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A line-replaceable unit (LRU) or Lowest Replacable Unit is a modular component of an airplane, ship or spacecraft (or any other manufactured device) that is designed to be replaced quickly at an operating location. An LRU is usually a sealed unit such as a radio or other auxiliary equipment. LRUs are sometimes described as "black boxes" despite the fact that they aren't necessarily black.
LRUs improve maintenance operations, because they can be stocked and replaced quickly from on-site inventory, restoring the system to service, while the failed (unserviceable) LRU is undergoing maintenance. Because they are modular, they also reduce system costs and increase quality, by centralizing development across different models of vehicles.
While the term LRU has been in use for decades, MIL-PRF-49506, Notice 1 of 18 Jan 05, the Performance Spec for Logistics Management Information defines an LRU as:
An LRU is an essential support item which is removed and replaced at the field level to restore the end item to an operational ready condition. Conversely, a non-LRU is a part, component, or assembly used in the repair of an LRU, when the LRU has failed and has been removed from the end item for repair.
LRUs are designed to specifications to assure that they can be interchanged, especially if they are from different manufacturers. Usually a class of LRUs will have coordinated environmental specifications (i.e. temperature, condensation, etc.). However, each particular LRU will also have detailed specifications describing its function, tray size, tray connectors, attachment points, weight ranges, etc. It is common for LRU trays to have standardized connections for rapid mounting, cooling air, power and grounding. The mounting hardware is often manually-removable, standard screw-detent quick-release fittings. Front-mounted electrical connectors are often jacks for ring-locked cannon plugs that can be removed and replaced without tools. Specifications also define the tools necessary to remove and replace (R&R) the unit. Many require no tools, or a standard-sized Frearson screwdriver. Frearson is specified for some vehicles, and many marine systems because Frearson screws are not designed to cam-out, and the same screwdriver can be used on many sizes of screws. Most LRUs also have handles, and specific requirements for their bulk and weight. LRUs typically need to be "transportable" and fit through a door or hatchway. 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, such as ARINC 404 and ARINC 600. LRUs are also defined by manufacturers like Airbus and Boeing and by various military organizations. In the military, electronic LRUs are typically designed to interface according to data bus standards such as MIL-STD-1553. On the International Space Station, LRUs are referred to as Orbit Replaceable Units.
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