On October 21, 1993, at 1840 hours mountain standard time, a Beech BE-76, N5148M, collided with terrain while attempting to go-around at Scottsdale Airport (SDL), Scottsdale, Arizona. The pilot was conducting a local visual flight rules instructional flight. The airplane, operated by Aero/Mech Inc., Scottsdale, sustained substantial damage. Neither the certificated commercial pilot/flight instructor (CFI), the certificated commercial pilot/dual student, nor the nonrevenue passenger was injured. Visual meteorological conditions prevailed. The flight originated at Scottsdale Airport at 1710 hours. The CFI reported that the dual student was practicing a simulated single-engine landing with the left engine throttle retarded to a zero thrust setting. When the flight was on final approach at 300 feet above ground, the local controller instructed the flight to go-around. The dual student applied full power on the operating engine and began a singe-engine go-around. The operating engine did not produce sufficient power and the dual student then applied full power on the idling engine. The engine did not respond to the power application. When the CFI realized that the airplane was unable to maintain sufficient altitude to return to the airport, he elected to land, gear-up, in an open field. The CFI also said that during the initial descent the student did not "clear the engine", nor did he instruct him to do so. Mr. Michael Brown, Aviation Safety Inspector, Federal Aviation Administration (FAA), Scottsdale Flight Standards District Office, told Safety Board investigators that he spoke with the operator's maintenance manager. The manager told him that both engines operated normally when tested after the accident. He said that the left engine spark plugs contained extensive carbon and soot deposits. These signatures are indicative that the engine was idling for a prolonged period at a rich mixture setting.