Abstract: A measuring system for the detection of ice buildup on aircraft in flight is described. It depends for its action on a well-known physical phenomenon called "heat of fusion" and senses the heat required to melt ice back to liquid water. The energy signal that is measured can only be produced by an ice-to-water transition, hence this system uniquely identifies the presence or absence of ice without regard to time, temperature, altitude, pressure or other factors. The system is capable of generating a rapid, real-time electrical signal that can be displayed to and readily understood by crew members, so that corrective action may be taken to safeguard the operation of the craft.