The flight instructor was giving instruction in a multi-engine, retractable landing gear-equipped airplane. On the downwind leg in the pattern he and asked the student to perform a simulated single-engine emergency landing with the left engine shutdown. It was the student's first training flight in a multi-engine airplane, and he asked a series of questions to the flight instructor during the procedure. The instructor reported that they both became distracted and forgot to extend the landing gear. The airplane landed with the landing gear retracted and came to rest on the runway. The airplane sustained substantial damage to the wing-spar and longerons. Per the National Transportation Safety Board Pilot Aircraft Accident Report, in the Recommendation section the instructor reported that the accident could have been prevented by implementing a sterile cockpit during critical phases of flight, per the flight school's operations manual. The pilot reported that there were no mechanical malfunctions or failures with the airplane that would have precluded normal operation.