Under a program funded by the Air Force Research Laboratory (AFRL), Advanced Cooling Technologies, Inc. (ACT) has developed a series of passive thermal management techniques for cooling avionics. Many avionics packages are often exposed to environment temperatures much higher than the maximum allowable temperatures of the electronics. This condition prevents the rejection of waste heat generated by these electronics to the surrounding environment and results in significant ambient heat gain. As a result, heat must be transported to a remote sink. However, sink selection aboard modern aircraft is limited at best. Often, the only viable sink is aircraft fuel and, depending on mission profile, the fuel temperature can become too high to effectively cool avionics. As a result, the electronic components must operate at higher than intended temperatures during portions of the mission profile, which reduces component lifetime and significantly increases the probability of failure. To address this issue, ACT developed two passive thermal management approaches for avionics packages: heat pipe assemblies to reduce the internal temperature gradient and a Loop Heat Pipe (LHP) to transport thermal energy to alternative sinks. Laboratory testing demonstrated that the heat pipe assemblies were capable of reducing the internal temperature gradient by approximately 25 °C (45 °F). This reduction translates directly to an increase in the allowable sink temperature that will still provide sufficient cooling for the electronic components. To provide additional temperature margin, ACT developed a LHP design to cool the fuel prior to entering the avionics enclosure. This approach was determined to be more reliable than cooling the avionics directly. The LHP was designed to transport thermal energy from the fuel to two heat rejection sections. Two heat rejection sections were necessary as aircraft sink conditions can vary considerably throughout the flight envelope. Since these sinks can approach temperatures much higher than the intended operating temperature of the LHP, the condenser sections were separated by a unique flow balancer design that provided passive deactivation of the high temperature sink while maintaining flow to the low temperature sink. This passive sink selection technique and overall LHP performance as a pre-cooler were demonstrated through laboratory testing. The LHP was shown to reduce inlet fuel temperature by 5 °C. Together with the internal thermal management system, laboratory testing indicates the potential for an increase of 30 °C (54 °F) in the allowable sink temperature for a generic avionics package. This increase allows for a wider selection of potential sinks and significantly reduces the sensitivity of avionics packages to fuel temperature. As a result, electronic components can be maintained below their maximum allowable temperatures despite high fuel temperatures.