This paper deals with studying the effects of both online and off-line test during flight critical missions where safety is a major issue. The on-line test, in this context, is a test performed on a digital airborne system during some specified windows in time while it is still performing its intended task. An off-line test is a test that is performed on the digital system once it is taken off-line because of a suspected failure. Both the on-line and the off-line tests are performed during flight. The difference between the two is that the off-line test can be made more effective than the on-line test due to the longer amount of time available for testing. Moreover, the off-line test may be designed to have diagnosis and repair capabilities built-in. Upon successful repair, the faulty processor may be reconfigured back into the system. This capability will undoubtedly increase the mission reliability.