Aircraft Systems and Line Replaceable Units (LRUs)

Aircraft System Testing

• A power-up test is performed when an aircraft is airborne, focusing on essential items to enable quick system operation.
• The test typically includes:
• Microprocessor test
• Memory test
• Data line and input/output circuit test
• Configuration test

Cyclic Tests

• Cyclic tests (In Operation Tests) are continuous, non-disruptive tests that monitor system operation without affecting performance.
• Tasks include:
• Watchdog test (restarting microprocessor if software fails)
• RAM test
• Data line (ARINC 429) message validity test

System Test

• The system test is a troubleshooting tool for maintenance crews, performed after system restoration or LRU replacement.
• It is similar to a power-up test, ensuring system integrity.

Specific Tests

• Specific tests are designed for particular systems, generating stimuli for command devices (e.g., actuators, valves).
• Examples include:
• Movement of slats or flaps
• Engine dry cranking

Maintenance Control Display Unit (MCDU)

• The MCDU is a screen-based interface for data display and command input.
• It allows access to system components, including:
• Flight Management System (FM)
• Aircraft Condition Monitoring System (ACMS)
• Central Maintenance System (CMS)
• Satellite Communication System (SAT)
• Air Traffic Service Unit (ATSU)

Aircraft Diagnostics

• Built-In Test Equipment (BITE) is essential for troubleshooting complex digital aircraft systems.
• BITE systems are self-contained, monitoring electronic/avionics systems for faults.

BITE Systems

• BITE systems range from simple go/no-go indicators to advanced multi-character displays and centralized monitoring systems.
• Advanced systems include:
• Central Maintenance Computer System (CMCS - Boeing)
• Centralized Fault Display System (CFDS - Airbus)
• These systems offer enhanced BITE features, fault monitoring, and data transmission capabilities.

Line Replaceable Units (LRUs)

• LRUs are individual components of a system, such as black boxes, sensors, or actuators.
• Each LRU is controlled by digital computers and permanently monitored for safety and testing purposes.

BITE Functionality

• BITE continuously monitors internal LRU circuits, inputs, outputs, and system links.
• When a failure occurs, BITE identifies the possible failed LRU, determines failure type (intermittent or permanent), and captures system environment data for later analysis.
• BITE information is stored in system memory and sent to a centralized maintenance device for analysis and troubleshooting.

This quiz covers the basics of aircraft systems, including Line Replaceable Units (LRUs) and their components, such as sensors and actuators, and how they are monitored and tested.