Airframe Study Guide

Aircraft Electrical Systems

Electrical systems are a significant portion of the FAA Airframe written exam. Understanding system architecture, circuit protection, and wire sizing will help you both on the test and in the shop.

1. AC vs. DC Electrical Systems

Most general aviation (GA) aircraft use a 28-volt DC electrical system (or 14V for lighter aircraft). DC is simpler to generate and distribute and works well for the avionics, lighting, and motors typically found in small aircraft.

Transport category (airline) aircraft use both AC and DC:

AC (Alternating Current) — Typically 115V, 400 Hz on transport aircraft. Used for high-power loads: avionics, flight instruments, large motors (hydraulic pumps, fuel pumps), galley equipment. AC at 400 Hz allows smaller, lighter transformers than 60 Hz utility power.
DC — Used for battery power, emergency systems, and loads that require DC (certain motors, relays). DC is derived from AC through transformer-rectifier units (TRUs).
Generation: GA aircraft use engine-driven alternators (AC → regulated to DC via integral rectifier). Transport aircraft use integrated drive generators (IDGs) or variable-frequency generators (VFGs) to produce AC.

2. Bus Architecture

Aircraft electrical systems distribute power through buses(busbars) — common connection points that feed multiple circuits. Buses are segregated by criticality to ensure that a fault in one area does not cascade to critical systems.

Main Bus — Feeds most normal electrical loads. Powered by the alternator/generator. Non-essential loads can be shed here if generation is lost.
Essential Bus — Feeds flight-critical systems (primary flight instruments, communication radios, warning systems). Stays powered when the main bus is shed. Fed directly from battery or a dedicated source.
Avionics Bus — Dedicated bus for avionics with voltage regulation and noise filtering. Often protected by an avionics master switch to prevent startup/shutdown voltage spikes from damaging sensitive equipment.
Hot Battery Bus — Always connected directly to the battery regardless of master switch position. Used for emergency equipment that must function without any switches (some fire detection, some ELT systems).

Bus segregation also simplifies fault isolation: if a circuit breaker pops, only the circuits on that bus segment are affected.

3. Circuit Protection

Every circuit must be protected against overcurrent to prevent wire overheating and potential fire. Aircraft use two types of overcurrent protection devices:

Fuses — One-time devices. When the rated current is exceeded, the fusible element melts and opens the circuit. Must be replaced (not reset). Used on general aviation aircraft and as backup protection. Always carry spare fuses of the correct rating.
Circuit Breakers (CBs) — Resettable devices. A tripped CB can be reset by pushing it back in. Used on most transport category aircraft and modern GA aircraft because they can be reset in flight without requiring spare parts.
Important rule: If a CB trips in flight, it should be reset once after a brief cooling period. If it trips again, do not reset it — the fault is still present and forcing current through a shorted circuit can cause fire. Report the fault for maintenance.
What CBs protect: Circuit breakers protect the wiring, not necessarily the equipment. The CB rating must be equal to or less than the wire's ampacity rating.

4. Wire Sizing

Selecting the correct wire gauge requires satisfying two independent constraints. The final wire selection must satisfy both — pick the larger (more conservative) gauge:

Constraint 1 — Ampacity: The wire must carry the required current without overheating. Ampacity depends on wire gauge, insulation type, ambient temperature, and whether the wire is in a bundle or conduit.
Constraint 2 — Voltage Drop: The wire resistance must be low enough that the voltage drop along the circuit does not exceed the allowable limit. Typical limits: 1–2% for avionics and sensitive equipment, up to 5% for lighting and motors. The formula for required circular mils (CM): CM = (K × I × 2L) / ΔV where K ≈ 10.75 (for copper), I = current, L = one-way length, ΔV = allowable drop.

Use the Wire Gauge Calculator to determine the correct AWG size for your circuit based on current, run length, and voltage drop constraints.

Wire is identified by its AWG number — lower numbers are larger wire. AWG 22 is thin (suitable for signal circuits); AWG 4 is heavy wire for high-current buses. Aircraft wire gauge is always selected from AC 43.13-1B Table 11-9 or the aircraft wiring diagram manual.

5. Common Faults and Troubleshooting

Most aircraft electrical problems fall into one of three categories. Knowing how to identify each saves diagnostic time on the flight line:

Open Circuit — A break in the circuit path. The load receives no power. Caused by a blown fuse, tripped CB, broken wire, or failed connector. Test with a voltmeter — check for voltage at the load side of the suspected break.
Short Circuit — An unintended path to ground (or to another conductor). Causes high current flow, trips CBs, or blows fuses. Often caused by chafed insulation where a wire contacts the airframe. Locate with an ohmmeter after removing power.
High-Resistance Connection — The most insidious fault. The circuit appears intact but a corroded terminal, loose connector pin, or cold solder joint introduces resistance that reduces current and voltage to the load. Symptoms: intermittent operation, dim lights, slow motors, unexplained avionics resets. Test by measuring voltage drop across connectors under load. Any connector should show <0.1V drop.

General troubleshooting approach: Half-split the circuit — start in the middle, determine which half the fault is in, then repeat. Measure voltage under load (not just with a test light) for accurate results.

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