On June 16, 1999, at 2200 hours Pacific daylight time, a Rockwell International 690B, N480K, had a left gear collapse upon landing at Monterey, California. The aircraft sustained substantial damage; however, the commercial pilot, the private certificated second pilot, and three passengers were not injured. The aircraft was owned and operated by SB Aircraft Sales of Boulder, Colorado, under the provisions of 14 CFR Part 91 for the cross-country business flight. An instrument flight rules (IFR) flight plan was filed and instrument meteorological conditions prevailed at the airport at the time of the accident. The flight originated at Tri-County Airport near Boulder at 1900 mountain daylight time, and was destined for the Monterey Peninsula airport. The pilot of the flight stated that during their approach to the airport they were using the approach and landing checklists. He stated that both he and the owner, a private pilot who was functioning as the copilot, observed three green gear down lights and a lack warning horns prior to passing the outer marker. Both pilots stated they observed hydraulic pressure of about 1,000 psi during the landing checks. The pilot said he landed the airplane with a slight tailwind, observed two beta lights, and pulled the power levers into ground idle/reverse. Shortly thereafter, the left landing gear collapsed, and the airplane stopped near the runway centerline with the gear warning horn, red unsafe light, and two green lights showing. All occupants exited the airplane via the main cabin door and both pilots coordinated the lifting and towing of the airplane with the fire crew. They stated that the left gear drag strut snapped into a down and locked position when the left wing was raised. A representative of Twin Commander Aircraft Corporation examined the airplane on July 8, 1999, at the request of Safety Board investigators. He said he noticed extensive damage to the lower left side of the fuselage. He observed that the left main gear was down and locked. He noticed that there was a section missing from the inboard gear-forging boss. He stated that the boss houses the clevis for the hydraulic cylinder attachment. He observed that the clevis remained bolted to the hydraulic cylinder, and that the hydraulic cylinder had no apparent damage and was in the full-extended position consistent with gear down. He observed that the outboard hydraulic cylinder was in place and had no apparent damage. The nitrogen bottle pressure gauge needle was in the green. The separated section of forging was recovered. He was not able to determine if the hydraulic cylinder installation had been adjusted per the maintenance manual. The strut body with a separated piece from the boss which houses the inboard retract cylinder clevis and the retract cylinder clevis were sent to the Safety Board's Materials Laboratory for examination. Bench binocular microscope examination of the strut body and separated boss piece revealed that the fracture faces contained features typical of overstress separation. No pre-existing cracks were noted on the fractures. X-ray energy dispersive spectroscopy analysis of the fracture from the piece of the boss produced a major peal of aluminum and minor peaks of magnesium, silicon, manganese, iron, copper, and carbon; all elements were consistent with an aluminum alloy. A review of the maintenance manual for the 690B indicates that an adjustment must be made to the cylinder over travel to obtain drag brace preload. The main landing gear must be fully extended, and the center hinge point of the drag brace assembly is forced over center by the hydraulic-pneumatic actuating cylinders and bungee cords, to provide a positive down lock. The adjustment procedure outlined in the maintenance manual applies to both of the main gear actuating cylinders and is necessary to assure adequate drag brace preload to lock the main landing gear in the down position. The maintenance manual also states that preloading of the drag brace also assures adequate clearance between the end of the actuating cylinders and pistons, when the gear is fully extended. A review of the aircraft maintenance logs indicated that the airplane was inspected in accordance with a 100-hour inspection on November 3, 1998 with a total time of 4,653.30 hours. On November 4, 1998, the airplane was inspected by the Federal Aviation Administration and issued a Standard Airworthiness Certificate as it entered the United States from Argentina after being purchased by the current owner. On January 21, 1999, the left hand hydraulic cylinder, main landing gear aft wheel well doors, and related hydraulic components were replaced and a landing gear check was performed. The airplane had accrued 34.2 hours since the last 100-hour inspection in November 1998.