The failure of the helicopter pilots and the airplane pilot to maintain an adequate visual lookout for known traffic in the traffic pattern, which resulted in a midair collision. Contributing to the accident were the airplane pilot's descent below the published airplane traffic pattern altitude (TPA) and the helicopter pilot's climb above the proper helicopter TPA as prescribed in the Federal Aviation Administration's Aeronautical Information Manual for airports without published helicopter TPAs. Also contributing to the accident were the lack of a published helicopter TPA, the absence of radar equipment in the tower, and the controller's inadequate task prioritization.