Electrical components in the aircraft
Switches
Functions:
- Master switch (Battery Master / ALT Master) — turns the bus on/off, often split (separate for battery and alternator, so the alternator can be isolated in an emergency).
- Magneto switch (ignition switch) — selects magneto (L / R / BOTH / OFF) and often combined with the starter (START position).
- Load switches — radio, pitot heat, landing light, position lights, beacon, etc.
- Toggle vs rocker: toggle or rocker switches, both common.
Fuses and circuit breakers
Fuses
- One-time fusible elements that blow on excess current.
- Ratings: typically 1–30 A.
- Replacement: on a C172 and similar types, spare fuses are often behind a flap in the cockpit.
- After a fuse blows: find the cause before replacing (loose wiring, short circuit).
Circuit breakers (CBs)
- Resettable protection — the knob pops out on overload.
- Reset: push the knob back in (POP/RESET); if it trips again, do not push it back.
- Advantage: no replacement parts needed.
- Ratings: typically 1–30 A; main-bus CBs can be 50–80 A.
Trip behaviour:
- Fast on short circuit (many times rated current).
- Delayed on moderate overload (e.g. pitot heat at low bus voltage draws 20% over).
Master solenoid (main relay)
- Electromagnetic main relay between battery and bus.
- Connects battery to bus when the master switch is ON.
- Disconnects battery from bus when OFF → no discharge through loads.
Starter solenoid
- Separate high-current relay for the starter motor.
- Driven by the magneto switch (START position).
- Switches the starter current (100–300 A) directly from the battery.
Avionics master / avionics bus
- Separate switch for the avionics bus (radio, GPS, transponder).
- Purpose: disconnect the avionics bus during engine start to keep voltage transients away from sensitive electronics.
- Switch on after the bus voltage has stabilised (after start).
Ammeter and voltmeter
Ammeter:
- Centre-zero — shows current in and out of the battery. Positive = charging, negative = discharging.
- Load indication — shows alternator output (always positive).
Voltmeter:
- Shows bus voltage; should read 13.8–14.2 V on a 14 V system.
- Below 13 V: alternator problem; above 15 V: voltage regulator failure.
Lights and lighting
External lights:
- Position (navigation) lights: red left, green right, white tail. Mandatory at night and in poor visibility.
- Anti-collision light (strobe / rotating beacon): white flashing or red rotating; visible from all sides.
- Landing light: usually in the wing; single or dual.
- Taxi light: weaker than landing light; for ground operations.
Internal lighting:
- Instrument lighting (red, green or white, dimmable).
- Cabin reading light.
LED lighting is now standard for low power draw and long life.