On July 14, 2012, at 2320 central daylight time, a Piper model PA-44-180 airplane, N580ND, was substantially damaged during a wheels-up landing at Grand Forks International Airport (KGFK), Grand Forks, North Dakota. The flight instructor and pilot receiving instruction were not injured. The airplane was registered to and operated by the University of North Dakota, under the provisions of 14 Code of Federal Regulations Part 91 without a flight plan. Night visual meteorological conditions prevailed for the local instructional flight that originated from KGFK about 2200. The flight instructor reported that the accident occurred while the pilot receiving instruction was demonstrating a landing with a simulated single engine failure of the twin-engine airplane. He recalled that the airplane tail impacted the runway shortly after the pilot receiving instruction reduced engine power during the landing flare. The flight instructor noted that he heard the landing gear warning horn shortly after the airplane made contact with the runway. The airplane slid 200 to 300 feet on the lower fuselage before coming to a stop. The pilot receiving instruction reported that during downwind, after the flight instructor had simulated the single engine failure, he began to fall behind on completing required checklist items, and as result, he inadvertently forgot to extend the landing gear. He recalled hearing the landing gear warning horn while on crosswind, but reportedly was too distracted with flying the airplane to extend the landing gear at that point. He reported seeing sparks and hearing a grinding noise upon touchdown, at which time he also recalled hearing the landing gear warning horn. A postaccident examination revealed that the landing gear selector handle was in the "UP" position. The landing gear extended uneventfully into a down-and-locked position after the airplane was lifted off the ground and the landing gear selector handle was placed into the "DOWN" position. The operator reported that there were no mechanical anomalies that would have prevented the landing gear from extending normally. Additionally, the operator stated that several lower fuselage structural components were damaged during the wheels-up landing. The pilot receiving instruction held a private pilot certificate with an airplane single engine land rating. He was not endorsed to act as pilot-in-command of a complex airplane. A complex airplane is equipped with a retractable landing gear, flaps, and a controllable pitch propeller. According to the operator, the pilot was receiving flight instruction towards a complex airplane endorsement in conjunction with the multi-engine training that he was receiving. He had accumulated 6 hours in a complex, multi-engine airplane when the accident occurred. At 2253, the airport's automatic weather observing station reported the following weather conditions: wind from 040 degrees at 6 knots, visibility 10 statute miles, broken ceiling 25,000 feet, temperature 25 degrees Celsius, dew point 19 degrees Celsius, altimeter 29.99 inches of mercury.