The quest for improving the thermal efficiency of gas turbines has raised the turbine entry temperatures (TET) and hence the operating pressure ratios (OPR). However, the combination of high TET and high cooling flow temperature makes the cooling of the hot sections more challenging. A cooled cooling air heat exchanger (CCAHX) can be used to cool the high-temperature compressor offtake, showing improved steady aerothermal performance in terms of pressure and temperature. Presented here is the experimental testing of the CCAHX and the process used to generate a highly representative transient simulation of its dynamic performance between idle and Maximum Take-Off (MTO) flight conditions. Said simulation is primarily used to predict the performance during further experiments. Due to the harsh experimental conditions, the simulation guarantees safe operations of the test facility while advising on the right procedures to use during real scale tests. Due to the complex internal structure of the heat exchanger, a one-dimensional computational fluid dynamics (CFD) commercial software was employed to validate the models using results from steady-state data for a single CCAHX unit. Derived procedures from the 1D analysis are used to guide the transient testing at real scale flight conditions, where the core and fan flows are thermally-linked to two CCAHXs in a back-to-back configuration. Results show that the approach considered here can correctly predict the performance of a complex heat exchanger system in transient, fast-changing operations, as calculations in real flight conditions showed good agreement with experimental results of a real scale test.