11.4 Electrical System and Alternator Failure

ACS PA.I.G.K1f is the electrical system. PA.I.G.K2 is what you still have when it quits. PAR’s favorite wrong picture is a dark cockpit that must also be a silent engine. The magnetos were never on this bus.

Battery, alternator, bus

Most light airplanes use a 14- or 28-volt DC system. PHAK’s usual parts:

• Battery — starts the engine and is the emergency reservoir if the alternator dies.
• Alternator (or, on older airplanes, a generator) — supplies the system in flight and recharges the battery. Alternators still make usable current at low RPM; generators often do not, so idle taxi can drain the battery.
• Voltage regulator — holds output a little above battery voltage so the battery stays charged without being cooked.
• Bus bar — the distribution strip that feeds radios, lights, pumps, and instruments.
• Master switch, often split: a BAT half that connects the battery to the bus, and an ALT (or GEN) half that brings the alternator online. Turning the alternator half off leaves the entire electrical load on the battery.

The master powers almost everything electrical except the ignition system: position and anti-collision lights, landing and taxi lights, cabin and instrument lights, radios, electric turn coordinator, fuel gauges, electric fuel pump, stall-warning electronics, pitot heat, and the starter. Magnetos are not on that list.

Many modern panels split the bus further in concept if not in name: an essential (or battery/emergency) bus for the few items you keep in an electrical emergency, and an avionics bus behind an avionics master so you can spin the engine without a voltage spike hitting the radios. Know which switch kills the glass and which switch still leaves you a transponder or a comm, from that AFM.

Breakers, fuses, and 91.205 at night

Circuit breakers and fuses protect wires and equipment from overload. A breaker can be reset — once, if the AFM allows, after a cooling pause — if the trip was nuisance. A fuse must be replaced. A breaker that trips again stays out; you just found a short.

14 CFR 91.205(c)(6) adds a night-VFR item: one spare set of fuses, or three spare fuses of each kind required, accessible to the pilot in flight. That rule is about airplanes that use fuses. A breaker-only panel does not require a pocket of spare fuses for circuits that have no fuses. Day VFR (91.205(b)) does not add the spare-fuse line. The FLAPS memory aid (Fuses, Landing light, Anti-collision, Position lights, Source of electrical power) is a study handle for 91.205(c), not a substitute for the regulation.

Ammeter versus loadmeter

You cannot manage a failure you cannot read.

They are not the same gauge. An ammeter talks about battery charge direction. A loadmeter talks about alternator output demand.

Overvoltage (a failed regulator, often that full-scale plus) can cook the battery and the avionics. The systems move is to shed the alternator — ALT half of the master OFF — then treat the rest of the flight as a battery-only electrical problem.

Alternator failure: shed load, know what still works

When the ammeter goes full minus, the low-voltage light comes on, or the loadmeter collapses, assume the alternator is no longer feeding the bus. AFM flows differ in button order; the knowledge is stable:

1. Confirm the ALT side is on and the breaker is in (a popped alternator breaker is a cheap save).
2. If it will not come back, ALT off so a sick alternator cannot put junk on the bus.
3. Shed electrical load immediately: pitot heat if you do not need it, extra lights, unused radios, electric fuel pump if the engine-driven pump is doing the job, cabin power. You are spending a finite battery.
4. Plan a landing with enough battery for a radio call, a transponder squawk, and lighting if it is night.

What still works after a total electrical (battery + alternator) failure, on a typical trainer:

• Engine — dual magnetos, independent of the bus
• Engine-driven fuel pump
• Vacuum attitude and heading gyros, if they are driven by an engine-driven vacuum pump rather than by electricity
• Pitot-static airspeed, altimeter, and VSI (those instruments do not eat bus voltage)

What stops: radios and audio, transponder and ADS-B out, GPS and glass screens, electric turn coordinator, electric flaps, fuel gauges, interior and exterior lights, pitot heat, electric boost pump, stall-warning electronics. Night, IMC, or a busy Class B arrival on a dying battery is a land soon problem, not a “the engine will quit in four minutes” problem.

Starter still engaged, and the split master

After start, listen. A starter that remains engaged (grind, scream, or a starter annunciator) is trying to motor an already-running engine. It will overheat, can weld or fling the starter drive, and is a firewall fire candidate. This is not a “taxi in and write it up” item. Typical immediate action: mixture idle cutoff, master OFF, and get the engine stopped before the starter becomes a torch. Do not keep the master on “so the radios still work.”

The split master is also how you isolate a failed alternator without killing the battery bus, and how you crank on battery alone with the ALT side off if the AFM wants that for start. Know which half you are holding.

Scenario: Jordan’s midnight ammeter

Jordan is on a night VFR hop when the ammeter slams to full minus and the low-voltage light comes on. He does not lean or fuss with the mags — those were never on this bus. He checks the alternator breaker, then turns the ALT half off, kills pitot heat and the second radio, and points at a lighted airport 15 miles ahead while the battery still runs one comm and the lights. The vacuum attitude indicator is still up. The engine never noticed. In the same airplane, a full-scale plus would have been the opposite story: regulator/overvoltage, ALT off for a different reason. And if this airframe used fuses, 91.205(c) already required the spare set within reach — not in the hangar.