HISTORY OF FLIGHT On October 21, 2009, about 1015 mountain daylight time, an Aero Commander 690, N690TR, experienced the collapse of its right main landing gear during landing rollout at the Cedar City Regional Airport (CDC), Cedar City, Utah. The airplane was operated by Kolab Canyon Air Services, LLC, on a positioning flight to pick up passengers in Milford, Utah. Visual meteorological conditions prevailed at the time, and a company flight plan had been filed. The airplane was substantially damaged, and the commercial pilot was not injured. The flight was performed under the provisions of 14 Code of Federal Regulations Part 91, and it originated from CDC about 0709. The pilot reported to the National Transportation Safety Board investigator that he attempted to lower the landing gear on approach to Milford Municipal Airport. The nose gear and the left main landing gear locked in the down position. The right main landing gear did not fully extend. The pilot stated that, although he performed the emergency procedures specified in the airplane's flight manual to remedy the situation, the right landing gear failed to lock down. Thereafter, the pilot flew back to CDC where he landed. During rollout, the right main landing gear collapsed. As the airplane decelerated, it slid off the runway and came to rest in an adjacent dirt area. A subsequent examination of the airplane revealed several bent bulkheads and stringers. TESTS AND RESEARCH The operator reported that it maintained the airplane using the manufacturer's approved inspection program. No anomalies, including preexisting damage, was reported with the landing gear extension system. Following the accident, the airplane's landing gear operating system was examined. Two components associated with the gear extension system were bench tested for functionality. The accumulator/regulator (part number EA1563) and the priority valve (part number 2979-3 (790301-501)) did not meet specifications. Specifically, when the accumulator/regulator was serviced with nitrogen to the specified test pressure, the unit would not pressurize. When the priority valve was pressurized, it operated only below specified limits for its part number. According to the Federal Aviation Administration (FAA) coordinator, the inability of the accumulator/regulator to be pressurized resulted in the landing gear's failure to properly extend. Also, the priority valve that had been installed in the airplane was found to be an incorrect part. Twin Commander's technical service manager reported that the priority valve should have been part number 790301-505. Accordingly, installation of the improper valve "...most likely contributed to the failure of the landing gear to fully extend to the down and locked position." (See the report from Twin Commander and the Higher Planes bench test report, included in the Safety Board’s Document Management System, for additional information.)