Electrical systems

021 Aircraft General KnowledgeChapter 6

Basic electrical theory

Formula — Ohm's law and power

V = I × R    P = V × I = I²R = V²/R
Most common AC frequency in aviation: 400 Hz
Resistance of most conductors increases with temperature

Key fact

CIVIL (reactance phase): In a capacitor, current leads voltage. In an inductor, voltage leads current.

Capacitive reactance XC = 1/(2πfC) — ↑frequency → ↓XC → ↑current. Inductive reactance XL = 2πfL — ↑frequency → ↑XL → ↓current.

Wiring and protection

Key fact

Unipole (earth return): Airframe is return path — saves weight and wiring; easier fault tracing; short circuits more likely. Dipole: Used on non-metallic structures.

Trip-free CB: Cannot hold contacts closed during fault. Non-trip-free: Can be held in — may cause damage. CB may be reset once after fault rectified, if necessary for safe flight.

Device	Feature
Fuse (light duty)	Ceramic, thin wire — one replacement in flight
Circuit limiter	High melting point — allows transient overload
Thermal CB	Prolonged overcurrent (heating delay)
Magnetic CB	Faster trip — no heating delay

Bonding and static

Key fact

Bonding purposes: Equalise potentials; single earth for unipole system; safe lightning path; reduce radio interference; safe charge/current distribution.

Static dischargers (wicks): Wing and tail tips — dissipate charge in flight; limit charge transfer with electrified clouds. Corona discharge if charge fails to dissipate — causes glow and interference.

Batteries

Key fact

Main function on transport aircraft: Emergency electrical power source. Capacity: Ampere-hours (Ah). Must be ≥80% rated capacity when checked (~every 3 months).

Lead-acid: ~2–2.2 V per cell; one broken cell = whole battery unserviceable; test on-load vs off-load voltage. NiCad: Lighter, stable voltage before rapid discharge; potassium hydroxide electrolyte; thermal runaway risk — monitor temperature on decreasing voltage.

Formula — Series and parallel

Series: voltages add, capacity unchanged
Parallel: capacities add, voltage unchanged

Exam trap

Battery control unit isolates battery when: charging complete, overheat, internal short circuit. Thermal runaway (charging): Temp↑ → resistance↓ → current↑ → temp↑.

High charge rate after engine start is normal; not normal if it persists.

DC generation

Key fact

Series wound: Voltage varies with load — high starting torque (starter motors). Shunt wound: Voltage falls slightly with load — needs minimum RPM and existing field. Compound wound: Near-constant voltage.

Differential cut-out (DCO): Connects generator to bus when output sufficient; isolates on reverse current (~20–30 A). Reverse current CB: Protects at 300–500 A reverse current — manual reset only.

AC alternators and CSD

Key fact

Alternator: Fixed stator windings (AC output); rotor field coils fed DC. Frequency wild if directly engine-driven (varies with RPM).

CSD/IDG: Hydromechanical — constant generator speed → constant 400 Hz. Can disconnect in flight; reset on ground only after shutdown. Red arc = disconnect, unusable rest of flight. Monitors: oil temperature, low oil pressure.

Formula — Frequency and 3-phase

RPM = Frequency × 2 × 60 ÷ number of poles
f = N × P ÷ 60

Star: Line voltage = √3 × phase voltage; Line current = phase current
Delta: Line voltage = phase voltage; Line current = √3 × phase current

Key fact

Typical aircraft AC: 115 V phase / 200 V line, 400 Hz. Star connection used in aviation (neutral absorbs excessive current — no earth available).

Distribution and paralleling

Key fact

Hot bus / DC bus: Directly connected to battery — emergency lights, fire extinguishers, landing gear. Bus tie breaker (BTB): Ties generators; opens on phase imbalance.

GCB: Connects generator to busbar. GCR (exciter breaker): Switches field excitation on/off — closes when generator voltage greater than battery; opens when battery voltage greater.

Exam trap

Paralleled generators require: equal voltage, frequency, phase rotation, same phase voltage — current unimportant. Unparalleled generators: phase unimportant.

Load sharing: Real load (kW) via CSD torque/equalising circuit; reactive load (kVAR) via voltage regulator/excitation. Underspeed: generator breaker opens only. Over/under-excitation, over-voltage: exciter + generator breakers open.

Key fact

Load shedding: Switch off non-essential loads to prevent generator overload. TRU: AC to DC. Inverter: DC to AC (static ~75% efficient; rotary ~50%). GPU: Short auxiliary pin prevents battery overcharge when ground power connected.

Feeder fault on DC circuit: Flux unbalance between voltage coil and series winding.

Motors and actuators

Key fact

DC series motor: High torque at low RPM — ideal starter. Self-regulating: on load = lower speed; off load = higher speed. Actuators: High-speed reversible series motors; direction reversed by reversing stator or rotor polarity (not both).

Squirrel cage (induction) motor: Loss of one phase — reduced torque, may overheat; cannot start if not already running.