On April 4, 2005, about 1225 mountain standard time, a Cessna 421B, N637SC, had the left main gear collapse after landing at Lake Havasu City Airport (HII), Lake Havasu City, Arizona. Sun Western Flyers, Inc., d.b.a. Sun Care Air Ambulance, was operating the airplane under the provisions of 14 CFR Part 91. The airline transport pilot and the two medical crewmembers were not injured; the airplane sustained substantial damage. The cross-country repositioning flight departed HII, about 1215, with a planned destination of Las Vegas, Nevada. Visual meteorological conditions prevailed, and a company flight plan had been filed. The approximate global positioning system (GPS) coordinates of the primary wreckage were 34 degrees 34.16 minutes north latitude and 114 degrees 21.29 minutes west longitude. The National Transportation Safety Board investigator-in-charge (IIC) interviewed the pilot via telephone. The pilot reported that after takeoff he had an unsafe gear indication; he recycled the landing gear two or three times until the gear appeared to be secured. He then noticed that the airspeed was too slow for the airplane's current flight configuration. The pilot suspected a landing gear malfunction and returned to HII. He then made a low approach to the runway and was told by his maintenance personnel that the left gear was still extended. The pilot cycled the gear down and observed that the landing gear lights were illuminated, indicating that the gear was down and locked. The pilot made a soft field landing onto runway 32. The pilot let the airplane roll to the end of the runway, and started to turn to the left, when the left main gear collapsed. The airplane's left wing came to rest on the runway. The reported damage to the airplane was confined to the left engine nacelle and the left wing in the area of the main landing gear. The airplane was removed from the runway and secured for further investigation. Federal Aviation Administration inspectors and maintenance personnel from Sun Western Flyers examined the airplane's left main landing gear assembly following the accident. The examination revealed that the roll pin (part number NAS561P4-28) that retains the attaching shaft to the trunnion assembly had fractured and failed. This allowed the landing gear attaching shaft to shift from its installed position, resulting in the trunnion assembly separating from the landing gear support structure.