaining fractures were either damaged so severely during the accident that the original fracture surface could not be viewed, or displayed characteristics of a "...complete overstress separation."

A review of the K-FLEX shaft's records revealed that this shaft had originally been used by the United States Army. Army records show that the shaft was removed from an AH-1F (attack model) helicopter when the shaft had accumulated a total of 459 hours since new. It was then released to the civilian market, and accumulated another 432 hours in logging operations prior to failure. The records also indicated that prior to being put into civilian service, the entire shaft was inspected in accordance with Technical Manual 55-1520-210-23.