In modern aircraft, one of the most difficult issues has been how to provide avionics with adequate cooling. Future versions of the fighter aircraft JAS 39 Gripen is equipped with new applications that have increased heat loads. In previous versions of the JAS 39 Gripen avionics was cooled by zero degree air and fuel, but in the next version a liquid loop will be installed to cool the new radar.The fluid in the liquid loop is cooled to proper temperature by pressurized bleed air from the engine which is cooled by ram air. The air to cool the avionics is produced the same way and this is a very expensive process for the airplane which lowers its performance. It is important to minimize the production of cooling air and therefore three new adjustable valves that provide various components of cooling air are installed in the next version of the JAS 39 Gripen. The cooled and pressure controlled air from the engine is distributed between different avionic shelves, each containing a set of components. Depending on the type of tasks performed and current flight mode of the aircraft the requirement of functions which should be active varies and therefore also the cooling demand to avionics. The first part of this thesis studies the overall priority of how the engine bleed shall be used. This part of the thesis results in a decision basis for the distribution of cooling air to be regulated in the absence of full cooling capacity. The amount of cooling which must be distributed to the radar is proportional to its developed power which varies widely depending on the radar’s operational mode. Since the pump which determines the liquid flow velocity operates at a constant speed is the regulation of cooling to the radar is controlled by varying the bleed air flow into the heat exchanger which cools the fluid and thus the temperature of the fluid has when it reaches the radar. This part of the thesis creates a control algorithm for controlling the airflow into the heat exchanger. The regulation keeps the fluid inlet temperature to the radar within the range of +25 ± 5 ˚ C and the gradient of the temperature less than 0.5° C per second. The PI-controller with the feed-forward filter succeeded in controlling the temperature of the liquid as it reached the radar within +25 ± 1° C, the temperature gradient requirement, 0.5° C per second, was also passed in all flight cases which were used to evaluate the controller. The PI-controller with feed-forward has a low convergence time and no static error. It also performs well when the measurement signals contain a lot of noise because of the controllers integrated low pass filter.   The three new adjustable valves saves 12 to 97 g/s of cooling air for the different valve positions studied in this thesis, this corresponds to 9 - 70% of the total amount of controllable air to the avionics. Since the production of cooling air is a costly process for the aircraft, the use of all 3 valves is recommended.