Line-replaceable unit

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A line replaceable unit (LRU) is a complex component of an airplane,^[1] ship or spacecraft^[2] that is designed to be replaced quickly at the flightline or airport ramp area. An LRU is a black box (sealed unit), such as a radio or other auxiliary equipment. An LRU is also known as a line replaceable module (LRM) or shop replaceable unit (SRU).^[3]

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.^[4] Although an LRU is called a "black box", the box itself is often painted gray in colour.

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.^[5]