On September 16, 2018, at 1606 eastern daylight time, a Cirrus SR22, N161DL, was substantially damaged during collision with trees and terrain during takeoff from Montauk Airport (MTP), Montauk, New York. The pilot was not injured, and a passenger sustained minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight, which was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.The pilot provided a written statement and was interviewed by telephone. He said he completed the preflight inspection, engine run-up, and before-takeoff checks with no anomalies noted, and then positioned the airplane for takeoff from runway 24. The pilot advanced the throttle and tracked the runway centerline during the takeoff roll. At rotation, the pilot's seat "abruptly slid backwards to the outermost distance from the controls." The pilot said that he could no longer reach the pedals to maintain directional control, and that aileron input was inadequate to counteract the airplane's left-turning tendency. The airplane departed the left side of the runway, struck trees and shrubs, and came to rest upright. According to the pilot, except for the pilot seat, the airplane performed as designed with no other anomalies noted. The pilot stated that he had adjusted his seat prior to the flight "as normal" and had never had difficulty with the seat at any time in the past. He added that this was the third Cirrus SR22 that he had owned and had no history of seat issues with either of his previous two airplanes. The pilot held a private pilot certificate with ratings for airplane single engine land and instrument airplane. His most recent Federal Aviation Administration (FAA) third class medical certificate was issued January 4, 2018. He reported 750 total hours of flight experience, all of which was in the accident airplane make and model. According to FAA records, the airplane was manufactured in 2014. Its most recent annual inspection was completed October 15, 2017 at 187.9 total aircraft hours. At 1654, the weather recorded at MTP included clear skies and variable winds at 4 knots. The temperature was 24°C, and the dew point was 19°C. The altimeter setting was 30.21 inches of mercury. Examination of photographs revealed that the wings, fuselage, and empennage all sustained substantial impact damage. An FAA inspector who responded to the site said he could not enter the cockpit, as the door was either locked or impinged by impact damage. Examination of the pilot's seat was performed by a manufacturer's representative under the supervision of an NTSB air safety investigator, and cabin safety specialist. The examination revealed no anomalies with the installation, dimensions, or operation of the pilot's seat. The seat moved freely fore and aft, with no binding or anomalous operation noted. While seated in the pilot's seat, the manufacturer's representative applied fore, aft, and twisting forces to the seat to "unlock" the seat from a locked position. When "twisted to right while being slid forward" partial seat position locking pin engagement could be induced, but not all pins would seat and the control handle would not be fully down, and neither could it be forced into position. Straightening or forward movement of the seat resulted in full pin engagement with the control handle in the fully-down position. According to the Pilot Operating Handbook and the placard on the center console, "CREW SEATS MUST BE LOCKED IN POSITION AND CONTROL HANDLES FULLY DOWN BEFORE FLIGHT"