On June 14, 2018, about 0950 eastern daylight time, a Piper PA-34-200, N56213, was substantially damaged during a runway excursion while landing at Umatilla Municipal Airport (X23), Umatilla, Florida. The flight instructor was not injured while the pilot undergoing instruction (PUI) sustained a minor injury. The airplane was owned by Elite Florida AC LLC, and operated by Spaceport Aviation under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the instructional flight, which originated from Space Coast Regional Airport (TIX), Titusville, Florida, about 0845, and was destined for X23.The PUI and/or the flight instructor stated that the flight proceeded to X23 where the downwind leg of the airport traffic pattern was extended, and the required before landing checks were performed. The PUI stated that he extended the landing gear, and the flight instructor confirmed there were 3 green lights displayed indicating all landing gears were down and locked. After touchdown on runway 01 at 80 miles-per-hour, the airplane suddenly began an uncommanded and violent swerve to the right, which the PUI could not correct with full left brake, rudder, and aileron inputs. The airplane departed the runway and impacted a drainage ditch. The PUI stated that while assessing the airplane with the mechanic from X23, they noticed the securing hardware for the upper and lower torque links of the right main landing gear was not in place. According to a Federal Aviation Administration inspector who responded to the accident site, a search was made for the missing right main landing gear hardware and it was not located. He inspected both disconnected torque links and did not see damage to either, nor did he see scoring damage to the inner diameter of the right main landing gear torque link brass sleeve that the attachment bolt passes through to secure the torque links together. The brass sleeve was measured and found to be round and not elongated. He also inspected the left main landing gear and found its torque link secured with the bolt, washer and castellated nut, but the cotter pin was missing. A review of the airframe maintenance records revealed during the airplane's last annual inspection which was signed off as being completed on May 16, 2018, new hardware was installed in both main landing gear torque links.The mechanic who signed off the inspection stated that as part of the new hardware at the torque links, cotter pins were installed, but because of clearance issues during subsequent gear retraction testing, the hardware at both torque links was loosened to adjust each brake hose, then the hardware was tightened after adjustment, but the cotter pins at both gear torque links were not installed. A review of the airplane's Pilot's Operating Handbook revealed that as part of the preflight inspection, it specified a visual inspection of the right main landing gear for leaks, the tires for inflation and wear, and the landing gear strut piston for proper extension. There was no direct mention to inspect the torque links for security.