The pilot of the gyrocopter reported that after flying over his passenger's house to take pictures, he headed back to the airport. He added that while in level flight, he noticed that "the trees on the ground under [him were] rapidly getting closer". He initially presumed something was wrong with the engine, so he reduced throttle and then successfully applied full throttle in an attempt to troubleshoot the situation. He made a left 180° turn and the gyrocopter struck trees and impacted terrain. The gyrocopter sustained substantial damage to the empennage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the gyrocopter that would have precluded normal operation. The pilot reported that he had presumed the terrain he was flying over was flat, but had later realized he had flown into rising terrain.