Avionics Systems and Aircraft Electrical/Electronic Maintenance
What this domain tests
This domain tests whether you understand how aircraft electrical power, data buses and avionics line-replaceable units (LRUs) work together, and whether you can use that knowledge for fault isolation, repair and testing rather than simply swapping boxes. Expect questions on power distribution, circuit protection, bonding and shielding, connector and wiring practices, and digital bus behaviour.
Core rules and concepts
- Aircraft electrical systems commonly use 28 V DC and 115 V AC at 400 Hz; know which buses are essential, non-essential and emergency, and what is shed after a generator failure.
- ARINC 429 is a point-to-point, 32-bit-word bus with octal labels and high/low speeds of 100 kbit/s and 12.5 kbit/s. ARINC 629 and AFDX (ARINC 664) are higher-rate shared-media systems used on later types.
- Bonding protects against lightning, static and RF interference. A poor bond can produce intermittent faults that look like LRU failures.
- Electrostatic discharge can destroy CMOS devices. Use grounded wrist straps, conductive bags and approved test equipment.
- Wiring work must follow the manufacturer's standard practices and approved wiring diagrams: correct wire gauge, crimp tool, contact retention, heat-shrink and identification.
Worked example
A heading flag appears intermittently. The trainee replaces the AHRS twice and the fault returns. The reasoning should be: confirm power at the LRU connector under load, check the ARINC 429 output at source and destination, then inspect the connector for pushed-back pins and check the shield and bond. The evidence points to a connector or wiring defect, not the AHRS. The correct action is to repair the wiring and retest the system.
Traps that cost marks
- Treating every fault as an LRU problem.
- Ignoring bonding and shielding.
- Confusing ARINC 429 word/label structure with other buses.
- Handling line cards without ESD protection.