Aircraft Electrical System Overview

Main Components of Aircraft Electrical Systems

Aircraft electrical systems are designed to supply both AC and DC power to various onboard systems. The primary sources are engine-driven generators or alternators (which convert mechanical energy into electrical energy), and batteries that provide backup or emergency power. In modern aircraft, AC power is often converted to DC using transformer rectifier units (TRUs), supplying 28V DC to most systems.

Busbars and Power Distribution

Electrical loads are organized and distributed via busbars—central points that connect multiple circuits. Busbars are classified based on the criticality of the systems they power:

• Vital/Essential Busbars: Supply flight-critical systems with redundancy and backup.
• Non-Essential Busbars: Power systems that can be shed during emergencies.
• Hot Busbars: Remain powered at all times for emergency or safety functions (e.g., fire detection, APU start).

Monitoring and Control

Cockpit panels display system parameters such as voltage, current, and frequency. Pilots can monitor the health of both AC and DC systems, select specific busbars for detailed readings, and control the connection of batteries and other major components.

Aircraft Structure as Common Earth

Most aircraft use the airframe as a common electrical return path (single-pole earth return), reducing wiring weight. However, this requires careful electrical bonding to equalize potential across the structure, preventing sparking, interference, and fire hazards.

Electrical Failures and Redundancy

Loss of a generator or busbar can impact system availability. Essential busbars are designed with multiple power paths to maintain supply even if one source fails. In total power loss scenarios, the battery (via static inverter if AC is needed) powers only the most critical systems for a limited time.

Generator vs Alternator

A generator produces DC power directly, while an alternator generates AC, which is then rectified to DC. Modern aircraft often use alternators due to efficiency and reliability.

Battery Monitoring

Continuous monitoring of battery charge and discharge status is vital. Ammeters show if the battery is being charged or is supplying power, helping detect faults or abnormal loads.

Grounding During Refuelling

Before refuelling or defueling, the aircraft must be grounded to prevent static discharge, which could ignite fuel vapours.