On April 14, 2009, about 1935 Alaska daylight time, a Piper PA-14 airplane, N4246H, sustained substantial damage during takeoff at Merrill Field, Anchorage, Alaska. The airplane was being operated by the pilot as a visual flight rules (VFR) personal local flight under Title 14, CFR Part 91, when the accident occurred. The solo private pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on April 15, the pilot said when he applied power for takeoff, the seat slid back as the airplane accelerated, keeping him from reaching the rudder pedals to maintain directional control. He said the airplane ground-looped to the left, striking the right wing on the ground. The pilot said there were no known mechanical problems with the airplane prior to the accident. The airplane sustained structural damage to the right wing. During a conversation with the IIC on May 7, an FAA inspector who examined the airplane, said the front seats of the accident airplane had been modified with adjustable seat tracks. The modification was done with an FAA field approval. He said it appeared that the mechanical stop, which was supposed to be installed on the left track, had been installed on the right track. The stop was designed to keep the seat from being pulled too far forward and preventing it from latching properly. He said because the stop was installed on the wrong track, the pilot was able to pull the seat locking pin forward of the last locking hole. The locking pin fell between two screw heads, and with the pilot's weight forward on the seat, it created the feeling that the seat was locked in position. He said during acceleration, with the pilot's weight on the back of the seat, the pin jumped over the screw head, and the seat slid back.