Electrical system & avionics

Your electrical system

Definition

The alternator supplies the airplane's electrical power while the engine is running. The battery provides power for starting and is the reserve if the charging system fails — not an endless backup. Buses and circuit protection distribute that power to the equipment. Fill in your airplane's electrical system below.

Explanation

Think of it as a producer, a reserve, and a distribution network: the alternator produces, the battery stores and reserves, and the buses (with their circuit breakers) hand power out to the radios, lights, and other loads. Knowing your system voltage, alternator output, and battery is the baseline for understanding what a failure leaves you with.

Figure 7-34. Electrical system schematic. — Figure 7-34 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 192)

Why it matters

Every electrical-failure decision — how long you have, what to shed, whether night or weather makes it urgent — depends on knowing your own system. This is the baseline for all of it.

From the Right Seat

Ask, for your airplane: if the alternator fails, what still works, and for how long on the battery? Knowing that before it happens turns a scramble into a plan.

A DPE might ask

What supplies the airplane's electrical power in flight? What's the battery for?
What is your system voltage and alternator output?

PHAK ch. 7 — Electrical system

The alternator: AC → DC

Definition

The alternator is really an AC generator — it produces alternating current (three-phase). But the airplane's electrical system runs on direct current (DC), at 14 or 28 volts, so the alternator's output is rectified to DC by diodes before it reaches the bus. A voltage regulator adjusts the alternator's field current to hold the output voltage steady regardless of engine RPM or electrical load, preventing over- and under-charging.

Explanation

The rectifier (a set of diodes) converts the AC to DC; the voltage regulator is the automatic control that keeps the charging voltage where it belongs as your RPM and loads change. Compared with an older generator, an alternator makes AC that's rectified to DC, produces its rated output even at low or idle RPM, and is lighter and more efficient. A generator makes DC directly but needs higher RPM to charge and is heavier, older technology.

Alternator

Makes AC, rectified to DC by diodes
Rated output even at low/idle RPM
Lighter and more efficient
The modern standard

Generator

Makes DC directly
Needs higher RPM to charge
Heavier, older technology
Found on some older airplanes

Why it matters

Understanding that your 'DC system' is fed by an AC alternator through a rectifier and regulator is exactly the depth a good oral goes to — and it explains why the alternator charges even at idle when a generator wouldn't.

A DPE might ask

The alternator produces AC — how does the airplane's DC system use it?
What does the voltage regulator do? How does an alternator differ from a generator?

PHAK ch. 7 — Alternator, rectifier & voltage regulator

Buses & circuit protection

Definition

A bus is a power 'highway' — a common conductor that distributes electricity from the battery and alternator out to the airplane's equipment. Circuit breakers (or fuses) protect the wiring and equipment by opening the circuit if too much current flows. Many airplanes split equipment across main, avionics, essential, or standby buses.

Explanation

A popped circuit breaker is a clue, not an invitation to keep resetting it — resetting into a fault can start a fire. The proper action is to reset it once, if the POH allows; if it pops again, leave it and manage without that circuit. Knowing which equipment lives on which bus tells you what you'll still have after an alternator failure.

Buses & protection	Detail
Bus	A power 'highway' that distributes electricity to the equipment.
Circuit breaker / fuse	Protects the wiring and equipment from too much current.
Popped breaker	A clue, not an invitation to keep resetting. Reset once per the POH; if it pops again, leave it.
Know your buses	Which equipment survives an alternator failure, and for how long.

Why it matters

The breaker rule is a real cockpit decision that separates a minor event from a fire, and knowing your bus layout is what makes an alternator failure manageable.

Common error

It's tempting to keep resetting a breaker that won't stay in until it 'holds.' Don't. A breaker pops because too much current is flowing — often a fault. Reset it once if the POH permits; if it pops again, leave it off and fly without that circuit.

A circuit breaker pops in flight. What's the correct action?

A DPE might ask

What is an electrical bus? What protects the circuits?
What do you do if a circuit breaker pops in flight?

PHAK ch. 7 — Buses & circuit protection

Avionics

Definition

Avionics may include the radios, GPS, transponder, ADS-B equipment, audio panel, autopilot, electronic flight displays, and engine monitors. They can dramatically reduce workload and improve situational awareness — but they also create setup, database, and distraction risks. Know what each item is powered by and what you lose when a bus fails.

Explanation

Do the head-down work while stopped: programming, troubleshooting, and reading menus during taxi or on the takeoff roll are classic ways to lose situational awareness. Glass or round-dial, you remain the system integrator — the avionics serve your plan, not the other way around.

Avionics	Detail
What they are	Radios, GPS, transponder, ADS-B, audio panel, autopilot, glass displays, engine monitors.
Benefit	Reduce workload and improve situational awareness.
Risk	Setup, database, and distraction errors — program and troubleshoot while stopped.
Know	What each item is powered by, and what you lose when a bus fails.

Why it matters

Most avionics-related incidents are distraction and mismanagement, not equipment failure. Setting up while stopped and knowing your power sources is basic modern airmanship.

From the Right Seat

Build your flows so the avionics are set before you move. If something needs reprogramming in flight, hand-fly or autopilot first, then look down — never both heads-down at once.

A DPE might ask

What avionics does your airplane have, and what's each powered by?
When should you program the GPS and avionics?

PHAK ch. 7 & AFH — Avionics & cockpit management

Alternator failure & load shedding

Definition

An electrical abnormal usually shows up as a low-voltage light, an ammeter or loadmeter reading a discharge, or displays dimming. If the alternator fails, the battery is the only source left — and its reserve is measured in minutes, not hours. So shed nonessential electrical load early, preserve the essentials you need to fly and land, and make a conservative landing decision.

Explanation

The instinct to 'the battery will be okay' is wrong — think in minutes and amps. Turn off what you don't need (extra radios, lights you're not using, nonessential avionics) to stretch the reserve, keep what you need to navigate and communicate for the approach, and land before the battery runs down. Night or IMC makes this a different category of problem, because you depend on electrically powered lighting and instruments.

Alternator failure	Indication → action
Low-voltage light / ammeter discharge	The alternator has failed — you're now on the battery.
Battery reserve	Minutes, not hours. Shed nonessential electrical load early.
Preserve essentials	Keep what you need to fly and land; make a conservative landing plan.
Night / IMC	A different category — you can lose lighting and instruments you depend on.

Why it matters

How you handle an alternator failure — early load shedding and a prompt landing — decides whether you land with working essentials or in the dark. It's a core systems emergency.

Common error

A common mistake is assuming the battery will last a long time after the alternator fails. It won't — the usable reserve is typically minutes, and it drains faster the more you're running. Shed nonessential load immediately and plan to land soon, not eventually.

The low-voltage light illuminates and the ammeter shows a discharge. You should:

Safety

At night or in IMC, an alternator failure is urgent — you rely on electric lighting and instruments. Have a plan for which items you'd keep and where you'd land before it ever happens.

A DPE might ask

How would you recognize an alternator failure? What would you do?
How long does the battery last, and how do you make it stretch?

PHAK ch. 7 & AFH — Electrical failure & load shedding

Electrical fire / smoke

Definition

The smell of hot insulation or visible smoke in the cockpit is an emergency. The general priority is to remove the electrical source — turn the master switch off — then isolate the problem (shed the offending circuits, ventilate as the POH directs) and land. Follow your airplane's specific checklist; an in-flight electrical fire outranks the inconvenience of losing your electrics.

Explanation

Master off removes the power feeding the fire. From there you follow the POH: some procedures have you bring essential items back one at a time to find and isolate the faulted circuit, with fire extinguisher and ventilation steps as needed. The theme is the same — kill the source, control the smoke, and get on the ground.

Electrical fire / smoke	Detail
Recognize	The smell of hot insulation or visible smoke — treat it as an emergency.
Remove the source	Turn the master switch off to kill the electrical power.
Isolate	Shed the offending circuits; ventilate per the POH.
Land	Follow the checklist and land — a fire outranks losing electrics.

Why it matters

An electrical fire is one of the few things more dangerous than losing your electrical system entirely. Knowing the master-off-then-isolate priority — and your airplane's checklist — is critical.

A DPE might ask

What would you do if you smelled electrical smoke in flight?
Why is turning the master switch off the first priority?

AFH & POH — In-flight electrical fire