Line-replaceable Unit - Specifications

Specifications

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 (R&R) without tools. Specifications also define the supporting tools necessary to remove and replace 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.

LRUs may be designed to ARINC 700-series standards. The form factor of LRUs comply to ARINC Standards, 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.