Aerial firefighting is a common method of fighting large scale wildfires and for remote fires when ground systems are not deployable. Wildfires are always risky operations, even for helicopter pilots. Obstacle detection systems now being developed for unmanned air vehicles show promise for helicopter applications, either as sense-and-alert systems for piloted aircraft or as sense-and-avoid systems for unpiloted aircraft. Our team customized a commercially available rotorcraft UAV to demonstrate autonomous obstacle avoidance in flight, while carrying a payload of water to be used in extinguishing a flame. Using a requirement-based development cycle, we selected a variety of lightweight, commercially available sensors to assist the UAV with obstacle avoidance, fire detection, and flight control systems. We installed and interfaced sensors with the existing UAV flight control system, and programmed closed-loop flight control algorithms. System performance was independently proven through requirements driven test procedures.