Incident Overview
On June 1, 2018, at approximately 1118 Pacific daylight time, a Cessna 210 airplane, registration N9410M, sustained substantial damage during a landing at Ramona Airport (RNM) in Ramona, California. The pilot and the passenger were uninjured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Pilot Account
The pilot reported that after a four-hour flight, while approaching Ramona Airport, he selected the landing gear handle to the down position. The green gear-down light did not illuminate, and the main landing gear did not lock down. He then selected the gear handle up, but the landing gear did not retract. Attempts to manually pump the landing gear down using the emergency hand pump were unsuccessful; the handle became very difficult to move and eventually would not move. The pilot conducted a visual approach and landing to the runway.
Accident Sequence
During the landing, the right main landing gear (MLG) collapsed, followed by the right wing and horizontal stabilizer contacting the ground. The airplane veered right, departed the runway, and came to rest on the parallel taxiway.
Post-Accident Examination
A mechanic who recovered the airplane reported observing a significant amount of hydraulic fluid on the airplane's belly. Examination revealed substantial damage to the right horizontal stabilizer and elevator. The landing gear system was tested using an external hydraulic pump, and the gear extended and retracted normally; however, during manual operation, the right MLG down-lock did not engage consistently.
The airplane's engine-driven hydraulic pump was mounted on an external electric-driven system. When the hydraulic system was pressurized, fluid leaked from a gap in the diaphragm seal between the rear housing assembly and front plate assembly, with fractured pieces of the diaphragm seal visible in the gap. The right MLG would not cycle when powered by the leaking pump. An airworthy pump allowed the gear to function properly.
Findings on the Hydraulic Pump
Four cap screws were installed holding the pump assembly together. However, the two front plate cap screws on the side where the seal gap was located were loose and could not be tightened. These screws had only three or four threads engaged with the rear assembly. One screw's shank length was 0.173 inches shorter than the manufacturer's specified minimum, and the other was 0.158 inches shorter. The corresponding rear housing internal threads were sheared and displaced. All four screws showed evidence of locking compound on the threads, inconsistent with the aircraft service manual instructions. Only two of the four cap screws had torque markings present.