Avionics is assuming an increasingly important role in modern aircraft. Avionics functions are more and more complex and critical to aircraft safety and performance. Against this background, avionics maintenance deserves special attention since the ability to detect and diagnose avionics faults has become a major issue.Maintenance of all modern airplanes relies on Built-In Test Equipment (BITE). Ideally BITE should report accurate and confident information permitting quick isolation of failures. Unfortunately, BITE is not infallible and this expected goal has not been reached. Many of the reasons explaining BITE unreliability have been identified and have solutions that can make a significant contribution toward a usable On-board Maintenance System.