Aircraft Electrical Distribution (Bus Structure)

Electrical distribution in the aircraft

The electrical distribution system routes power from the source (alternator/battery) to the loads. The central node is called the bus (busbar).

Bus structure in PPL aircraft

A typical C172 or PA-28 has a simple, hierarchical bus structure:

        BATTERY ────┬──── MASTER SOLENOID ─────┐
                    │                            │
        ALTERNATOR ─┘                            │
                                                  ▼
                                          MAIN BUS (14 V)
                                                  │
                ┌─────────────────────────────────┼─────────────────────────┐
                │                                 │                          │
        AVIONICS MASTER                         CBs                        CBs
                │                          (loads)                    (loads)
                ▼
        AVIONICS BUS
                │
        ┌───────┴───────────────────┐
        │           │           │
       Radio       GPS      Transponder

Components:

• Main bus — feeds all non-avionics loads (pitot heat, landing light, beacon, cabin lighting, heater fan).
• Avionics bus — separately switchable, feeds sensitive avionics (radio, GPS, transponder, autopilot).
• CB panel — every load has its own CB; CBs are grouped (communication, navigation, cockpit lighting, etc.).

More complex bus structures (multi-bus)

Larger aircraft or IFR-certified GA aircraft have multiple separate buses for higher safety:

1. Battery bus

• Directly connected to the battery, independent of the master switch.
• Feeds only a few essential loads (emergency radio, ELT, emergency lighting).

2. Essential bus / emergency bus

• On alternator failure the pilot can manually select this bus.
• Only the minimum loads: one radio, one transponder, one display, standby instruments.
• Extends battery endurance.

3. Main bus

• Full power supply in normal operation.

4. Avionics bus

• As in simple types.

5. Crossover switch / cross-tie

• In dual-generator systems a bus can be fed by either generator.

Bus control

Master switch ON:

• Master solenoid closes → main bus connected to battery/alternator.
• CBs feed individual loads.

Avionics master ON (after start):

• Avionics bus connected to main bus.

On alternator failure:

• Disconnect ALT switch if appropriate, shed loads, bus voltage drops slowly (battery discharging).
• Possibly switch to essential bus.

Fault-current protection

Fuses/CBs:

• Every circuit has its own fuse or CB.
• On a short in one load, only that circuit trips — others remain functional.

Fault current detection:

• On modern types: ground fault interrupter (GFI) for wet environments (rare in light aircraft).

Bonding and grounding

Ground point:

• In metal aircraft the airframe is the common ground.
• Battery negative directly bonded to airframe.
• Loads have a positive lead via CB and a single negative point on the airframe.

Bonding connections:

• All metal parts (engine components, control surface hinges) electrically connected to the airframe → no potential differences, avoids sparking and static buildup.