On February 24, 2003, about 1432 eastern standard time, a Cessna 172RG, N5177R, registered to and operated by Comair Aviation Academy, experienced collapse of the main landing gear during the landing roll at the Orlando Sanford Airport, Sanford, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 instructional flight. The airplane was substantially damaged and the certified flight instructor (CFI) and pilot-rated student sustained minor injuries. The flight originated at 1130 from the Orlando Sanford Airport. Before the flight departed, hydraulic fluid was added to the landing gear hydraulic system. The CFI reported that after takeoff the landing gear retracted without incident. The flight proceeded to the practice area where airwork was performed. The flight continued and when the landing gear was extended prior to performing additional airwork, it failed to extend. The emergency checklist was followed but failed to extend the landing gear. The flight returned to the departure airport and the flight crew contacted dispatch in an attempt to lower the landing gear which was unsuccessful. Additionally, the CFI attempted to manually extend the main landing gear but was unable. The flight returned for landing and with the nose landing gear extended and the main landing gear in trail, the main landing gear collapsed at touchdown. Examination of the airplane by an FAA airworthiness inspector revealed the hand pump suction line assembly part number 2480001-23, was damaged. Hydraulic fluid covered the area. The line was retained for further examination. The damaged line was examined by the NTSB Materials Laboratory located in Washington, D.C. Examination of the line revealed at least eight transverse cuts in the tube section of the line adjacent to the end ferrule. A cut directly adjacent to the end ferrule penetrated the wall thickness and was slightly less than .03 inch wide. The sides of that cut were straight into the tube but the bottom of the cut had a curved surface progressing to a sharp bottom. The other cuts did not penetrate the wall thickness, and loss of metal at the cut locations appeared to be primarily from metal flow as opposed to being from grinding or cutting away material. There was no evidence that the edges of the ferrule or B-nut created the cuts in the tubing. The airplane minus the retained hand pump suction line was released to John McGann of Comair Aviation Academy, on March 10, 2003. The retained line was released to Jose Lema, of the Federal Bureau of Investigation, Maitland, Florida, on March 29, 2004.