This paper summarizes the result of an effort to develop a unified approach to design-driven testability
evaluation of avionic systems. These systems include both internal diagnostic equipment referred to as built-intest
(BIT) and external off-line test equipment. At the designing stage an adequate database to evaluate the
quality of the BIT is the failure mode and effect analysis. In the paper various mathematical indices are
suggested and constructed to quantify testability of avionic systems. The indices provide the needed flexibility
for representing structural and reliability properties of the controlled system. Analytical model for evaluation
BIT performance impact on the system’s reliability is discussed.