What happened
A glider was in flight with a private pilot operating the controls and a certified flight instructor (CFI) occupying the rear seat. The CFI's intent for the flight was to observe the pilot's technique and judgment, as no formal instruction had been scheduled.
During the flight, the private pilot flew the aircraft beyond its gliding distance back toward the departure airport. Although the CFI stated he protested this decision, he permitted the pilot to continue the maneuver. While the private pilot remained in control, the aircraft was configured for a precautionary off-airport landing in a field. During this approach, the glider encountered an area of sink. The aircraft subsequently struck a tree, spun around, and impacted the ground in a nose-first orientation. There were no injuries reported.