On November 14, 2007, about 1345 Pacific standard time, a Piper PA-44-180, N274ER, sustained substantial damage following a right main landing gear collapse and a loss of control during landing roll at the Byron Airport (C83), Byron, California. The flight instructor, pilot-rated student and sole passenger were not injured. Visual meteorological conditions prevailed for the 14 CFR Part 91 instructional flight, and a flight plan was not filed. The flight departed the Hayward Executive Airport (HWD), Hayward, California, about 1200. In a statement dated November 27, 2007, the flight instructor reported that this was to be the fourth full stop landing with his student. The flight instructor stated that at the midfield point he verified that the landing gear was down by observing the 3 green landing gear down-and-locked lights illuminated. The flight instructor further stated the after touching down on the centerline and during the rollout, and after about 300 to 400 feet, "...I felt a 'clunking' sound, and the airplane dropped slightly to the right and started to pull to the right. I would estimate we were rolling 50 to 60 knots at this point." The flight instructor further reported that he then brought both throttles to idle and helped on the controls with the student to maintain directional control. The pilot stated, "Soon after there was a second 'clunk' and the right wing dropped to the runway, resulting in an uncontrollable slide off the right side of the runway." Examination of the airplane by a Federal Aviation Administration inspector revealed that the right main landing gear trunnion had failed. The component was forwarded to the Safety Board's materials laboratory for further examination. The examination revealed that the right main landing gear aft trunnion attach fitting (p/n 67042-13) was fractured at 3 locations intersecting the center bearing boss area. The outboard, forward and lower fracture surfaces each exhibited a rough and irregular appearance consistent with overstress in a cast aluminum alloy. It was also noted that a portion of the outboard fracture had a relatively flat plane of fracture with a curving boundary, features consistent with fatigue.