On February 5, 1997, at 2116 mountain standard time, a Cessna 421C, N45MV, experienced a main gear collapse during a landing at Cedar City Municipal Airport, Cedar City, Utah. The airline transport pilot and his passenger were not injured, but the aircraft sustained substantial damage. The Part 91 business flight, which departed Las Vegas, Nevada about 70 minutes earlier, was operating in instrument meteorological conditions at the time of the accident. The flight had been conducted on an IFR flight plan, and there was no report of an ELT activation. According to the pilot, who was executing a night ILS approach in a snow storm, both main gear failed to extend when he put the gear handle down at glide-slope intercept. He therefore recycled the gear four or five times. Each time, the nose gear down-and-locked light came on, but the down-and-locked light for the main gear did not illuminate. At about 400 feet above the ground (AGL) the pilot attempted to get the main gear to extend by using the emergency landing gear blow-down system, but the right main gear down-and-locked light still did not come on. Because the gear could not be retracted after using the blow-down system, and because of the severity of the night IFR weather conditions, the pilot made the decision to complete the landing with the right gear not in the down-and-locked position. After touchdown, as the aircraft slowed, the right wing contacted the runway, and the aircraft began to slide sideways. The aircraft then slid off the right side of the runway into a snow bank near the runway edge. As the aircraft was sliding off the runway, the left main gear and nose gear collapsed, and the right main gear was torn from the wing. During a post-accident test of the landing gear extension and retraction system, all three gear extended and retracted normally. After the extension/retraction test was completed, the landing gear control valve, the shuttle valve, hydraulic system loading valve, and the right main landing gear actuator were removed from the aircraft and tested. All of the valves operated normally with no noted discrepancies. Fluid that was removed from the right main gear actuator prior to testing was sent to Hample Oil Distributors of Wichita, Kansas for analysis. The laboratory tests completed a Hample Oil Distributors determined that the sample contained 30% water contamination by volume. The investigation was unable to determine when or how the hydraulic system in the 19 year old aircraft had become contaminated. At the time of the landing, the temperature was approximately 23 degrees Fahrenheit, and the aircraft had been at altitude for over an hour prior to the approach. It is believed that due to the low ambient temperature, the water in the actuator froze while the aircraft was en route, and because the temperature was still below freezing near the surface, the water in the actuator remained frozen, keeping the actuator from functioning properly when the pilot attempted to put the gear down.