Numerous electronic systems are installed onboard modern aircraft. Tasks performed by these systems cover engine and flight control, navigation, communications, data recording, lighting, radar detection, threat warning and jamming, fuel management, weapon systems and more. Onboard electronic systems utilize combination of different technologies like analog and digital electronics, Radio Frequency (RF) transmit and receive, and electro-optics. Some systems are flight-critical, that means, failure, malfunction, or performance degradation may cause severe results for flight safety, while others are mission-critical, meaning that similar problems may risk mission accomplishment. Awareness about the instantaneous status of an onboard system by flight crew is of key importance. This makes Built-In-Test (BIT) indispensable for any airborne system. An effective BIT will fulfill pre-flight and in-flight system monitoring and error reporting tasks, take protective measures against potential catastrophic failures, and besides, with additional diagnostic features, will also support flight-line maintenance, hence contribute to system sustain ability. BIT capability shall be designed such that its operation while performing the fore-stated tasks will not adversely affect in-flight functional performance. While an onboard system is at stake, effect on size, weight and power consumption shall also be considered. Different approaches can be followed when designing BIT capability for different systems implying varied technologies like RF, analog or digital electronics. Those depend on the system testability requirements and criticality of system performance for flight safety or mission accomplishment. This paper examines considerations about proper BIT design for systems implying different technologies and operational needs. Measures of test coverage, diagnostic capabilities for flight-line maintenance support, effects of coverage on system performance, system complexity and cost are discussed. BIT implementation for an RF threat warning system for military aircraft is presented as an example. Design approaches for specific system requirements, techniques practiced, software structure and implementation results are given. It is intended to raise awareness about good practices for Built-In-Test design.