CASA Part 66: LRU Connection and Maintenance

Avionics Equipment Connectors

• During connection, ensure plug covers are removed and inspect pins for damage, bent pins, contamination, and corrosion.
• Set the LRU onto the rack, ensuring it is flush and centralized, then push it back into the rack until the support pins and connector keys engage.
• Apply firm pressure to fully engage the LRU and secure it with a knurled knob.

Cockpit LRU Fitment

• Cockpit control panels are held in place with a Dzus fastener and rail system.
• Routine preventative maintenance involves inspecting, adjusting, lubricating, and replacing components to prevent breakage.

BITE (Built-In Test Equipment)

• BITE is a test capability built into aircraft components, modules, or subsystems to facilitate diagnosis and repair.
• BITE ranges from simple self-tests to sophisticated systems that record performance data and provide instructions for repair.

Troubleshooting Flow Chart

• A flow chart is a printed or computerized guide that directs maintenance technicians along a logical testing and diagnosis path.
• The chart branches to another test or conclusion based on previous test results.

No Fault Found (NFF)

• Repairing or replacing avionics components is expensive, especially when unexpected.
• NFF is a frustrating maintenance problem where no fault is found despite troubleshooting efforts.

Tunnel Vision

• Tunnel vision occurs when maintenance technicians assume a familiar fault and overlook other possible causes.
• Researchers have found that people tend to stick to a single diagnosis even when told to expect a different malfunction.

Half-Split Test

• The most efficient test is one that provides the most information, ideally eliminating half the possible failed components.
• This type of test is called a "half-split" test.

Test-Induced Failure

• Testing a system or component can cause a failure, creating safety risks if undetected.
• Test-induced failures can occur, for example, when a connector is disconnected and a pin is bent, creating an open circuit.

Troubleshooting Example

• To troubleshoot a system, eliminate large chunks of the system by using knowledge of complex systems.
• Ask questions to confirm the fault and eliminate possible causes before testing.
• Perform a functional test to confirm the fault and review symptoms to find clues leading to the cause.

Dzus Fasteners and Rail System

• Dzus fasteners are used to secure LRUs (Line Replaceable Units) in aircraft instrument panels
• To remove the fastener, rotate it 90 degrees anticlockwise and a spring will push the clip out; be careful not to force it further as it may damage the clip
• To fit the fastener, push it down and rotate until locked; the lock is only 90 degrees from where it pushes down
• The slot of the LRU will be across the aircraft or horizontal in a vertical panel when locked, and along the rail or vertical when unlocked

Cockpit LRU Dzus Fastener

• The Dzus fastener in the cockpit has a cam mechanism in the handle that disengages from the rear connector and secures the display into the panel
• The handle is secured by 2 spring clips

Alternative LRU Securing Methods

• Some light aircraft use a single captive Hex screw that, when unscrewed, will push the LRU out of its rack and disconnect it
• The crew may need to use a long Allen Key of the appropriate size to unscrew the Hex screw
• The electrical connector may be central or offset to one side, depending on its location

Maintenance Practices

• Preventative maintenance involves inspecting, adjusting, lubricating, replacing, and upgrading/overhauling systems and components to keep them from breaking
• Maintenance tasks are performed in response to regulatory requirements or to take advantage of technological advances
• The efficiency and effectiveness of a maintenance organization and individual workers are judged on their ability to find and fix problems efficiently

Built-In Test Equipment (BITE) and Automated Test Equipment (ATE)

• BITE is included in the design of a component, module, or subsystem to aid in diagnosis and repair
• ATE is a range of partially or fully automated test capabilities that require the LRU to be removed from the aircraft and sent to the avionics bay
• ATE is programmed to perform a series of tests without human intervention or can be set up to do so

Troubleshooting Methods

• "Shot gunning" involves randomly replacing LRUs until the symptoms of trouble disappear, but it is an inefficient and expensive method
• A hypothesis is an assumption that must be proven or disproven by objective testing
• Psychic blindness and tunnel vision can hinder effective troubleshooting
• Half-split testing is a method that provides the most information by eliminating roughly half the items from a set of possibly failed components
• Test-induced failures can occur, and it is essential to balance the need for testing against the likelihood of a test-induced failure

Troubleshooting Example and Lesson Learned

• A troubleshooting example involving a radar system shows the importance of not taking shortcuts and assuming the first problem found is the only one
• It is essential to try to falsify your theory instead of just trying to prove it correct