Aircraft Electrical and Electronics Systems
This domain tests DC and AC power generation, distribution and protection, battery and bus behaviour, bonding, and the electronic systems used for communication, navigation and indication. Questions often involve current, voltage drop, circuit protection and the difference between generator and alternator construction.
DC power
A DC generator uses a commutator to rectify its output. An alternator has a rotating field and a stationary armature; its AC output is rectified by diodes. The voltage regulator controls field current to hold bus voltage constant. A battery provides standby power and absorbs transients; its capacity is stated in ampere-hours, while cranking ability is a short-duration current rating. Reverse-current protection prevents the battery from discharging through the generator.
Distribution and protection
Power is distributed through bus bars. Circuit breakers and fuses protect wiring against overload; a breaker should not be reset repeatedly without finding the fault. Bonding provides a low-resistance path between metallic parts to control static electricity and provide a return path. Good bonding also reduces radio interference.
Electronics
Avionics often use ARINC 429 for digital data transfer. Shielded cables, proper grounding and separation of power and signal wiring reduce electromagnetic interference. Static dischargers on trailing edges bleed accumulated static charge into the air.
Worked example
A 28 V DC bus supplies a 56 W lamp. Current = power / voltage = 56 / 28 = 2 A. If the supply wire has 0.5 Ω resistance, voltage drop = 2 × 0.5 = 1 V. The lamp therefore receives 27 V, and the wire dissipates 2 W as heat.
Traps
Do not confuse alternator and generator construction. Ampere-hour capacity is not the same as cold-cranking current. A circuit breaker that trips again after reset indicates a fault, not a weak breaker. Bonding is not the same as grounding the neutral of an AC system.