On December 14, 1998, approximately 1100 Pacific standard time, a Garlick UH-1H helicopter, N94NW, owned by Northwest Helicopters, and operated as a 14 CFR Part 133 aero-logging operation, impacted the terrain after experiencing a separation of the vertical fin in flight. Visual meteorological conditions prevailed, and no flight plan was filed. The aircraft sustained substantial damage, and the commercial pilot received serious injuries. The flight originated at a nearby helicopter service pad. According to the pilot, he was in a hover at the end of a 150 foot long-line that had just been attached to a set of logs. He reported that he had not yet began to lift the logs, but had taken the slack out of the line when he heard a loud bang. The aircraft immediately began spinning to the right and settled into the terrain. The aft portion of the tail boom was sent to the NTSB materials laboratory for examination. That examination revealed the presence of fatigue cracking in all five layers of the left side of the vertical fin spar cap. According to the laboratory, the cracks initiated from the hole for the fourth rivet above where the fin protrudes from the tail boom, and propagated through most of the left side of the spar cap prior to final separation. During the investigation, it was determined that the vertical fins of at least two other UH-1 helicopters had previously separated in flight, although not through the fourth hole. The FAA had previously issued Airworthiness Directive (AD) number 97-20-09 to address the problem of fin separation from UH-1 helicopters used in repeated heavy-lift operation, but the FAA Rotorcraft Directorate subsequently determined that the AD was not adequate. As a result, the Directorate initiated the formulation of a Priority Letter Airworthiness Directive calling for the replacement of the original equipment manufacturer's (OEM) fin spar with one that "...has been proven to account for the effects of repeated heavy-lift operations."