On July 1, 2009, at 0830 mountain daylight time, a Cessna T210L airplane, N2118S, was substantially damaged during landing at Dona Ana County Airport (5T6), Santa Teresa, New Mexico. The private pilot was not injured. The flight was being conducted under the provisions of Title 14 Code of Federal Regulations Part 91 without a flight plan. The flight originated at Cielo Dorado Estates Airport (TA50), El Paso, Texas, and was terminating at the time of the accident. Visual meteorological conditions prevailed at the time of the accident. The pilot reported he lowered the landing gear handle on downwind to land and heard the gear cycle down. He told the investigator he could not recall if he saw green, gear down indicator lights. While in the landing flare he heard someone call "gear, gear" on the radio, but it was too late for him to go around. The left main gear collapsed on touchdown and the airplane exited the side of the runway. The pilot did not recall hearing the gear warning horn prior to landing. Examination of the airplane revealed substantial damage to the horizontal stabilizer and the elevator. The right main and nose landing gear were found in a down and locked position and the left main was in the gear well. During initial inspection by FAA investigators the left landing gear did not extend fully and lock into position during manual extension, but the gear operated correctly and did lock down when hydraulic power was applied to the system. The hydraulic system was found to be properly serviced and the three gear indicator lights operated properly when the gear was extended and locked. The airplane was taken to a repair station and the landing gear was examined further. No abnormalities were found with the airplane at that time. The witness who made the radio call of “gear, gear” could not be located. In the months after the accident, and following the repairs to the airplane, the pilot had four additional landing gear malfunctions and failures to extend. He said he had to manually extend the gear during each instance. After the fourth occurrence, the nose gear up-lock actuator and the landing gear accumulator were removed and resealed. The seal in the landing gear accumulator was found to leak. The accumulator requires gaseous nitrogen or dry air. The seals from the accumulator were not retained by maintenance personnel and were not available for examination by investigators. The reason for the failure of the seals was not determined.