On July 16, 2015, about 1115 eastern daylight time, a Cessna 162, N6047W, was substantially damaged when it impacted trees and terrain after takeoff from Chautauqua County/Dunkirk Airport (DKK), Dunkirk, New York. The private pilot received minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the local personal flight. The airplane was registered to and operated by a private individual under the provisions of Title 14 Code of Federal Regulations Part 91. A witness located at the airport observed the airplane during its initial climb after takeoff. He stated that, at an altitude about 100-200 feet, the airplane made an "abrupt" left turn, lost altitude, then pitched to a nose-up attitude. He then observed the airplane lose altitude and pitch up a second time, and stated that the airplane was in an "aggressive" nose-up pitch attitude as it sank below the tree line and out of his view. The pilot reported to a Federal Aviation Administration (FAA) inspector that he conducted a preflight inspection of the airplane and observed no anomalies. He then conducted two touch-and-go takeoffs and landings in the airport traffic pattern before the accident takeoff. On the third takeoff, during climbout from runway 6, the airplane's right cabin door opened at an altitude about 400 feet. The pilot attempted to reach across the right seat and close the door, but could not reach the handle. He stated that the airplane "did not want to fly" and "began to sink." The airplane subsequently impacted trees and the pilot egressed. The airplane came to rest upright, in a nose-down attitude among trees about one mile northeast of runway 6 at DKK. Postaccident examination of the airplane by an FAA inspector revealed substantial damage to both left and right wings and the fuselage. The fuel tank sight glasses indicated that each fuel tank was about one-quarter full. Control continuity was established from the cockpit to all flight control surfaces. Impact damage precluded a functional test of the right door latches; however, visual inspection revealed no anomalies. The pilot held a private pilot certificate with a rating for airplane single engine land. He reported 420 total hours of flight experience, of which 90 hours were in the accident airplane make and model. The airplane was manufactured in 2011 and was equipped with a Continental Motors Inc., O-200 series, 100 hp reciprocating engine. Its most recent condition inspection was completed December 14, 2014. At the time of the accident, the airplane had accrued 90 total hours of operation. According to the pilot's operating handbook for the airplane, a pilot should not attempt to close an open cabin door inflight. The written procedure stated that the pilot should leave the door open, reduce throttle as necessary for an airspeed below 80 knots, secure seatbelts and loose items in the cockpit, and land as soon as practical. Landings with an open door were to be performed normally.