A general method for the comparison of hydraulic and thermal performance of different liquid coolants in complex systems is offer ed. As a case study, the performance of polyalphaolefin (PAO) and a silicate ester-based fluid (Coolanol 25R) used as liquid coolants in avionic systems is presented. Thermophysical property expressions for the variation of density specific heat, thermal conductivity, and kinematic viscosity with temperature for PAO and Coolanol 25R were developed. The range of temperature for this study was from -54 to 135 degrees C. Based on the results, the hydraulic performance of Coolanol 25R is much better than that of PAO at low temperatures (below 0 degrees C) and in the laminar flow regime. In the turbulent region, PAO outperforms Coolanol 25R hydraulically over the entire temperature range. The thermal performance of PAO at temperatures below 61 degrees C and in the laminar flow region is slightly better than that of Coolanol 25R. In the low-temperature turbulent region Coolanol 25R outperforms PAO thermally. Ah other temperatures, the performance of the two liquid coolants is reasonably close and fairly independent of the flow regime.