On April 8. 1996, at 1730 central daylight time, a Cessna 182Q, Mexican registration XBCHP, was substantially damaged upon impact with trees while on initial takeoff climb from the Boerne Stage Field, near Boerne, Texas. The commercial pilot sustained serious injuries, while the non-rated mechanic aboard sustained minor injuries. The airplane was owned by a private owner in Nuevo Laredo, State of Tamaulipas, Mexico. Visual meteorological conditions prevailed for the maintenance test flight conducted under Title 14 CFR Part 91. The flight was originating at the time of the accident. At the time of the accident, the airplane was being operated by Texas Skyways Inc., of Boerne, Texas. The pilot reported that a supplemental type certificate (STC) was applied to the airplane to replace the engine and propeller. A complete maintenance test flight was previously completed by the pilot. The propeller RPM was found to be out of adjustment, and the pilot elected to take the owner's mechanic with him on the flight to verify the propeller RPM as well as to show the improved performance of the airplane with the new engine-propeller package. The pilot told the FAA inspector that he "used poor judgment" by electing to take off beyond the mid field point of the 3,400 foot runway. The pilot added that he had previously performed this feat under different conditions. During the accident flight, the weight of the airplane was considerably higher since not only he carried a passenger, but the airplane was serviced with 75 gallons of fuel. Additionally, the density altitude was established at 3,200 feet, and the winds at the accident site were reported as "light from the south." Witnesses observed the airplane initiating the takeoff run to the north with 600 foot of usable runway remaining. On the enclosed pilot/operator report, the pilot stated that "once airborne the airplane continued to accelerate but failed to clear the trees past the end of the runway." He further stated that after the right wing impacted a tree, "the airplane continued until striking a power line, stalled and fell nose down into other trees." The airplane came to rest in the nose down attitude atop the trees. The pilot told the FAA inspector that the engine was performing well and reported no maintenance anomalies prior to the accident. Performance figures were not available for the STC modified aircraft. Repeated attempts to obtain a completed pilot/operator report from the owner of the airplane, and a passenger statement from the mechanic were unsuccessful.