On March 5, 1994, at 1100 mountain standard time, a Piper PA-23- 180, N2142P, collided with terrain following an in-flight loss of control near Fairfield, Arizona. The accident sequence was precipitated by the flight instructor inadvertently feathering the left propeller. The aircraft was operated by Proflight of Las Vegas, Nevada, and was on a dual instructional flight. Visual meteorological conditions prevailed at the time of the instructional flight. The airplane was substantially damaged upon colliding with terrain about 10 miles south of the airport in the state of Arizona. The airline transport certificated instructor pilot and the second pilot, who was receiving instruction for a multiengine flight instructor rating, were seriously injured. The two passengers were not injured. The flight originated from Las Vegas, Nevada, on March 5, 1994, at 0815 Pacific standard time as a dual instructional flight. According to the pilot under instruction and the passengers, during the accident flight the flight instructor was in the final process of preparing the second pilot to take his multiengine flight instructor check ride. At the completion of the (accident) flight, the second pilot had planned to use the airplane for the check flight. The occupants further reported that after completing a touch-and- go landing at St. George, Utah, the airplane climbed between 200 and 300 feet above the 2,938-foot mean sea level field elevation. Then, the flight instructor attempted to retard the left engine's throttle, but accidentally retarded the left engine's propeller control. The left engine's propeller blades feathered. The flight instructor maneuvered the airplane over lower elevation terrain while attempting to restart the left engine; however, the aircraft ended up crossing the Arizona border and overflying hilly terrain. The pilot lost control of the airplane as the airspeed decreased. The left wing lowered and the airplane collided with hilly terrain. The pilot under training reported that the instructor retarded the left engine propeller control as the landing gear was retracting. The only engine-driven hydraulic pump is located on the left engine and the flight instructor had to pump the landing gear completely up by hand. During this time, the aircraft continued a gradual descent toward the ground. The pilot under training and the two passengers reported that the flight instructor by-passed several suitable forced landing sites in his attempt to restart the left engine. After recovery of the aircraft, the left engine and propeller combination was removed from the aircraft and installed in an engine test cell. Under supervision of a Federal Aviation Administration (FAA) inspector, the engine was started three times, twice with the propeller feathered. During all three test runs, the engine started immediately and was exercised throughout its normal operating range. The propeller governor was noted to function smoothly and maintained control of the propeller rpm; the propeller blades were exercised into and out of the feather position easily with no hesitation.