Power cables present a persistent threat to helicopter pilots and have been responsible for a number of crashes in recent years. Active sensors are currently being developed to detect obstacles such as exposed power wires. However, the ability of passive sensors, such as forward-looking infrared (FLIR) systems currently employed on many rotary-wing aircraft, to detect such wires remains unclear. This study details observations, assumptions, and models that support the conclusion that wires are essentially reflective in nature, thus their signatures on passive thermal sensors depend principally on their surroundings. Because of this irregular nature and inconsistent appearance, predicting the performance of FLIRs in detecting wires is difficult and the utility of FLIRs as wire warning sensors is open to question.