7.2 Airframe Electrical Power, Wiring, and Protection

Generation, the Bus, and the Battery

Airframe electrical systems include DC and AC generation, voltage regulation, batteries, distribution buses, lighting, wiring, connectors, switches, circuit protection, bonding, shielding, and measurement. The exam may show a simple fault, but the safe mechanic sees the whole path: current leaves a source, passes through protection and control, reaches a load, and returns through ground or a return conductor.

Most light aircraft use a 14-volt system (12-volt battery) or a 28-volt system (24-volt battery); the charging source runs about 1–2 volts above battery voltage so the battery charges. An alternator produces AC internally and rectifies it to DC; it is lighter, produces useful output at low engine RPM, and is the modern standard. A DC generator produces DC directly through a commutator but needs higher RPM for output. A voltage regulator holds output voltage steady across engine speed and load.

Large turbine aircraft commonly use 115-volt, 400-Hz AC generators, with a transformer-rectifier (TR) unit producing 28-volt DC for DC loads and an inverter producing AC from DC for emergency/instrument loads.

The bus bar is the common metal strip from which power is distributed; loads tap off the bus through individual circuit protectors. Buses are often split (main, essential/battery, avionics) so critical loads can be isolated. The battery supplies starting current, provides backup if the generator fails, and dampens voltage transients. When a symptom appears, identify the source and the affected bus first — do not chase a single landing light if the entire bus is dead.

Wire Sizing, Protection, and Bonding

Wire is sized by American Wire Gauge (AWG) — and the counter-intuitive rule the test loves is that the smaller the AWG number, the larger the conductor. Heavy primary cables (battery, starter, alternator, bus feeders) run 8 AWG down to 00 AWG; small instrument and lighting circuits use 18–22 AWG.

Selection depends on the current carried, the allowable voltage drop (continuous-load circuits permit a smaller drop than intermittent ones), conductor length, insulation temperature rating, and whether the wire is bundled or in free air (bundled wire derates because heat cannot escape). Routing and securing protect wire from chafing, fluid, heat, and vibration; clamps and grommets are inspection items.

Circuit protection protects the wire, not the load. A breaker or fuse opens before the conductor overheats and ignites. This is why installing a higher-rated breaker to stop a nuisance trip is unsafe — it lets the wire carry more current than its insulation can survive. Circuit breakers can be reset (and are "trip-free," meaning they trip even if the button is held), while fuses must be replaced; spare fuses (commonly 50% of each rating, at least one) are carried. Find the cause of a trip before resetting; repeated resetting of a tripped breaker is poor practice and can start a fire.

Connectors, splices, terminals, and soldering are exam favorites because small workmanship errors cause large faults. A poor crimp creates resistance and heat; a loose terminal gives intermittent failures under vibration; excess solder wicks up stranded wire and creates a stiff stress point that fatigue-cracks; wrong flux promotes corrosion. Bonding jumpers provide a low-resistance path so all metal parts share one potential — this carries static off the airframe, gives lightning a controlled path, and provides an EMI-quiet ground for radios.

Shielding drains electromagnetic interference. Both must be reconnected after any repair. Finally, respect energized-circuit hazards: de-energize and tag when practical, and be cautious with external power, where wrong polarity or voltage can damage equipment and arcing can injure personnel.

Ohm's Law and the Electrical Load

The Airframe test assumes the General-level math. Ohm's law is E = I × R (voltage = current × resistance), and power is P = E × I. Worked example: a 28-volt landing light draws 7 amps. Its resistance is R = E ÷ I = 28 ÷ 7 = 4 ohms, and it consumes P = E × I = 28 × 7 = 196 watts. To protect that circuit you would choose the next standard breaker above the 7-amp continuous load (and size the wire for 7 A at the run length and allowable voltage drop).

In a series circuit current is the same everywhere and resistances add; in a parallel circuit voltage is the same across each branch and the total resistance is less than the smallest branch. Most aircraft loads hang in parallel off the bus, which is why one failed lamp does not kill the others.

The electrical load must be managed against the generator/alternator rating. The mechanic (and an electrical load analysis) totals continuous loads, intermittent loads, and the battery charge demand and confirms the charging source can carry them with margin. An ammeter shows charge/discharge (or total load), and a loadmeter shows the fraction of rated output in use. If the load exceeds generation, the battery discharges and bus voltage falls.

When troubleshooting, read whether one load or many loads are affected to locate the fault: many loads on one bus point to a source, bus-feed, or protection fault, while a single load points to its own branch — control, wiring, ground, or load. Never repair an aircraft wire with an automotive connector or unapproved splice; restore the approved design.