I'm studying the Integrated Modular Avionics (IMA) and the Line-Replaceable Unit (LRU), but I don't get the difference.


A Line Replaceable Unit is a piece of hardware that can be exchanged for a replacement part in a relatively short time by only opening and closing fasteners and connectors. You will find the term LRU in avionics but also in ATC hardware. Examples are FMC, transponder, etc.

When you have a complex avionics system you tend to end up with a great variety of LRUs. All these LRU have a very specific function. To be able to quickly replace malfunctioning parts, maintenance needs to keep a large stock of spares for all these LRU's.

The Integrated Modular Avionics concept attempts to reduce the variety of LRU's by designing the avionics with common components. E.g. one type of power supply, one type of memory unit, one type of processing board and only a few variations of input/output modules. Instead of working with very function-specific LRUs, the IMU works with more generic Line Replaceable Modules (LRMs). This reduces the number of replacement parts that need to be kept on stock. The LRMs of IMA have more generic functions than in a traditional avionics system.


An IMA is a computer network utilising the ARINC bus system that interconnects various devices. This is in contrast to Line Replaceable Units LRU'S where they are standalone systems that can be replaced. Basically they are electronic boxes typically in the aircrafts electronics bay that can be swapped out quickly. Because they can be swapped out quickly they can be done on the line. Ie. When the airplane is on the tarmac or in the hangar

  • $\begingroup$ IMA is designed with various replaceable modules that can be swapped out quickly. Both the systems have this characteristic in common, isn't it? May I say that IMA uses LRU, and each unit is a Module? $\endgroup$ – Gianni Alessandro Jul 21 '15 at 12:33
  • $\begingroup$ Yes but the whole system cannot be replaced easily. Cards can be removed but the underlying system will still be there. $\endgroup$ – Mark Jul 21 '15 at 12:36
  • $\begingroup$ Also it is easier to deal with LRU'S in troubleshooting because you would just replace the whole unit. Less troubleshooting. $\endgroup$ – Mark Jul 21 '15 at 12:38
  • $\begingroup$ They decided to substitute the old computers with LRU, easily replaceable. In a second moment they decided to put everything in a single computer, not replaceable, but partitioned into many modules, featuring the decomposition of the avionic devices into its basic functional elements. Am I right? IMA is younger than LRU, isn't it? $\endgroup$ – Gianni Alessandro Jul 21 '15 at 12:40
  • $\begingroup$ A lot of the LRU'S do not have computers. I worked on a lot of LRU'S that just have analog devices in them. Ie transformers, wiring circuits that distribute power etc... so not all LRU'S have computers in them. $\endgroup$ – Mark Jul 21 '15 at 12:43

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