On July 30, 2013, about 1143 mountain daylight time, a Piper PA-44-180 Seminole, N5367Y, experienced a left main landing gear collapse during landing at Billings Logan International Airport, Billings, Montana. The airplane was registered to and operated by Rocky Mountain College under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certified flight instructor (CFI) and the commercial pilot undergoing instruction were not injured; and, the airplane sustained substantial damage. The local instructional flight departed from Billings about 1020. Visual meteorological conditions prevailed and no flight plan had been filed.In both a written statement and a telephone interview with a Safety Board investigator, the CFI reported that the flight was intended to follow a lesson plan in the multi-engine commercial pilot syllabus for Rocky Mountain College. After performing numerous maneuvers in the local practice area, they returned to the airport and performed a simulated single-engine inoperative landing to runway 28R. After touchdown, the commercial pilot taxied back to the runway and departed, maneuvering onto the downwind leg of the traffic pattern. He configured the airplane with the landing gear extended and visually confirmed that the nose gear was down by utilizing a mirror affixed to the wing; all three landing gear indicator lights in the cockpit were illuminated confirming the down and locked position. The CFI further stated that with the landing gear appearing to be extended, the commercial pilot continued with the practice short-field landing. After the landing flare, the airplane touched down on the runway surface and the left main landing gear collapsed. The propellers contacted the runway and the airplane skid about 800 to 1,000 feet. During the accident sequence the airplane incurred substantial damage to the wing spar and a section of the fuselage skin was wrinkled. The left wheel assembly had separated at the strut. The airport supervisor stated that he thoroughly inspected all airfield surfaces before and after the accident. He did not observe any evidence or anomalies that would indicate the airplane struck any objects. He did note that the airplane's propeller made slashes in the asphalt where the wheel was found. Based on the pilots' statements about the landing, the separated landing gear was shipped to the National Transportation Safety Board Materials Laboratory. A Senior Materials Engineer performed an examination of the fracture surfaces. He observed that the landing gear was fractured through the lower end of the strut tube just above its connection with the fork. The tube was bent, the fracture faces had a rough appearance, and the fracture faces were oriented at 45 degrees to the longitudinal axis of the tube. The fork was bent so that the top of the fork was impinging on the top of the tire. The appearance of the strut tube and the fork were consistent with an overstress event. A representative from Piper Aircraft looked at photographs of the airplane and stated that the damage was consistent with that of a hard landing.