On July 7, 1994, at 1125 Pacific daylight time, a Robinson R-22M, N42WC, sustained substantial damage during an autorotational forced landing to Ross Island beach, Portland, Oregon. The student pilot and certified flight instructor sustained minor injuries. No flight plan was filed for the instructional flight, which had departed Hillsboro, Oregon about 1105. The pilot stated that he had lost tail rotor effectivness and attempted an autorototation from about 75 feet AGL. The pilot stated that the aircraft continued turning through two or three rotations before toching down. The aircraft landed near shoreline, with the tail rotor and boom striking the water. Inspection revealed that the tail rotor gearbox (P/N A021-1) output shaft pinion (A545-1) retaining key (A114-2) was not installed at the time of factory assembly, allowing the tail rotor hub and blade to become disengaged.