Abstract: A high reliability cooling system for avionics provides a low-weight tertiary backup. A first source of cooling airflow (20) is directed through a first duct (28) to a first compartment (30, 32). A second source of cooling airflow (58) is directed through a second duct (68) to a second compartment (70, 74, 76, 78). A check valve (104) is located in the first duct (28) and permits airflow from said first source (20) to said first compartment (30, 32) and prevents a back-flow of air therethrough. An interconnecting duct (84) is provided between the first and second ducts (28, 68). The interconnecting duct (84) connects to the first duct (28) at a location between the check valve (104) and the first compartment (30). A shutoff valve (86) is located in the interconnecting duct (84) and is operable to open or close airflow therethrough. The invention provides three modes of operation. First, with the shutoff valve (86) closed, there are two separated avionic cooling systems.