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P O L Y T E C H . N U

Avtech Remote Electronics Unit (LRU)

imageThis article is about the Avtech Remote Electronics Unit LRU (Line Replacement Unit). Official data nor documentation is found, so the original use is an educated guess. This LRU was origianlly placed in a mounting rack in the avionics bay in an aircraft. There are three slots at the front panel with adjustable resistors. Each slot is marked with the text: OBS, COPILOT and PILOT. And the variable resistors are marked with:

PA ST; likely means public address audio strenth
IMPH ST; likely means interphone audio strenth
HF ST; likely means high frequency (3...30MHz) radio audio strenth
UHF ST; likely means ultra high frequency (300...3.000MHz) radio audio strenth
VHF2 ST; likely means very high frequency (30...300MHz) radio audio strenth
VHF1 ST; likely means very high frequency (30...300MHz) radio audio strenth
SPRK ST; likely means speaker audio strenth
SPKR; likely means speaker

This indicates that this LRU is used for audio signal handling. It's expected that the audio signals (like from the radio's) are fed into this LRU. The audio level (amplification/attenuation) are set for each input by the trimmer resistors. Based on the text and controls it's therefore expected that this LRU is used for conditioning/routing the audio signals.

Since this board is very complex to reverse engineer, only a 'walktrough' of this LRU is shown.

LRU: Line Replaceable Unit (general)
LRU module. An aircraft LRU is a Line Replaceable Unit. An Aircraft LRU is in general a component or module within an aircraft that can be quickly and easily replaced on the line (at the maintenance site) without extensive aircraft downtime. LRUs are designed to be modular, allowing for efficient maintenance and troubleshooting. In the context of aircraft, LRUs can include a wide range of systems and components, such as avionics equipment, sensors, communication devices, navigation systems, and more. These units are typically designed to be plug-and-play, meaning they can be removed and replaced without the need for extensive recalibration or reconfiguration. The use of LRUs in aircraft maintenance helps reduce turnaround times and overall maintenance costs. If a specific component fails or needs maintenance, technicians can quickly swap out the faulty LRU with a replacement unit, allowing the aircraft to return to service more rapidly. Additionally, LRUs facilitate easier troubleshooting since technicians can isolate and address specific faulty components without dismantling the entire aircraft system.

The housing is a rather straight forward LRU housing. It's not uncommon that the mounting frame of the LRU is equipped with an air duct for cooling the electronics. At the bottom of the LRU are ventilation holes so this indicates that this LRU indeed needs some ventilation. Electronics generates heat and to prevent failure cooling is required. Altough it's not expected that this device generates a huge amount of head compared to power electronics LRU's



ARINC 404 data bus
imageThe LRU is equipped with an ARINC 404 connector. The ARINC 404 connector is a specific type of rectangular electrical connector commonly used in aviation and aerospace applications. The term "ARINC" stands for Aeronautical Radio, Incorporated, an organization that sets technical standards for the aviation industry. The ARINC 404 specification outlines the design and dimensions of a standardized rectangular connector commonly used for avionics equipment.

Key features of the ARINC 404 connector include:
Rectangular Shape: The ARINC 404 connector is rectangular in shape, which helps distinguish it from other types of connectors.
Standardized Dimensions: The connector follows specific dimensions and configurations outlined in the ARINC 404 standard. This standardization ensures compatibility and interchangeability of avionics equipment across different aircraft and manufacturers.
Metal Shell: The connector typically has a metal shell for durability and to provide shielding against electromagnetic interference.
Modular Design: Like many connectors used in avionics, the ARINC 404 connector is designed to be modular, allowing for easy installation, removal, and replacement of avionics LRUs (Line Replaceable Units).
Used in Rack-Mounted Equipment: ARINC 404 connectors are often associated with rack-mounted avionics equipment. These connectors are used to connect avionics LRUs to the aircraft's systems.

The ARINC 404 standard has been widely adopted in the aviation industry to ensure consistency and compatibility between different avionics components. It plays a crucial role in the design and integration of avionics systems in various types of aircraft.

The LRU mounting has usually a fixed plug that is connected to the receptacle of the LRU. By inserting the LRU, the electrical connections are established. There are two alignment holes on the rear panel preventing damage to the pins. The ARINC 600 is the successor to the ARINC 404 connector for many of the new avionics designs. Compared to the ARINC 404, the ARINC 600 features lower mating force contacts, increased contact count, a front release and a floating keying system.

Between the connectors are three hexagonal holes. Each hexagonal hole has a locking insert. In this case the inserts are placed at it's default (bottom) position. Each pin can be rotated to achieve six possible position. The result is that 6 x 6 x 6 = 216 possible insert configurations are possible. The idea is that if there are multiple LRU's of the same size and same connectors, the LRU's can't be placed in the wrong mounting frame. If there are physical identical LRU's but are internally different and may not be interchanged, each LRU has it's own insert arrangement. By rotating an insert, the LRU won't fit into the 'wrong' mounting frame. If there are multiple LRU's of the same function, the insert arrangement may be the same since the LRU's are in that case interchangeable.

inside view
There are (judged by the eye) three identical boards installed which are connected to backplane. The boards are coated to prevent damage from environmental conditions like high humidity. Based on the production dates on the microchips the device is built in the year 1994.