AC vs DC Power: Characteristics and Aircraft Applications

Walk through the electrical bay of any modern transport-category aircraft and you will find two fundamentally different kinds of electrical power sharing the same airframe. Alternating current (AC) and direct current (DC) each have physical properties that make them suited to specific jobs, and aviation maintenance technicians must understand both to troubleshoot circuits, read wiring diagrams, and ensure airworthiness. This article explains what AC and DC are, how each behaves, why aircraft designers choose one over the other for particular loads, and what the FAA knowledge test expects you to know cold.

What Is Direct Current?

Direct current flows in one direction only — from the negative terminal of a source, through the circuit, and back to the positive terminal (conventional current flows from positive to negative, but the electron flow is the reverse). The voltage and polarity remain essentially constant over time. A simple battery is the classic DC source: chemical energy creates a stable potential difference that drives current in a single, unwavering direction.

In aircraft, DC power is generated by generators and alternators, and stored in lead-acid or nickel-cadmium batteries. Most light general aviation aircraft operate on a 14-volt or 28-volt DC system. The 28-volt standard (nominally 24 V battery, regulated to about 28 V by the alternator) is common in smaller aircraft and serves as secondary and battery-bus power on larger aircraft, but it is not the dominant primary power standard on transport-category aircraft — those aircraft rely primarily on AC generation, as discussed below.

What Is Alternating Current?

Alternating current periodically reverses direction. The voltage rises from zero to a positive peak, falls back through zero, reaches a negative peak, and returns to zero — completing one cycle. The number of complete cycles per second is the frequency, measured in hertz (Hz). Commercial power in the United States runs at 60 Hz; aircraft AC systems almost universally use 400 Hz.

The choice of 400 Hz is deliberate and important. Because transformers and motors are smaller and lighter at higher frequencies, the 400 Hz standard allows the same power to be handled by equipment that weighs far less than equivalent 60 Hz hardware. In a world where every pound costs fuel and payload, that difference is significant. The trade-off is that 400 Hz power cannot travel long distances without unacceptable losses, which is why aircraft AC buses are kept physically close to their sources.

The magnitude of AC voltage is expressed as root-mean-square (RMS) voltage, sometimes called effective voltage. RMS is the AC equivalent that produces the same heating effect as an equal DC voltage. The peak voltage of a sine wave is higher than its RMS value by a factor of approximately 1.414 (the square root of 2). For a standard 115-volt AC aircraft bus, the actual peak voltage is about 163 volts. When the FAA or an aircraft manual states a voltage, it refers to RMS unless otherwise noted.

How AC and DC Differ in Circuit Behavior

DC circuits are governed entirely by resistance. Ohm's law (E = IR) applies directly, and resistive loads like heating elements and incandescent lamps behave the same on DC as they do on AC of equivalent RMS voltage.

AC circuits introduce two additional opposition forces: inductive reactance and capacitive reactance. Inductors (coils, transformer windings, motor windings) resist changes in current; their inductive reactance increases with frequency. Capacitors resist changes in voltage; their capacitive reactance decreases as frequency rises. The combined opposition of resistance and reactance is called impedance (Z), measured in ohms. These reactive properties are what make transformers possible — they only work on AC because the constantly changing magnetic field in the primary winding induces a voltage in the secondary. On DC, a transformer passes no steady-state energy; it acts like a short circuit (resistively) or simply blocks current (through inductance).

Another AC-specific concept is phase. In a single-phase system, one AC voltage source feeds the circuit. Aircraft commonly use three-phase AC, in which three separate voltages are generated 120 degrees apart. Three-phase systems deliver power more smoothly, allow smaller and simpler motors, and are more efficient for high-power applications. Large transport aircraft use three-phase, 115/200-volt, 400 Hz AC as the backbone of their primary electrical distribution.

Aircraft Applications: Where Each Type Is Used

DC Applications

DC remains the power of choice wherever stored energy, polarity sensitivity, or simplicity is required. Common DC loads and systems include:

Batteries and emergency power: Batteries are inherently DC devices. The aircraft battery provides starting power and an emergency backup if all generators fail.
Avionics and flight instruments: Most avionics, radios, and glass-panel displays are designed around regulated DC (typically 14 V or 28 V) because their internal solid-state circuits require stable, polarity-specific voltage.
Ignition systems (reciprocating engines): Magneto-fired ignition does not depend on the aircraft electrical system, but starter motors and many auxiliary systems are DC-powered.
Fuel and oil quantity indicators, position lights, warning lights: Simple resistive circuits that operate cleanly on DC.
Solenoids, relays, and contactors: Used throughout DC distribution to switch high-current loads remotely.

AC Applications

AC shines where high power, motor-driven loads, or transformer-stepped voltages are needed. Common AC loads include:

Autopilot servo motors and actuators: AC motors are simpler, lighter, and more reliable than equivalent DC motors because they have no brushes to wear.
Gyroscopic flight instruments (older designs): Attitude indicators and directional gyros driven by air-turbine motors in older aircraft were replaced in many cases by AC electric motors, typically operating at 26 V AC, 400 Hz.
Large transport aircraft primary distribution: Engine-driven AC generators (often called integrated drive generators, or IDGs) produce 115/200-volt, three-phase, 400 Hz AC to power galleys, hydraulic pump motors, environmental control packs, and other high-demand loads.
Transformer-rectifier units (TRUs): Convert AC to DC for avionic buses, demonstrating that most modern transport aircraft actually use both types simultaneously — AC as the primary generation medium, DC as the distribution medium for sensitive loads.
Inverters: Convert DC to AC for aircraft that generate power primarily as DC but need AC for certain instruments — the opposite of a TRU.

Key Numbers and Rules

Standard light aircraft DC systems: 14 V or 28 V nominal.
Aircraft AC frequency: 400 Hz (versus 60 Hz for household power).
Standard transport AC bus voltage: 115 V single-phase or 115/200 V three-phase.
RMS vs. peak: Peak voltage = RMS × 1.414; for 115 V RMS, peak ≈ 163 V.
Three-phase phase separation: 120 degrees between each of the three phases.
Why 400 Hz: Transformer and motor cores can be smaller and lighter — critical for airborne use.
Impedance (Z): Total opposition to AC flow; includes resistance, inductive reactance, and capacitive reactance.
Power factor: Ratio of true power (watts) to apparent power (volt-amperes) in an AC circuit; equals 1.0 for purely resistive loads, less than 1.0 when reactance is present.

Common Test Traps

Confusing RMS with peak voltage. The FAA asks about the effective (RMS) value of AC. If a question gives you a peak voltage and asks for the effective voltage, divide by 1.414 (or multiply by 0.707). Do not use peak voltage as if it were RMS.
Thinking transformers work on DC. They do not. A transformer requires changing flux, which only AC provides. Applying DC to a transformer primary will simply pass resistive current (or blow a fuse); no stepped voltage appears at the secondary.
Forgetting that aircraft use 400 Hz, not 60 Hz. A question may list several frequencies; 400 Hz is always the correct answer for aircraft AC systems.
Assuming all aircraft power is DC. While light GA aircraft often are entirely DC, transport-category aircraft are predominantly AC with DC derived through TRUs. Know both architectures.
Mixing up TRUs and inverters. A TRU converts AC to DC. An inverter converts DC to AC. These devices do the opposite jobs; swapping them on an exam is a common error.

Mastering the distinction between AC and DC — their behavior in circuits, their advantages, their limitations, and the specific voltages and frequencies used in aviation — gives you the foundation to understand every electrical system you will encounter on the flight line. Whether you are inspecting a Cessna 172's 14-volt alternator system or troubleshooting a transport jet's IDG-fed AC bus, the underlying principles remain the same.