On February 13, 1995, approximately 1020 Pacific standard time, a Sikorsky S-58JT, N1099T, sustained substantial damage during a forced landing after a main rotor mast failure while conducting helicopter logging operations. The commercial pilot, who was the sole occupant of the aircraft, was seriously injured. There was no fire, and no flight plan was filed for the flight, which was conducted near Marblemount, Washington, and had departed approximately 1010. The ELT actuated, but did not aid in locating the accident site. A witness stated that the pilot had just hooked a load, then released it and descended rapidly to impact. He described hearing an engine sound increasing in rpm before the crash. The pilot stated that he had a 150 foot long-line attached, and about ten minutes into his cycle, heard a loud bang while picking up a load. He said he tried to move the load downhill, away from the ground crew person, released the load and long-line, and turned the aircraft to head down slope. The aircraft impacted the terrain. Post accident investigation revealed that the main rotor shaft, P/N S1635-20059-2, S/N HR243, which had been installed new on August 16, 1993, during the last main gearbox overhaul, had separated through the shaft-to-flange radius. The part had 952 hours total time since new. The normal transmission overhaul period and main rotor shaft retirement time are both 2500 hours.