On December 6, 2004, about 1243 eastern standard time, a Piper PA-28-181, N294PA, collided with a Piper PA-28R-201, N283PA, while both aircraft were waiting for takeoff on runway 9, at Fort Pierce-St. Lucie County Airport, Fort Pierce, Florida. Both flights were on 14 CFR Part 91 instructional flights. Visual meteorological conditions prevailed at the time and no flight plans were filed for either flight. The commercial-rated flight instructor and the private-rated dual student on N283PA, and the student pilot on N294PA were not injured. N283PA sustained substantial damage and N294PA sustained minor damage. The flights were originating at the time of the accident. The student pilot in N294PA stated he completed his engine run-up in the designated engine run-up area and then taxied to the holding position for runway 9, at A-4. He brought his airplane to a stop behind N283PA. He stated he usually sets the parking brake, selects 1,000 engine rpm, and switches off the landing light. He does not remember doing these steps. He does remember holding pressure on the toe-brakes. As he waited, he turned around to look for a bottle of water in the back seat. When he did, he took his feet off the toe-brakes, thinking the parking brake was set. A moment later, while still turned around, he heard a loud bang and felt his airplane shaking. He immediately looked in front of him and saw that his propeller had hit the tail of N283PA. The flight instructor on N283PA stated he and his student had been holding second in line for departure for a few minutes prior to the accident. They felt an impact and their airplane skidded forward a few feet. He opened the cabin door and looked back to see N294PA behind them with the engine stopped, and the propeller spinner buried in the empennage of his airplane.