**This report was modified on December 22, 2016. Please see the docket for this accident to view the original report.** The pilot of the parked airplane reported that he and his co-pilot had just re-entered the airplane and were having a conversation when he felt the airplane shake and heard a loud noise. He further reported that he looked out of the right window and saw an aircraft with its propeller hitting the right wing. The airplane sustained substantial damage to the right wing. The pilot of the other airplane reported that, after engine start and before taxi for takeoff, he was looking at his navigation communications and did not notice the airplane rolling forward. The airplane rolled forward and collided with the airplane in front of it. The pilot further reported that he was unaware of the airplane's forward movement and that the thought that the hand brake was fully engaged. The airplane sustained minor damage. Neither pilot reported any preaccident mechanical malfunctions or failures with the airplanes that would have precluded normal operation.