Due to the increasing power density of onboard electric and electronic equipment and heat dissipation in civil and military aircraft, more efficient ways of transferring heat and new cooling techniques are necessary. A passive heat transfer prototype was developed and experimentally evaluated in laboratory and on ground and flight tests in an Embraer test aircraft. The passive heat transfer device consists of a loop-thermosyphon with two condensers and a common evaporator, using water as the heat transfer working fluid. An electric resistance and a variable power source were used to dissipate heat inside the evaporator simulating heat transfer from an onboard electronics bay. The fuselage/external air stream and the air flow inside an air conditioning system duct were used as heat sinks. Prior to flight test, laboratory tests were conducted simulating ground and flight operations. During flight test, the fuselage/external air stream was responsible for dissipating over 80% of the heat generated by the resistance. Even when the fuselage condenser loop was not operating as a loop thermosyphon due to water freezing, the device was able to properly remove heat maintaining vapor temperatures between 40 °C and 50°C. Moreover, it was shown that angular velocities (pitch, yaw and roll rates) and aircraft accelerations did not affect the prototype thermal performance. The prototype is a concept proof of a novel and improved passive heat transfer system which has been subjected to a patent application.