The internal structure of a typical Line Replaceable Unit (LRU) is illustrated in Figures 11 and 12. An LRU is a modular component of an avionic system designed to perform specific onboard functions and to be quickly replaced during maintenance operations. The modular design of LRUs significantly reduces aircraft downtime and simplifies troubleshooting procedures.
Fig. 11. Internal structure of LRU
The power supply module of an LRU converts the aircraft electrical network voltage, typically 115 VAC or 28 VDC, into lower stabilized voltages such as 5 VDC and 15 VDC required for powering microprocessors, memory devices, interface circuits, and other electronic components. Stable and reliable power conversion is essential because modern avionics systems are highly sensitive to voltage fluctuations and electrical disturbances. A typical LRU contains input and output digital communication channels for exchanging information with other onboard systems and avionic units. One of the most widely used communication standards in civil aviation is ARINC 429, which provides reliable serial data transmission between avionics equipment. Through these interfaces, LRUs exchange navigation data, flight parameters, control commands, system status information, and warning messages with other aircraft subsystems.
Fig. 12. Functional structure of LRU
To communicate with external sensors and actuators, LRUs may also include several analog input and output channels. These channels allow the processing of signals from various aircraft sensors, including temperature sensors, pressure transducers, gyroscopes, accelerometers, fuel quantity sensors, and engine monitoring devices. In addition, LRUs often include discrete input and output connectors used for receiving digital status signals and transmitting one-time control commands to other aircraft systems.
At the core of the LRU is the computing processor, which receives information from input interface modules, processes the data according to embedded software algorithms, and generates output signals for other systems. The processor executes control logic, performs calculations, monitors system parameters, and stores operational data within internal memory modules. Modern LRUs commonly employ high-performance microprocessors and digital signal processors capable of real-time data processing and fault management.
An important feature of modern avionics equipment is the integration of Built-In Test Equipment (BITE) or built-in diagnostic systems. Each LRU continuously monitors its own operational condition and, in the event of a malfunction or abnormal operation, generates a failure indication or warning signal. These diagnostic capabilities greatly improve maintenance efficiency by enabling rapid fault detection and isolation without extensive manual testing.
Since avionics equipment generates a considerable amount of heat during operation, thermal management is another critical aspect of LRU design. Excessive temperature can negatively affect electronic component reliability and reduce system lifespan. Therefore, some LRUs are equipped with specialized cooling and ventilation systems designed to maintain acceptable operating temperatures. Such cooling solutions may include forced-air ventilation, heat exchangers, or dedicated aircraft environmental control interfaces, as illustrated in Figure 11. Effective thermal management contributes to higher reliability, improved operational stability, and extended service life of avionics equipment.
