On March 22, 2010, about 1430 Pacific daylight time, a Willams Yakovlev 3UPW, N153U, sustained substantial damage following the collapse of the left main landing gear during landing roll at the Half Moon Bay Airport (HAF), Half Moon Bay, California. The private pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed for the local flight, which was being operated in accordance with 14 Code of Federal Regulations (CFR) Part 91, and a flight plan was not filed. The airplane departed HAF about 1420. In a statement submitted to the Safety Board investigator-in-charge (IIC), the pilot reported that after taking off and returning to the airport traffic pattern, he extended the landing gear. The pilot stated, “Everything was in the green. Three lights for the gear and mechanical indicators showing the gear down. I made a normal landing, and when the full weight of the plane was on the mains the left [landing] gear collapsed.” The pilot added that the airplane went off the left side of the runway and came to rest on a heading of 120 degrees after landing on runway 30. The pilot reported that the outer one-third of the left wing, the left aileron and the left flap were bent. The pilot further reported that the propeller blades were bent, the tail wheel had broken off, and the left landing gear doors were bent. The pilot revealed that the airplane was manufactured in 1995 and not assembled until 2005, when he purchased it. On April 27, 2010, under the supervision of a Federal Aviation Administration (FAA) airworthiness inspector, a FAA certified airframe and powerplant mechanic conducted a teardown examination of the airplane’s left main landing gear pneumatic actuator. After removing the actuator and applying shock air to the component, the mechanic reported that operation of the actuator appeared to be normal, with no internal or external leaks noted. The mechanic stated that downline air pressure normally forces the down lock balls into the down lock track to lock the actuator in the down position. When the actuator was disassembled, the mechanic reported that he observed that the actuator’s down lock ball track was contaminated with dirt and that the steel down lock balls were corroded. The mechanic stated, "I believe a combination of dirt in the down lock track, corroded down lock balls, and low down lock pressure caused the gear to collapse.” The FAA inspector reported that he concurred with the mechanic’s statement.