On December 9, 2001, about 2020 eastern standard time, a Rockwell International NA-265-80 Sabreliner, N819GY, operated by Eagle Jet Charter, Inc., was substantially damaged when the left engine thrust reverser assembly separated from the engine, and struck the fuselage during landing at General Edward Lawrence Logan International Airport (BOS), Boston, Massachusetts. The two certificated airline transport pilots and a passenger were not injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the flight that originated from McCarran International Airport (LAS), Las Vegas, Nevada. The non-scheduled passenger flight was conducted under 14 CFR Part 135. The airplane was landing on runway 4L, a 7,861-foot long, 150-foot wide, asphalt runway. In a written statement, the pilot-in-command said: "...After a normal landing the thrust reversers were deployed in the prescribed manner and both deploy lights indicated a normal deployment. About a second after full reverse power was achieved the aircraft yawed violently to the right. I braked left and stowed both thrust reversers. After taxiing off the runway, it was noted that the left thrust reverser unlock light was still illuminated...." Examination of the airplane by a Federal Aviation Administration inspector revealed that the left engine thrust reverser assembly had separated from the engine and was found on the runway. Additionally, the thrust reverser punctured the fuselage aft of the left engine and subsequently cut a rib and stringer. The left thrust reverser was retained for further examination. The left thrust reverser inboard pinion gearbox, the outboard pinion gearbox with broken drive cable housing cover, and upper outboard T-fitting, were forwarded to the Safety Board Materials Laboratory. According to the metallurgist's report, visual examination of the outboard pinion gearbox revealed it was intact; however, three of the four attaching bolts at the forward flange were fractured, with the threaded ends remaining in the flange. Optical examination of the fracture surfaces on the threaded ends revealed that all fractures initiated from multiple origins. The fractures then propagated to the surface toward the interior with arrest marks and flat features indicative of fatigue propagation. The fourth bolt had pulled out of the flange and was contained in the T-fitting. The bolt was intact, slightly bent, and contained the helicoil and apparent remnants of the aluminum threads from the pinion gearbox. The inboard pinion gearbox was fractured into two sections. The fracture was located just aft of the forward flange. Two origin areas were observed at the base of the two rails in the radii near the forward flange. Examination of the fracture surfaces revealed damage indicative of fatigue. The fatigued regions were observed propagating from both the upper and lower surfaces of both rails, consistent with reverse bending loads. All fatigue regions initiated in the radii at the base of the rail with multiple origins. Visual examination of the forward face of the rails revealed signs of heavy impact/wear. Scanning electron microscope examination of the largest fatigue region on the rail revealed damage consistent with low-alternating stress high-cycle fatigue propagation. Review of maintenance records revealed that the left thrust reverser was installed on May 7, 1996. At that time, the airplane had accumulated approximately 6,717 total flight hours, and 5,715 landings. It was not determined if the left thrust reverser attaching bolts were replaced at that time. The airplane was maintained under a continued airworthiness inspection program. The thrust reversers were inspected on June 3, 2001, at 9226.8 flight hours, 8,026 landings. The thrust reversers were last lubricated on September 11, 2001, at 9353.5 flight hours, 8,157 landings. At the time of the accident, the left thrust reverser had accumulated 8,275 total landings, and about 2,560 landings since it was installed.