Electrical Systems and Components
Aircraft electrical systems and components
This domain tests the hardware between the bus and the load: relays and contactors, switches, circuit breakers and fuses, motors and actuators, solenoids, batteries, transformer-rectifier units (TRUs), inverters, and avionics power supplies. Questions often ask you to select a component rating, explain a control circuit, or identify why a protection device operated.
Core rules
- A relay or contactor is an electromagnetically operated switch. The coil circuit is low current; the contacts carry load current. Contactors are used for high-current bus and starter circuits, relays for smaller switching duties.
- Circuit protection must protect the wire. Breakers are resettable; fuses are one-shot. Never uprate a breaker to stop nuisance trips without finding the cause and checking approved data.
- DC motors draw high inrush current. A 28 V motor may need a slow-blow breaker or controlled start. Solenoids convert electrical energy into linear motion.
- Lead-acid and nickel-cadmium batteries have different charging and servicing requirements. Ni-Cd batteries can suffer thermal runaway and need specific capacity checks. Lithium batteries need dedicated protection and venting.
- A TRU converts AC to DC; a static inverter converts DC to AC. Check input and output ratings, cooling, and overload limits.
- Bonding and shielding are part of the system: they provide return paths and control electromagnetic interference.
Worked example
A 28 V DC fuel-boost pump draws 12 A continuously and has a 15 A breaker. The feed wire is 14 AWG with a derated bundle capacity of 18 A. The 15 A breaker protects the 18 A wire, and the 12 A load is below the breaker rating. If the pump seized and drew 20 A, the breaker should trip before the wire overheats. Replacing the 15 A breaker with a 20 A unit would leave the wire unprotected and is not acceptable without an approved modification.
Traps
Confusing relay coil and contact ratings; fitting a larger fuse; assuming a breaker that resets is serviceable; ignoring battery electrolyte and ventilation; and forgetting that a TRU does not store energy.