At 1930 eastern time, a passenger boarding a Cessna U206G was seriously injured when he walked into the spinning propeller as the airplane was standing with the engine running at Kelleys Island Airport. The pilot and second passenger were not injured. The pilot said they arrived from Sandusky, Ohio, and he parked the airplane with the engine running to disembark two passengers. He said he did not consider it an unsafe practice because he had seen commuter airliners do it. The pilot set the parking brake and exited the airplane. He saw the two passengers exit the airplane and proceed to their vehicle. As the pilot was boarding a new passenger to Sandusky, one of the previous passengers returned and walked into the propeller, breaking his collarbone.