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What is a line replaceable unit (LRU) in an aircraft data bus system?

What kind of components are defined as LRU in a data bus system?

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The components needed for a databus depend on the specific bus standards chosen. The most common of the recent air transport (B787, B777X, A350XWB) aircraft use the ARINC 664 network standard.

These aircraft also use ARINC 653 IMA computing resources hosted in a cabinet. Within the cabinet will be ARINC 664 network switch modules. These connect the IMA modules via a backplane bus and also connect it to external systems via fiber optic or quadrax network cabling.

ARINC 664 is a switched Ethernet type of bus with each A664 end system connected directly to a network switch, so there is no contention for media access at the end system. All network access control is controlled within the switches.

Beyond the IMA switches, the network typically has 4 or 6 remote switches spread along the aircraft. Aircraft systems with A664 end systems can be connected directly to these switches, but many systems use legacy interfaces such ARINC 429, ARINC 825 (CAN), and even analog audio or differential voltage signals.

These other networks can be connected via generic programmable network bridge devices called Remote Data Interface Units (RDIU) or something similar. Specialized interfaces are typically identified as gateways (Audio Gateway unit or A825 Gateway). These have an A664 connection as well as a number of the legacy bus connections. These have internal software that translates the data between the bus types. The key benefit of this architecture is a significant reduction in network cabling.

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Down to the smallest unit in a system that can be removed as a piece in the field (the line) is an LRU. So take a computer unit where the internals can only be serviced in a back shop but the computer itself can be swapped out at the gate. The computer is the LRU.

If you have a black box with an easy to remove cover, with sub-components mounted on quick change cards so techs and open the box on-aircraft, replace a card, close the box, ops test and go, then the card is also an LRU.

The easiest way to tell is, generally speaking if something is an LRU, there will be a removal/installation procedure in the Aircraft Maintenance Manual. Below the LRU level, it is covered in a Component Maintenance Manual and would be serviced in a back shop or at a 3rd party after being pulled from the aircraft.

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