On February 13, 1994, at 1433 central standard time, a Piper PA- 44-180, N190ND, registered to University of North Dakota, and operated by a flight instructor and student, experienced a collapse of the right main landing gear on touchdown at the Grand Forks International Airport, Grand Forks, North Dakota. The airplane sustained substantial damage. The occupants reported no injuries. The 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The local instructional flight departed Grand Forks, North Dakota, at 1345. The instructor pilot stated that when the landing gear was extended the right main landing gear light began flashing and an unsafe gear warning light was illuminated. Several attempts were made to assure that the landing gear was extended including repeated attempts at normal and emergency gear extension. Maintenance personnel were contacted by radio in an attempt to alleviate the problem. A low pass was made over the airport to attempt to determine if the gear appeared to be extended. The tower reported that the gear appeared to be down. Upon landing the right main landing gear collapsed. A subsequent examination revealed that a cotter pin on the right main landing gear scissors was open and caught a wiring bundle in the wheel well which prevented the down lock from engaging.