The 289-hour private pilot elected to takeoff from runway 35, a 2,450-foot long, by 55-foot wide grass airstrip that sloped downhill, with a known 15-knot tailwind. The pilot stated in the Pilot/Operator Accident/Incident Report (NTSB Form 6120.1) that he had previously departed from the same airport a few times during similar weather conditions. The pilot, who reported having accumulated a total of 200-hours in the same make and model aircraft, indicated that he configured the airplane for takeoff with 20 degrees of flap since the runway was "a rough grass airstrip." The pilot stated that the takeoff was normal and climb performance was normal until he reached tree-top level. At that time the airspeed dropped to about 40 miles per hour and the aircraft stopped climbing. The pilot added that it became apparent to him at that time that he was not going to clear the trees and began to maneuver around the trees in an attempt to turn the airplane into the wind. The pilot further stated that "we got boxed into a corner and wound up hitting some trees on the northwest side of the field." A large tree trunk impaled the windshield and the airplane came to rest nose-down and in an inverted position. There was no post crash fire. The pilot received serious injuries and his passenger received minor injuries. Both occupants were able to egress from the single-engine airplane through the broken windscreen unassisted. The pilot reported that "he had been taught that on runways with a very steep hill [slope], the pilot should always land uphill and takeoff downhill, regardless of wind direction." Recorded weather at the nearest weather reporting station, located 15 nautical miles to the south of the accident site, were reporting wind from 160 degrees at 14 knots, gusting to 22 knots, clear skies, and a temperature of 32 degrees Celsius. The Investigator-in-Charge calculated the density altitude at 2,928 feet.