On July 20, 1996, about 1159 Alaska daylight time, a Tundra tire equipped Piper PA-18 airplane, N2561N, sustained substantial damage while back taxiing for takeoff on runway 7 at the Mc Grath Airport, Mc Grath, Alaska. The solo private pilot was not injured. The local flight was departing for Palmer, Alaska, and had previously departed a rural mining airstrip located 52 miles northeast of Kotzebue, Alaska, at 1000. The 14 CFR Part 91 flight operated in visual meteorological conditions without a flight plan. During a telephone conversation with the NTSB investigator-in-charge on July 25, 1996, the pilot stated that he was ferrying the aircraft to Palmer for repairs. He said that while taxiing for takeoff at Mc Grath, the left tire went flat, and the left wing struck the ground, damaging the left wing spar. An FAA flight service station specialist who was on duty at the time of the Mc Grath accident, looked at the airplane after the accident. His statement is attached. The pilot had been involved in an earlier airplane accident on July 13, 1996, while landing N2561N at the aforementioned mining airstrip (reference NTSB Accident Report ANC96LA126). In the early accident, a portion of the right main landing gear collapsed, and the airplane sustained damage to the landing gear, right wing spar, right wing lift struts, and fuselage. The pilot made unauthorized repairs to the main landing gear by fashioning a right main gear leg strut out of iron water pipe, and made repairs to the lift struts by taping boards to them. The pilot then elected to make a flight from the mining strip to Palmer, Alaska, for repairs. En route, the pilot stopped for gas at Mc Grath. The ferry flight for repairs was not authorized by the FAA, and no authorized aviation mechanic had inspected or made any repairs to the airplane prior to the ferry flight, or while the airplane was at Mc Grath. Approved aviation repair facilities were available at Mc Grath. Conversations with an FAA inspector, and a manufacturer of over-sized Tundra tires, disclosed that proper landing gear geometry is essential for the tires to roll efficiently and to keep side loads to a minimum. If excessive side loads are placed upon the tire, the tire may rotate on the rim, slicing the valve stem, and allow the tire to deflate.