Chapter 1. Basics of avionics systems

1.3. Line replacement unit concept

Avionics modern aircraft consists of a number of fully redundant blocks (Line Replaceable Unit - LRU), which is easily replaced in case of failure or need for modernization. The block system allows to replace the functional elements of the systems with minimum time spent on installation. Each block of LRU has its own logic and functional boundaries and is monoblock structure. Modular LRU provides easy access to the components of the system for their testing and replacement. In case of a refusal built-in control block LRU, this block can be replaced by a new airport with minimum time spent.

The size and structure of the LRU standardized multiple standards developed by firm «Aeronautical Radio Incorporated» (ARINC) [39]:

  1. Air Transport Radio (ATR);
  2. Modular Concept Unit (MCU).

Block size ATR are described by standard of ARINC 404. According to this standard are allocated such basic types of blocks:

  • 1/2 ATR;
  • 3/4 ATR;
  • 1 ATR;
  • 1 1/2 ATR.

Each of these blocks is the same height 269.88 mm and dependent from it’s series length :318 mm (series «short») or 495.8 mm (series «long») (Fig. 6 and 7, respectively). Each of these blocks is the same height 269.88 mm and dependent on the length of 318 mm series (series «short») or 495.8 mm (series «long») (Fig. 6 and 7, respectively). In addition, each of these blocks placed on a specialized rack containing standardized wired connection to the LRU (Fig. 8) [52]. This allows different manufacturers of avionics systems elaborate fixed levels and relevant connections.

Fig. 6. ATR sizes series «short»

To provide greater flexibility to the system also developed options for mounting modules series «short» in place a series «long» (Fig. 9).

Fig. 7. ATR sizes series « long»

The development of digital technology needed to review the concept of ATR. Significant reduction of dimensions digital electronics compared to analog encouraged to develop a new standard for the size and structure of the LRU. This led to the development of the next standard - ARINC 600, which describes MCU (Fig. 10).

Fig. 10. Sizes of 1 MCU

According to the new standard 8 blocks of MCU in size roughly equivalent to one block of ATR. The size of a typical PC system is 2 MCU [52]. However, the size of large avionics systems, such as systems of air signal is 8 or 10 MCU. The width of MCU is calculated by the formula:

W = (N × 1,3) – 0,032″,

where N – the number of size of MCU.

AVIONICS training course materials represented only in demonstrative mode. If it useful, all grammar mistakes will be corrected and more documents will be added. Let me know if you interest in avionics.
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(20th of December, 2014)
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Avionics training in details by
Ostroumov IvanOSTROUMOV Ivan, PhD
Associate Professor, www.ostroumov.sciary.com