The pax stated that he and the plt were at a bar where they had about three beers prior to this flight. During the flight the pilot did several stalls and some aerobatic maneuvers. The pax was unnerved and told the plt that he wanted to return to the arpt. During the final apch leg to the arpt the plt undershot the rwy and the acft collided with trees located about 475 ft from and below the elevation of the rwy. A witness saw the acft apch the arpt in a normal manner with the engine running before the collision. The plt was comatose for several days afterwards and has no recollection of the accident. The pax reported no problem with the acft before the accident. Exam of the acft did not disclose evidence of malfunction. The barometric pressure was 29.75. The altimeter was set to 30.40. According to the plt's logbook, the plt's last flt and last night flt were respectively 6 and 5.5 months before the accident date. A blood alcohol test was not performed on the plt because of his injuries.