HISTORY OF FLIGHTOn September 20, 2017, at 1314 Pacific daylight time, a Cessna 210F airplane, N7330E, sustained substantial damage to the left horizontal stabilizer after the main landing gear collapsed during the landing roll at Sacramento Executive Airport, Sacramento, California. The private pilot and passenger, who held a flight instructor certificate, were not injured. The airplane had just been purchased by the private pilot, and at the time of the accident was registered to the previous owner. The airplane was being operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed, and no flight plan had been filed for the local flight. Both occupants provided differing accounts of their operational roles during the flight. The pilot, who was seated in the left seat, stated shortly after the accident that this was his first flight in the airplane, and that it was his understanding that it would be a training flight. The instructor had no previous experience flying the Cessna 210 series and stated that he had explicitly explained to both the pilot and the pilot's mechanic (who had arranged for the two to fly together), that he would not be operating the airplanes controls or providing instruction, and that he was simply acting as an observer. The pilot stated that his intention was to perform a flight in the traffic pattern and then perform a touch-and-go landing. He reported that the flight was uneventful, and that he flew the airplane during the takeoff and landing approach legs, and that the instructor operated the landing gear handle, and was assisting with the flight controls during the landing roll. He stated that prior to landing, he visually confirmed the landing gear had extended by viewing them through the gear mirrors. Shortly after landing, the airplane began to shake and then after traveling about 100 ft, veered left. He immediately applied right rudder and felt the instructor was doing the same. The airplane then dropped onto its belly. The pilot did not complete an National Transportation Safety Board Pilot/Operator Aircraft Accident/Incident Report Form 6120.1, provide any further updates to his statement, or respond to multiple requests to clarify the circumstances of the accident. The instructor provided an extensive statement, reporting that the mechanic who had approached him to fly requested that he be especially vigilant that the landing gear was down and locked before landing. The instructor reiterated in the statement that he did not manipulate any controls until the airplane began to veer off the runway, and that at no time did he operate the landing gear handle. The instructor reported that after the pilot extended the landing gear, he observed the green landing gear indicator light illuminate, and confirmed through the gear mirrors that they had extended. Photographs taken shortly after the accident revealed that the nose gear was in the extended position, and both main landing gear had partially retracted, with their gear doors remaining open. The flaps appeared to be fully extended. A series of three black skid marks were observed on the runway, swerving from the centerline through to the airplane's final resting location just left of the runway edge. The outer width of the skids marks were about 34 inches. TESTS AND RESEARCHLanding Gear Operation The landing gear and flaps are extended and retracted by hydraulic actuators, powered by an engine-driven hydraulic pump and a pressure accumulator. The nose gear retracts forward and up, with its doors remaining open when the gear is extended. The main gear rotates aft and up into wells under the fuselage, with the doors remaining closed except during gear transition. Both the main and nose gear have positive mechanical up and down locks, operated by separate hydraulic actuators. Limit switches control two position indicator lights, which show that the gear is either up (red), or down and locked (green). The limit switches are connected in series, so that all three gears must be locked before either indicator light comes on. During the extension sequence, the nose landing gear locks into position before the main gear. The landing gear is controlled by the pilot through a four-position gear handle. The handle positions are, "Up-Operating", "Up-Neutral", "Down-Neutral", and "Down-Operating". To reposition the gear, the handle is pulled out and moved to the desired operating position, then released. A detent holds the handle in the operating position until the cycle is completed, at which point the handle automatically returns to the cycle's respective neutral position. A safety switch, actuated by the nose gear strut, restricts the gear position handle to prevent inadvertent retraction whenever the nose strut is compressed by the weight of the airplane. The instructor stated that he could not specifically recall if the gear handle was in the "Down-Neutral" or "Down-Operating" position, but that the mechanic, who was one of the first to arrive on scene, immediately entered the cabin, turned on the master switch, and moved the gear handle. The mechanic provided a statement indicating that when he arrived at the airplane, the gear handle was in the "Down-Operating" position, and it was not until later when he lifted the airplane with a hoist that he turned on the airplanes master switch. As soon as he did, the gear completed its cycle, the gear handle moved to the "Down-Neutral" position, and the green indicator light came on. Landing Gear Testing The airplane was removed from the accident site and stored at the mechanic's shop, where it was mounted on jack stands and examined by an FAA inspector the following week. No mechanical anomalies were observed to the landing gear system, beyond damage to the gear doors. The gear was successfully extended and retracted multiple times utilizing the gear handle, and both the red and green gear position lights illuminated appropriately.