On September 11, 1999, at 2030 central daylight time, a Cessna 172RG airplane, N9724B, was substantially damaged while landing at the Fort Smith Regional Airport, near Fort Smith, Arkansas. The airplane was registered to Christiansen Aviation Inc., of Tulsa, Oklahoma, and operated by Flight Works, Inc, of Bethany, Oklahoma. The instrument rated commercial pilot, sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 personal flight, and an IFR flight plan was filed. The cross-country flight originated from the Wiley Post Airport, Oklahoma City, Oklahoma, at 1845. According to the pilot, the aircraft departed on an IFR flight plan and was destined for Little Rock, Arkansas. The pilot elected to divert from the filed flight plan and land at Fort Smith when the "oil pressure went beyond the green range." He contacted Fort Smith Approach Control, as the oil pressure continued to rise, and set up for landing on runway 25. When the pilot moved the gear lever to the down position, the "indicator light did not come on." He recycled the gear, and the light still did not illuminate. He informed the control tower of the problem, and they suggested that he execute a low flyby so they could observe the gear and try to determine if it was extended. After the flyby, the controllers stated that the gear was down, but they were unable to determine if it was in the locked position. Upon landing the right main landing gear collapsed, and the airplane veered to the right of the centerline. The propeller contacted a taxiway light, and the airplane exited the runway surface. The airplane came to rest upright in grassy terrain adjacent to the runway. The FAA inspector reported that the rear spar of the right horizontal stabilizer was structurally damaged. Additionally, the propeller was "nicked." The airplane, which had accumulated a total of 8,462 hours, was examined by Dawson Aircraft of Clinton, Arkansas. A gear retraction test was performed and "smoke" was observed coming from the electric pump motor/hydraulic power pack. The electric pump motor/hydraulic power pack was removed from the airplane and replaced with an overhauled electric pump motor/ hydraulic power pack. Another gear retraction test was performed, and the gear system operated within manufacturer's specifications. The reason for the excessive oil pressure indication was not determined.