On November 12, 1997, at 1130 hours Pacific standard time, a Cessna 172RG, N9451D, had a left main gear collapse on landing and came to rest off runway 28L at Montgomery Field, San Diego, California. The aircraft, operated by American Flyers, sustained substantial damage. The commercial rated flight instructor pilot and private pilot were not injured. Visual meteorological conditions existed for the local instructional flight and no flight plan was filed. The private pilot reported that he had been cleared for a straight-in landing on runway 28L by the control tower. He and the instructor visually checked that the landing gear was down for the short field landing. After landing, the aircraft started to vibrate and rotated to the left. The instructor pilot shut down the engine, and then right landing gear collapsed. The aircraft came to rest approximately 40 feet left of runway 28L. The pilot reported that, "at no time before, during or after touchdown was a gear warning horn heard from the overhead speaker." A Federal Aviation Administration (FAA) inspector from the San Diego, California, Flight Standards District Office examined the aircraft on-scene and found that the left main gear actuator piston was broken. An examination of the aircraft logbook by the Safety Board revealed that the main gear actuator piston had been replaced in November 1994. It had accumulated 1,530.2 hours since installation. Metallurgical and fractographic examinations were conducted under FAA supervision on the main landing gear actuator piston at the Cessna Aircraft Company facilities in Wichita, Kansas. The metallurgical examination revealed that the actuator piston was within manufactured specifications. During the visual examination it was noted that beach marks were present on the fracture surface that is characteristic of a fatigue crack growth. However, no other cracks were found in the actuator piston. The fractographic examination determined that the fracture outside of the fatigue region was the result of a single event overload and not due to the fatigue crack growth beach mark (see attached report).