Electricity fundamentals for aircraft

A few simple principles

Modern aircraft depend on electrical power for everything from engine starting and cockpit displays to flight controls, lighting, communication and navigation systems. To troubleshoot electrical faults, understand aircraft power systems and work safely around energized equipment, technicians need a solid grasp of the fundamentals: voltage, current, resistance, power, and the differences between AC and DC electricity.

While aircraft electrical systems can be highly sophisticated, they are all built on a few simple principles and equations.

Voltage (E or V)

Voltage is electrical pressure, measured in volts (V). It is the force that pushes electrons through a circuit. Without voltage, no current can move. A helpful analogy is water flowing through a pipe:

• Voltage = water pressure
• Current = water flow

Current (I)

Current is the flow of electrical charge through a conductor. It is measured in amperes (amps). Current is what actually powers electrical devices. For example:

• A small LED may draw milliamps.
• A landing light may draw several amps.
• An aircraft starter motor may draw hundreds of amps.

Resistance (R)

Resistance opposes the flow of current and is measured in ohms (Ω). Every circuit contains some amount of resistance. Resistance is produced by:

• Wire material
• Component design
• Connections
• Corrosion
• Damaged conductors

Power (P)

Power is the rate at which electrical energy is converted into useful work. It is measured in watts (W). Examples of electrical work include:

• Producing light
• Driving motors
• Heating components
• Powering avionics
• Charging batteries

Ohm's law

The relationship between voltage, current and resistance is called Ohm's law: E = I × R, where E is voltage in volts, I is current in amps and R is resistance in ohms. If any two values are known, the third can be calculated. Ohm's law is one of the most frequently used tools in electrical troubleshooting. For example:

• Voltage 28 V and resistance 14 Ω: current = 28 ÷ 14 = 2 A.
• Voltage 24 V and current 3 A: resistance = 24 ÷ 3 = 8 Ω.

Electrical power

Power combines voltage and current. The primary equation is P = E × I, where P is power in watts, E is voltage in volts and I is current in amps. For example, 28 V × 10 A = 280 W, so the device consumes 280 watts.

The importance of I²R heating

Power can also be written as P = I² × R. This equation explains why electrical connections get hot. Notice that current is squared: double the current and you get four times the heat, triple it and you get nine times the heat.

A slightly loose or corroded connection may add only a small amount of resistance, but if it carries high current, significant heating can occur. For aircraft technicians, understanding I²R heating is critical when inspecting high-current circuits. This can lead to:

• Burned connectors
• Damaged terminals
• Wiring failures
• Smoke events
• Electrical fires

Conductors and insulators

Electricity flows through conductors and is restricted by insulators. Insulation prevents unwanted current paths and protects personnel from electrical hazards.

• Common conductors: copper, aluminum, silver and gold. Copper is the most widely used aircraft wiring conductor.
• Common insulators: Teflon, rubber, plastic compounds and composite materials.

Series circuits

Aircraft circuits are built using combinations of series and parallel arrangements. In a series circuit, there is only one path for current flow. For example, in a circuit running from the battery through lamp A and lamp B to ground, if lamp A fails open, lamp B also goes out. Series circuits are relatively uncommon for aircraft loads because a single failure affects the entire circuit. Characteristics:

• The same current flows through every component.
• Individual voltage drops add up to the source voltage.
• One open component interrupts the entire circuit.

Parallel circuits

In a parallel circuit, multiple current paths are available. For example, a 28 V bus may feed a landing light, a radio, a fuel pump and a display unit, each operating independently. Aircraft electrical distribution systems are primarily parallel systems. Characteristics:

• Each branch sees full bus voltage.
• Branch currents add together.
• One failed load does not necessarily affect others.

Open circuits

Understanding open and short circuits is essential for troubleshooting. An open circuit occurs when the current path is broken, so no current flows and equipment stops operating. An open circuit is like a broken pipe in a water system. Causes include:

• Broken wires
• Failed switches
• Loose connectors
• Open fuses

Short circuits

A short circuit occurs when current bypasses the intended load and takes an unintended low-resistance path. Short circuits can cause serious damage if not properly protected. Effects include:

• Excessive current flow
• Overheating
• Tripped circuit breakers
• Blown fuses

Direct current (DC)

Direct current (DC) flows in one direction only. Sources include batteries, aircraft battery buses and many electronic power supplies.

Most transport-category and many other aircraft use approximately 28 V DC as their standard DC system voltage, while many light aircraft use 14 V DC. The term "28 V DC" comes from the nominal voltage supplied by a charging system operating a 24-volt battery system. Advantages of DC:

• Simple distribution
• Ideal for batteries
• Well-suited for avionics and electronics
• Easy energy storage

Alternating current (AC)

Alternating current (AC) reverses direction periodically. Instead of flowing continuously one way, current oscillates back and forth. Large aircraft commonly use 115/200 V AC, three-phase, 400 Hz power systems.

Why aircraft use 400 Hz

Most utility power systems use 50 Hz (much of the world) or 60 Hz (North America). Aircraft typically use 400 Hz, and the major advantage is weight reduction. At 400 Hz:

• Transformers become smaller.
• Motors become lighter.
• Generators become lighter.
• Filters become smaller.

Every pound matters

Since every pound matters in aviation, these weight savings are extremely valuable. A 400 Hz system can provide the same power while requiring significantly smaller and lighter components than equivalent 50/60 Hz systems.

Three-phase AC power

Large aircraft generally use three-phase AC power. Many high-power aircraft systems depend on it. Benefits include:

• More efficient power generation
• Smoother motor operation
• Reduced vibration
• Greater power capacity
• Smaller wiring for equivalent loads

Modern aircraft power systems

Modern aircraft increasingly use advanced electrical architectures. Aircraft such as the Boeing 787 use significantly more electrically powered systems than earlier generations, reducing reliance on pneumatic and hydraulic power where practical. The trend continues toward greater electrical efficiency and reduced aircraft weight. Examples include:

• Variable-frequency AC systems
• Higher-voltage DC systems
• Integrated power electronics
• More-electric aircraft designs

Aircraft electrical distribution

Electrical power is distributed through buses. A bus is simply a common electrical distribution point. Each system receives power from the appropriate bus through switches, relays and protective devices. Because buses operate as parallel distribution systems, individual loads can often fail without affecting other equipment. Examples include:

• Battery bus
• Essential bus
• Avionics bus
• Main AC bus
• Emergency bus

Circuit protection

Aircraft circuits are protected by circuit breakers, fuses and current limiters. Their purpose is often misunderstood. Circuit breakers and fuses primarily protect the wiring, not individual equipment. The goal is to prevent wiring from overheating and becoming a fire hazard.

Why breakers trip

The breaker's operation is a symptom of a fault somewhere in the circuit. A breaker generally trips because:

• Excessive current exists
• A short circuit exists
• A component has failed
• Wiring has been damaged

The danger of repeated resets

Repeatedly resetting a tripped breaker can be dangerous. A tripped breaker should be treated as evidence of a problem, not as a nuisance to be ignored. Maintenance personnel should identify and correct the fault before repeatedly resetting protective devices. If the underlying fault remains:

• Wires may continue heating.
• Insulation may degrade.
• Smoke or fire may develop.

Common electrical troubleshooting clues

Electrical faults often leave clues. Understanding the difference dramatically speeds troubleshooting.

• High-resistance faults may cause dim lights, slow motors, intermittent operation, hot connectors and voltage drops.
• Open circuits may cause complete loss of operation, dead components and no measured current flow.
• Short circuits may cause tripped breakers, blown fuses and excessive current draw.

Key takeaways

• Voltage is electrical pressure, current is charge flow, and resistance opposes current flow.
• Ohm's law: E = I × R.
• Power: P = E × I.
• I²R heating explains why poor connections become hot.
• Series circuits share the same current. Parallel circuits share the same voltage.
• Aircraft electrical buses are primarily parallel distribution systems.
• DC flows one direction. AC reverses direction periodically.
• Most larger aircraft use 28 V DC, and many use 115/200 V AC, three-phase, 400 Hz.
• 400 Hz systems reduce the size and weight of motors, transformers and generators.
• Circuit breakers and fuses protect wiring from overheating.
• Never repeatedly reset a tripped breaker without identifying the underlying fault.

The bottom line

Aircraft electrical systems may appear complex, but they are built on a few fundamental concepts: voltage pushes current, resistance limits it, and power does useful work. Understanding Ohm's law, power relationships, circuit behavior and the differences between AC and DC gives technicians the foundation needed to troubleshoot faults, maintain electrical systems safely, and understand why aircraft electrical designs use 28-volt DC systems alongside lightweight 400 Hz AC power generation.