On July 3, 2005, about 1615 Pacific daylight time, a Scheibe SF-28A Tandem-Falke, N12HS, experimental motor-glider, registered to and operated by the pilot as a 14 CFR Part 91 local flight, was substantially damaged during an off airstrip landing near North Plains, Oregon. Visual meteorological conditions prevailed at the time and no flight plan was filed. The private pilot and his passenger were not injured. The glider was operating in the local area and had departed from the North Plains airstrip about one hour prior to the accident. The pilot reported during a telephone interview and subsequent written statement, that he was performing an engine off landing to the west, with a three to four knot tailwind, at the North Plains Glider Port. The pilot stated that the approach was normal until final approach when the airspeed indicator suddenly indicated a drop in airspeed to 20 mph. The pilot initially thought that it was wind shear or a stall and pushed the nose down and closed the spoilers. The airspeed indicator then indicated 70 mph and the glider's descent rate decreased. Because of the decrease in descent rate, the pilot now determined that his airspeed and altitude were too high for a safe landing on the airstrip. The pilot opted to land in an open field adjacent to the airstrip. After making the decision to land in the field, the pilot, in an attempt to stay aloft longer, tried to start the engine even though he was aware that operating procedures did not recommend it. The pilot stated that he left the cowl flaps closed, left the fuel off because he believed that there was enough fuel in the system to start the engine, and he left the propeller feathered so as not to increase drag. The engine did not start and the pilot continued with the off airstrip landing. After the glider flew about 10 feet above ground level over a glider and tow plane on the ground waiting to takeoff, it suddenly stalled, dragging a wing and impacting the ground hard in a field next to the airstrip. The glider completed about a 120 degree turn before coming to rest. The field was covered with about three foot tall grass. In a subsequent written statement, the pilot reported that in May 2005, during stall testing, the airspeed indicator dropped to 20 mph and then jumped to 70 mph. After this event, the pilot had the line and airspeed indicator inspected. During this inspection, a couple of drops of water was found at the bottom of the drain plug. The line was cleared and there were no further problems. The pilot reported that the glider is kept outside and under a tarp. The pilot reported no anomalies with the airspeed indicator during the hour flight prior to the accident. The pilot reported a total flight time of 260 hours in all aircraft with 26.6 hours in gliders. The pilot reported a total flight time of 5.2 hours in the make and model glider involved in the accident, with 3.8 hours as pilot-in-command. The pilot reported under the recommendations on how the accident could have been prevented section of the NTSB Pilot/Operator Report Form five items. Three of the items indicated a lack of training in preparing the pilot for the emergency situation. The other recommendations identified back-up safety procedures that he might have done.