9.4 Engine Electrical Generation and Protection

Electrical Power Around the Engine

The engine-related electrical system does several jobs: it cranks the engine through a starter, it charges the battery and powers the bus through an engine-driven generator or alternator, it senses and indicates (CHT, oil temperature, tachometer, fuel flow), and it protects wiring with breakers and fuses. Most light reciprocating aircraft use a belt- or gear-driven alternator with a solid-state voltage regulator that controls field current to hold bus voltage near 14 V (12-V system) or 28 V (24-V system).

Older and larger aircraft may use DC generators with carbon-pile or transistorized regulators, and turbine aircraft commonly use a starter-generator that motors the engine for start and then becomes the generator.

Note an important independence: a certificated reciprocating engine runs on magneto ignition, which is self-powered once turning, so after a successful start the engine keeps running even with a total electrical failure. The electrical system is still needed for starting aids, indication, lighting, and avionics, but the running engine itself does not depend on it — a distinction the ACS tests directly.

Ohm's Law and a Worked Charging Calculation

Electrical troubleshooting rests on Ohm's law: E = I x R (voltage = current x resistance), and on the power relationship P = E x I. In a series circuit the current is the same everywhere and voltages add; in a parallel circuit the voltage is the same across each branch and the branch currents add.

Worked example. A landing-light branch on a 28-V bus draws 8.4 A. Its resistance is R = E / I = 28 / 8.4 = 3.33 ohms, and it dissipates P = E x I = 28 x 8.4 = 235 W. If a corroded connector adds 1 ohm of unwanted series resistance, total resistance becomes 4.33 ohms, current falls to 28 / 4.33 = 6.5 A, and the light dims — the classic symptom of high-resistance corrosion in a circuit. The same logic explains a slow-cranking starter: a weak battery or a high-resistance cable/ground drops voltage under the heavy starter load, so the starter sees far less than bus voltage and turns slowly while panel lights dim.

Charging Faults and Circuit Protection

When the charging system misbehaves, the alternator/generator, regulator, field circuit, wiring, drive, current limiter, and battery are all candidates. Undercharging (low bus voltage, battery discharging) often traces to an open field, a failed regulator commanding low output, a slipping/broken drive belt, or a blown alternator field breaker.

Overcharging (high bus voltage, hot battery, possible boiling electrolyte) most directly indicates a failed voltage regulator allowing unregulated field current, or a poor regulator sensing/ground connection — left unchecked it cooks the battery and the bus. The mechanic confirms by measuring field voltage and bus voltage and comparing to the regulator's setpoint.

Circuit protection — thermal circuit breakers and fuses — exists to protect the wiring from overheating in an overload or short, not to protect the connected device. A breaker that trips once may be a transient; a breaker that trips repeatedly is a fault to diagnose (a short to ground, a shorted component, or an overloaded circuit). It must never be tied closed, held in, or replaced with a higher rating, because that defeats wire protection and invites a fire.

The correct action is to isolate the circuit, find the short or overload, and repair it. Distinguishing open (no current, dead branch), short (excess current, tripping protection), and overload (current above rating from too much connected load) is the heart of electrical fault sorting.

Batteries, Starter-Generators, and Inspection Practice

Light aircraft use lead-acid batteries (flooded or sealed/valve-regulated) and some use nickel-cadmium (Ni-Cad) batteries common in turbine aircraft. A Ni-Cad can suffer thermal runaway, where overheating raises current draw, which raises heat further — a reason turbine aircraft monitor battery temperature and why charging-system condition matters. Battery condition is verified by a capacity (load) test, not by open-circuit voltage alone, because a sulfated or worn battery can read full voltage with no load yet collapse under the heavy starter draw.

A turbine starter-generator is a single machine that motors the engine for start, then is reconfigured to generate once the engine drives it. This saves weight but means a starter-generator fault can present as either a no-crank or a charging problem.

During inspection the mechanic checks the alternator/generator drive belt tension or coupling, brush wear on generator-type units, the security and corrosion of all terminals and grounds (a bad ground is one of the most common and most overlooked high-resistance faults), the condition of bonding straps, and the calibration of the voltage regulator. A loose or corroded ground will show up exactly like the Ohm's-law example above: added series resistance, dropped voltage at the load, dim lights, and slow cranking.

Always measure the voltage drop across a suspect connection under load rather than relying on a static resistance reading, because some high-resistance faults only appear when current flows.