On January 11, 2003, at 1130 eastern standard time, a Piper PA-28-140, N573PA, was substantially damaged during a forced landing to a private airstrip near New Middletown, Ohio. The certified flight instructor and the certificated commercial pilot sustained minor injuries. No flight plan was filed for the flight that originated at New Castle Municipal Airport (UCP), New Castle, Pennsylvania, about 1050. Visual meteorological conditions prevailed for the local instructional flight, conducted under 14 CFR Part 91. Prior to departure, the commercial pilot informed the flight instructor that there had been a discrepancy with the throttle, and the cable was replaced. During the pre-flight inspection, engine start, taxi, and engine run-up, there were no discrepancies with the throttle. The flight instructor and commercial pilot flew to the practice area and practiced flight maneuvers which required low and high power settings. According to the flight instructor, at the completion of one of the reduced power maneuvers, at an altitude of 2,500 feet msl (1,500 feet agl), the commercial pilot went to apply power, but the throttle was "completely jammed." The flight instructor was unable to maintain altitude due to the idle power setting, and elected to land at a private airstrip. While he flew the airplane, the commercial pilot made several attempts to move the throttle, but without any success. The mixture and carburetor heat controls worked normally. In preparation to land, the flight instructor extended the flaps to 40 degrees, and maintained an approach speed of approximately 80 mph. Prior to reaching the end of the runway, the airplane encountered a high sink rate, and collided with trees. Two Federal Aviation Administration (FAA) inspectors performed an on-scene examination. According to one of them, the airplane clipped a tree that was located about 50 feet from the end of the runway. After it clipped the tree, the nose of the airplane hit the ground, and airplane flipped over. Examination of the airplane revealed that the throttle bracket was crushed from the impact, and the throttle linkage was kinked. However, the linkage was still intact at the carburetor. The inspectors removed the carburetor to help free the throttle linkage. Once removed, the throttle linkage moved freely, and fuel sprayed from the accelerator pump. No fretting or rubbing of the throttle linkage was noted. The throttle quadrant was removed and examined. No discrepancies were noted with the system. Fuel was present in the carburetor, main fuel line, and both wing tanks. New Middleton Airport was a private airport that was oriented North and South. The snow covered, turf runway was approximately 3,000 feet long and 50 feet wide, with a field elevation of 1,000 feet msl. The certified flight instructor reported 3,073 flight hours, of which, 522 hours were in make and model. The commercial pilot reported a total of 435 flight hours, of which, 60 hours were in make and model. Weather at Youngstown Airport (YNG), Youngstown, Ohio, about 15 miles north, at 1151, was reported as winds from 230 degrees at 15 knots gusting to 20 knots, visibility 10 statute miles, scattered clouds 2, 800 feet, broken clouds at 3, 500 feet, temperature 46 degrees F, dewpoint 57 degrees F, and barometric pressure setting 30.01 inches HG.