line-replaceable unit

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. The name derives from the fact that the unit is replaced at the flightline or airport ramp area.LRUs speed up repair, because they can be stocked and replaced quickly from inventory, restoring the larger 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".
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