Incident Overview
A glider pilot reported that prior to departure, an unsecured seat cushion was provided to a passenger to elevate his seating position and improve his view. After releasing from the tow plane and performing a 30-degree bank turn, the pilot leveled the glider and found he could not move the control stick fully aft in the pitch axis.
Troubleshooting Attempt
The pilot attempted to troubleshoot the blocked flight control and asked the passenger if he noted any obstructions. The passenger reported none. During the subsequent forced landing, the pilot was unable to safely maneuver to the departure gliderport and landed in an adjacent field, where the glider struck a fence.
Examination Findings
The pilot reported that examination of the aft cockpit after the accident revealed the seat cushion had shifted during flight, blocking the control stick. The cushion's internal metal frame and the aft control stick displayed witness marks consistent with contact during flight. The accident resulted in substantial damage to the fuselage.
Pilot Actions
The pilot stated that the seat cushion was not secured prior to flight, and its shift caused the control obstruction. No other causal factors were identified in the pilot's report.