On September 6, 2003, at 2345 Pacific daylight time, the propeller of a Piper PA-28-161, N9141Y, which was standing in the tie down area with engine running, partially severed a bystander's arm at Sacramento Executive Airport (SAC), Sacramento, California. The private pilot operated the rental airplane under the provisions of 14 CFR Part 91. There was no damage to the airplane. The private pilot, who was in the airplane at the time, was not injured, and his friend, who was cut by the propeller, sustained serious injuries. The flight was preparing to depart SAC for Hayward Executive Airport (HWD), Hayward, California. Night visual meteorological conditions prevailed, and no flight plan had been filed. There were no witnesses to the accident; however, a corporate flight crew was conducting a preflight near the airplane. The Safety Board investigator-in-charge (IIC) interviewed the captain. He stated that the first officer had observed the pilot and passenger walking to their airplane. The flight crew then went about their business. The captain indicated that the auxiliary power unit (APU) was running at the time. He heard someone yell, and then heard a second yell. When he turned to look he saw the pilot and passenger walking towards the security office. He stated that something wasn't right with the passenger so he went over to check on them. The captain provided first aid assistance until the emergency medical service crew arrived. The captain further indicated that neither he nor the first officer touched the airplane during or after the incident. In the pilot's statement to the Safety Board he indicated that he asked the friend to bring some merchandise to the airplane. The pilot then told his friend that he was going to start the engine and to stay clear of the airplane. He started the airplane, and when he advanced the throttle to move forward he felt something sticking underneath the airplane. He asked his friend to come to the window so he could ask her to check and see if the airplane was stuck on anything. The friend replied that there was nothing stuck underneath airplane. She then turned to walk away from the airplane, and the propeller struck her right arm. He shut down the engine and then yelled for help, while attending to his injured friend. According to airport personnel, the day after the incident, it was noted that the right wing was secured with a tie down chain. The left wing and tail tie downs were not secured.