Line Replaceable Unit
Line Replaceable Units (LRU) are a general term for components, of complex systems such as aircraft, that can be replaced quickly, and generally with a minimum of tools. The term derives from maintenance that can be done on the flight line, rather than moving the aircraft into a maintenance facility.
Depending on the nature of the LRU, the unit replaced may be disposable, or it might be shipped to a factory or depot maintenance facility for overhaul. Modern electronic LRUs are unlikely to be repairable even at the factory, due to the complexity of multilayer printed circuit boards that cannot be taken apart. LRUs intended for severe environments may also be "potted", or filled with insulating material that cannot be removed.
In some cases, the LRU may be returned, not for reconditioning, but for destructive engineering testing that will reveal why the unit failed, and give information for improvements.
To simplify supply requirements, military units, airlines, and even automobile dealers prefer to have LRUs to be common among different "platforms". A radar power supply, for example, might work equally well in several different aircraft. The LRU philosophy emphasizes reducing the number of form factors and connectors required. Indeed, a program such as the Joint Tactical Radio System goes even farther, with the same physical device able to be placed in different applications, by being loaded with different software.