On June 1, 2018, about 1118 Pacific daylight time, a Cessna 210 airplane, N9410M, sustained substantial damage when it was involved in an accident at Ramona Airport (RNM), Ramona, California. The pilot and the passenger were uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot stated that, while approaching RNM after a 4-hour flight, he selected the landing gear handle to the “down” position; however, the green gear down light did not illuminate and the main landing gear did not lock in the down position. He then selected the landing gear handle up but the landing gear did not retract. He then attempted to manually pump the landing gear down using the emergency hand pump, but the handle was very difficult to move, and then it would not move. He conducted a visual approach and landing to the runway. During the landing, the right main landing gear (MLG) collapsed, followed by the right wing and horizontal stabilizer impacting the ground. The airplane then veered right, departed the runway, and came to rest on the parallel taxiway. The mechanic who recovered the airplane reported seeing a lot of hydraulic fluid on the airplane’s belly. Examination of the airplane revealed that right horizontal stabilizer and elevator had sustained substantial damage. The landing gear system was operated with an external hydraulic pump, and the landing gear extended and retracted normally; however, during the manual operation of the system, the right MLG down-lock did not engage consistently. The airplane’s engine-driven hydraulic pump was mounted on an external electric-driven system, and when the hydraulic system was pressurized, fluid started leaking from a gap in the diaphragm seal between the rear housing assembly and front plate assembly, and fractured pieces of the diaphragm seal were visible in the gap (see figure 1). The right MLG would not cycle when powered by the leaking pump. When an airworthy pump was used to test the MLG, it functioned properly. Figure 1. A photograph showing the engine-driven hydraulic pump before disassembly. Four cap screws were installed and holding the assembly together; however, the two front plate cap screws on the side with the leaking seal gap were loose and could not be tightened. The screws only had three or four threads engaged with the rear assembly, and the one screw’s shank length was .173 inch shorter than the manufacturer’s specified minimum shank length, and the other was.158 inches shorter than the minimum shank length. The corresponding rear housing internal threads were sheared and displaced. All four of the screws had evidence of locking compound on the threads, which was inconsistent with the aircraft service manual instructions. Only two of the four cap screws had torque markings present.