Aircraft Electrical and Avionics Systems
Electrical and Avionics Systems tests power generation and distribution, bus architecture, circuit protection and digital databuses. You must be able to calculate load, voltage drop, and explain redundancy.
Power architecture
Large aircraft use 115/200 V AC, 400 Hz, three-phase, with a neutral; small aircraft use 28 V DC. An IDG or VSCF supplies AC; TRUs convert AC to 28 V DC; batteries supply essential DC during start and emergency. Bus ties allow sources to be isolated or paralleled; generator control units protect against over/under voltage, overfrequency and feeder faults.
Avionics data
ARINC 429 is a point-to-point, one-way label bus; MIL-STD-1553 is a bidirectional command/response bus; AFDX uses switched Ethernet with virtual links and redundancy. Shielding, bonding and return paths control EMI and HIRF.
Worked example
A 2.5 kW DC load on a 28 V bus draws I = P/V = 2500/28 = 89.3 A. A 0.02 Ω feeder gives V_drop = 89.3 × 0.02 = 1.79 V, or 6.4% of 28 V - too high for many terminal loads, so a larger cable with lower resistance is required. For AC, 115 V phase voltage gives 200 V line-to-line: V_LL = √3 × V_phase.
Traps
Using 115 V as line-to-line, forgetting power factor in AC kW/kVA, or treating a bus tie as a generator parallel without checking frequency and voltage. A tripped generator is not automatically a battery failure.