On August 21, 1999, about 1059 eastern daylight time, a Cessna 172RG, N6231V, registered to SJG Enterprises, Inc., operated by Golden Express Aviation, experienced a collapse of the main landing gear on landing at the Kendall-Tamiami Executive Airport, Miami, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 personal flight. The airplane was substantially damaged and the commercial-rated pilot and one passenger were not injured. The flight originated about 5 minutes earlier. The pilot stated that he performed a preflight to the airplane before departure and after takeoff while climbing through 500 feet agl, the airplane abruptly and uncommanded, yawed to the left. He noted that the turn coordinator indicated a skid to the left. The flight continued and he noticed that the left turning tendency increased. The flight returned for landing and while attempting to apply manual rudder trim, he noticed that the trim wheel spun freely. While on short final approach, the tower controller advised him to goaround as the landing gear was not extended. He placed the landing gear selector handle in the "down" position and landed the airplane with the nose landing gear down and locked; the main landing gear was not fully down and locked at touchdown. He further stated to an FAA inspector that the landing gear warning horn sounded when the power was reduced on short final and the flaps were at approach setting. Postaccident examination of the airplane by an FAA airworthiness inspector revealed that the nose landing gear was bent slightly to the left and the main landing gears were collapsed. Also, damage to the right horizontal stabilizer tip and elevator was noted. The landing gear hydraulic pump circuit breaker was popped and the rudder trim wheel and the rudder trim actuator were found in the full left position. The chain for the rudder trim assembly was off the trim sprockets and the master link was noted to be bent. The master link lock clip which was noted to be fractured, was retained for further examination. A wire at the throttle warning micro-switch was noted to be broken with evidence that it had existed in that condition for some time. Reconnection of the broken wire revealed that the warning system tested satisfactory. Retraction and extension tests of the landing gear revealed no evidence or preimpact failure or malfunction; the gear warning horn operated with the flaps selected to the full down position. Metallurgical examination of the master link lock clip by the NTSB Materials Laboratory in Washington, D.C., revealed ductile dimples typical of overstress failure. The airplane minus the retain master link lock clip was released to Charles Maynard, the insurance adjuster, on October 4, 1999. The retained master link lock clip was also released to Charles Maynard, on October 26, 1999.