Line-replaceable unit

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A line-replaceable unit (LRU) is a black box of electronics, such as a radio or other auxiliary equipment for a complex engineered system like an airplane or ship. LRUs speed up repair, because they can be replaced quickly, restoring the big system to service. They also reduce the cost of systems, and increase the quality, by spreading development costs of the type of unit over different models of vehicles. Although an LRU is called a "black box", the box itself is often painted grey in color. The adjective "black" can be thought of as vestigial, being carried over from early LRUs that were painted black, or it can also be thought of in a metaphorical sense where the input and output for the box are observable connections whereas the inner workings are unobserved, or "black".

LRUs are designed to common specifications. These specifications define the inputs and outputs with simple twist-on/twist-off cannon plug-type connectors. They also define the tools required to replace the unit (usually nothing more than a #2 Phillips-head screwdriver) and the bulk and weight (they typically need to be carried by one person and fit through a door, if possible). There are also requirements for flammability, unwanted radio emissions, resistance to damage from fungus, static electricity, heat, pressure, humidity, condensation drips, vibration, radiation and other environmental measurements.

Many LRUs for commercial aircraft are designed according to ARINC specifications. ARINC (Aeronautical Radio Incorporated) is a company owned by a number of airlines that sells specifications and designates standards. LRUs are also defined by manufacturers like Airbus and Boeing and by various military organizations. Military LRUs are typically designed to interface according to data bus standards such as MIL-STD-1553.