Most light aircraft use a 14-volt or 28-volt direct current (DC) electrical system. An engine-driven alternator makes the power, a battery starts the engine and backs the alternator up, and a bus bar shares the current out through fuses or circuit breakers. Airliners add alternating current (AC), typically three-phase 115 volts at 400 hertz, because AC equipment is lighter and simpler.
This guide explains each part, how the two kinds of system differ, and what a pilot sees when something goes wrong. It's built from the FAA's Pilot's Handbook of Aeronautical Knowledge, its Aviation Maintenance Technician Handbook for airframes, and the transport category rule in 14 CFR 25.1351.
Key Takeaways
The FAA says most aircraft have either a 14-volt or a 28-volt DC system. A 12-volt battery is charged from a system running at about 14 volts.
Modern light piston aircraft use a DC alternator. Older ones use a DC generator, which often can't power the whole system at low engine rpm.
The engine's magnetos make their own ignition current, so the battery and alternator don't run the spark on a typical piston engine.
Transport category aircraft use AC alternators, one on each engine plus one on the auxiliary power unit, with the output held at about 400 Hz.
If the alternator fails, the battery carries the whole load. The FAA's advice is to switch off nonessential equipment and follow the flight manual.
The Power Sources, at a Glance
Sources of electrical power on aircraft (FAA Pilot's Handbook and FAA Aviation Maintenance Technician Handbook)
Source
What it does
Where it's used
Battery
Stores energy for engine starting and gives a limited supply if the alternator or generator fails.
Almost every aircraft. Most small private aircraft use lead-acid batteries, and most commercial and corporate aircraft use nickel-cadmium (NiCd).
DC generator
Engine-driven. It produces AC internally and a commutator turns it into DC. It needs a control circuit to hold the right voltage and current.
Older light aircraft. The FAA says many have been replaced by alternators.
DC alternator
Engine-driven, usually by a belt or gear. A rotating field makes three-phase AC in the armature, and diodes convert it to DC. It supplies enough current even at slow engine speeds.
Modern light piston aircraft.
AC alternator
Makes three-phase AC with no rectifying. A constant-speed drive keeps the frequency near 400 Hz when engine speed varies, though an alternator driven directly at constant speed, as on many APUs, needs none.
Aircraft that use a large amount of electrical power, such as transport category aircraft.
Inverter
A solid-state device that converts DC to AC. Many supply both 26-volt and 115-volt AC.
Light aircraft with a small AC system, and as a backup on transport aircraft.
Transformer rectifier
In the handbook's split-bus example, steps 115-volt AC down to 26-volt AC and rectifies it to 26-volt DC.
Transport aircraft, to make DC from the AC buses.
Ram air turbine
A small AC generator driven by airflow, used when the main generators are lost.
Some transport aircraft, as an emergency source.
External power
A ground power unit plugs into a receptacle on the aircraft.
Many aircraft. The FAA notes it helps with cold-weather starts.
How a Light Aircraft's DC System Works
The FAA lists eight parts in a basic aircraft electrical system: the alternator or generator, the battery, a master (battery) switch, an alternator switch, a bus bar with fuses or circuit breakers, a voltage regulator, an ammeter or loadmeter, and the associated wiring.
A simplified light aircraft system. Original diagram for PilotPassion, after the FAA handbooks. Real aircraft differ, so the flight manual's wiring diagram is the reference.
Following the diagram from the battery side: a battery can deliver a huge current, so it's connected to the system through a solenoid, a heavy-duty electrically operated switch. The maintenance handbook explains that the master switch in the flight deck controls that solenoid, which is why it turns off or on virtually all electrical power. The FAA's pilot handbook adds that the master switch in the ON position powers all the electrical circuits except the ignition system.
From the alternator side, the alternator switch feeds a control circuit, the voltage regulator, which sends current to the alternator's field. More field current means more output. The regulator holds the system voltage a little above the battery voltage, and that difference is what keeps the battery charged.
Everything else hangs off the bus bar, the common terminal the FAA describes as simplifying the wiring and giving a common point to distribute voltage. Most circuits are protected by their own breaker or fuse. The equipment on it includes position, anticollision, landing, taxi, cabin and instrument lights, the radios, the turn indicator, fuel gauges, the electric fuel pump, the stall warning system, pitot heat and the starter. Our guide to aircraft lights covers the lighting circuits in detail.
Many aircraft also have a separate avionics bus. The maintenance handbook says it's often advantageous to disconnect all avionics from power, for example by depowering the bus while the starter motor runs, to protect the sensitive electronic circuits. For the instruments and displays those circuits feed, see our guide to cockpit types.
Will the Engine Stop if the Electrics Fail?
On a typical piston engine, no. The pilot handbook says a magneto uses a permanent magnet to generate an electrical current completely independent of the aircraft's electrical system, and it keeps firing whenever the crankshaft is turning. Most standard certificated aircraft have two magnetos. That's why the master switch in the ON position powers everything except the ignition system.
The picture changes for engines under full authority digital engine control (FADEC). The FAA says FADEC eliminates the need for magnetos and may be powered by the aircraft's main electrical system, or in many aircraft by a separate generator connected to the engine. Either way a backup electrical source must be available, because failure of the FADEC could cause a complete loss of engine thrust. See our guide to aircraft engine types for how each engine type gets its ignition.
Other equipment does depend on the electrical system. The FAA's list of electrical loads includes the electric fuel pump, pitot heat, the stall warning system and the starting motor, so the flight manual's emergency procedures decide what happens next. Our fuel systems guide shows where the boost pump sits.
Voltage, Current and Resistance: A Worked Example
System voltage is the electrical pressure and current is the flow. Ohm's law links them with resistance: voltage equals current times resistance. The maintenance handbook gives this aircraft example: a 28-volt landing light circuit has a lamp with 4 ohms of resistance, so the current is 28 divided by 4, or 7 amps. A taxi light with 4.9 ohms drawing 2.85 amps runs on 14 volts.
The handbook notes that in almost all cases aircraft loads are wired in parallel, so every load sees the same voltage, and the current through each one depends on its resistance. It also says that low resistance means increased current, which helps explain why circuit breakers pop and fuses blow.
Battery voltage depends on the cell count. The handbook says an aircraft lead-acid battery has 6 or 12 cells in series, giving an open-circuit voltage of about 12 or about 24 volts. Open-circuit means with no load connected. In the air the alternator holds the bus higher than that, at roughly 14 or 28 volts.
Generator or Alternator?
Both turn engine rotation into electricity, and both charge the battery. They do it differently, and the pilot handbook is blunt about which one wins: alternators have several advantages over generators.
DC generator versus DC alternator (FAA Pilot's Handbook and FAA Aviation Maintenance Technician Handbook)
DC generator
DC alternator
Output at low rpm
Most don't produce enough current at low engine rpm to run the whole system, so the load comes from the battery, which can quickly be depleted.
Produces enough current to run the whole system even at slower engine speeds, and its output is more constant across a wide range of speeds.
How it makes DC
The armature rotates and induces AC. A commutator and carbon brushes deliver it to the loads as DC.
A field winding rotates inside a three-phase armature. The AC output is converted to DC.
Control
Needs a control circuit, such as a three-unit regulator with a voltage regulator, current limiter and reverse current relay.
Regulators are vibrating-relay or, on most modern aircraft, solid-state alternator control units. Depending on the unit, they can add over-voltage, under-voltage and overcurrent protection.
Reverse current
Reverse current sensing opens a relay and disconnects a failed generator, which would otherwise draw current from the bus instead of supplying it.
Diodes in the armature circuit stop current flowing from the bus back into the alternator.
Weight and age
Found on older light aircraft. Business jets and turboprops use a starter-generator, one unit that does both jobs.
In general lighter and more efficient. Found on modern light piston aircraft.
Reading the Ammeter and Loadmeter
The FAA says the ammeter shows whether the alternator or generator is producing an adequate supply of power and whether the battery is receiving a charge.
More current is being drawn from the battery than is being replaced.
Full-scale minus
A malfunction of the alternator or generator.
Full-scale plus
A malfunction of the regulator.
The FAA says to consult the flight manual for the right action in either case. Not every aircraft has an ammeter. Some have a warning light that comes on when the system discharges, and some have a loadmeter instead. A loadmeter starts at zero and shows the load on the alternator or generator as a share of its capacity. With everything switched off it reflects only the battery's charging demand.
One wrinkle: the maintenance handbook describes a second layout, with the meter in the alternator's output line rather than the battery line. That meter reads in one direction only, from alternator to bus, so a dead alternator shows zero and the battery is being discharged. A voltmeter helps too. A normally working system reads within its limits, approximately 14 or 28 volts.
When the Alternator Fails
With the alternator out of the system, the entire load falls on the battery. The FAA's instruction is that all nonessential electrical equipment should be turned off to conserve battery power, and the specific procedure comes from the flight manual. The handbooks give no rule of thumb for how long a battery will last, because that depends on the aircraft, the battery type and condition, and the load.
The maintenance handbook does define an emergency rate for battery ratings: the total essential load needed to support the essential bus for 30 minutes, with a lead-acid battery's voltage staying at or above 1.67 volts per cell. It's a rating set by specification, not a promise for any one flight.
Six or twelve lead-acid cells in series. The handbook says most small private aircraft use lead-acid batteries.
Charging releases oxygen and hydrogen, so the cells lose water and need periodic replenishment. A discharged battery can freeze, which damages the plates.
Valve-regulated lead-acid (VRLA)
A sealed battery with the electrolyte absorbed in glass-mat separators. Used in general aviation and turbine aircraft, and sometimes an approved replacement for NiCd.
No water loss in normal charging. A pressure relief valve may vent if the battery is overcharged.
Nickel-cadmium (NiCd)
A metal box of cells connected in series for 12 or 24 volts. Most commercial and corporate aircraft use them.
Thermal runaway is the risk. A shorted cell or a high temperature with a low charge can start a cycle of rising current and heat that ends in fire or explosion under constant-voltage recharge.
Three more points from the maintenance handbook. First, keep separate storage and servicing facilities for flooded lead-acid and NiCd batteries: the acid electrolyte destroys a NiCd, and the alkaline electrolyte destroys a lead-acid one, and the handbook calls mixing them extremely dangerous. Sealed batteries can be charged and capacity-checked in the same area. Second, a lead-acid battery freezes more readily when it's discharged, while NiCd electrolyte freezes at about minus 75 degrees Fahrenheit. Third, any change of battery type may be a major alteration, so it isn't a simple swap.
The handbook adds that in a normal charging system the alternator or generator restores a battery to full charge during a flight of 1 hour to 90 minutes.
Fuses, Circuit Breakers and Protection
The maintenance handbook divides faults into opens and shorts. An open circuit is a disconnection. A short circuit is an unwanted connection, and the most dangerous is a positive wire touching ground. Wires and components are protected mechanically, with covers and good installation, and electrically with fuses and circuit breakers.
A fuse is a strip of metal in series with the supply that melts when the current exceeds its rating. Fast-acting fuses open very quickly, and slow-blow fuses open only on a continued overload such as a short. A circuit breaker does the same job and can be reset instead of replaced. The handbook calls a breaker that opens the circuit whatever the position of its control trip-free, and says automatic-reset breakers shouldn't be used as circuit protection in aircraft. Placards at the panel name each circuit and its amperage limit.
The handbook adds guidance aimed at technicians. When a breaker trips, the circuit should be checked and the fault removed before it's reset. If a breaker trips for no apparent reason it can be reset once, and if it trips again there's a circuit fault. What a pilot should do in flight is a flight manual question, because the answer depends on which circuit it protects.
Some new aircraft designs use digital circuit protection, which monitors the current in each circuit and reroutes power away from it when the maximum is reached. The handbook lists weight savings and fewer mechanical parts as the advantages.
Starting With Ground Power
An external power circuit lets a ground source run the aircraft or start the engine without draining the battery. It has a receptacle, a solenoid that connects it to the bus and the wiring. The maintenance handbook says the receptacle connects through a reverse polarity diode, so a ground unit plugged in the wrong way round is blocked and the solenoid doesn't close. Wrong polarity, it says, could be catastrophic to the electrical system. For transport category airplanes, 14 CFR 25.1351(c) requires that no external supply with reverse polarity or reverse phase sequence can power the airplane's system if it can reach equipment other than the starter circuit.
AC Systems on Transport Aircraft
Why bother with AC? The maintenance handbook gives several reasons. AC can be sent over long distances more readily and economically, because transformers raise and lower the voltage. AC devices, especially motors, are smaller and simpler than DC devices, and most AC motors need no brushes. Circuit breakers operate satisfactorily at high altitude in an AC system, whereas arcing is so excessive in DC systems that breakers must be replaced frequently. And most airplanes with a 24-volt DC system have special equipment that needs 400-cycle AC, which an inverter supplies.
The traditional transport category arrangement, after the FAA Aviation Maintenance Technician Handbook. Original diagram for PilotPassion. Individual aircraft differ.
The handbook describes the arrangement like this. A typical airliner has a main AC generator on each of two engines, a third on the auxiliary power unit (APU), and a smaller emergency generator driven by a ram air turbine. The handbook says an AC generator produces three-phase 115-volt AC at 400 hertz. The system must hold 115 to 120 volts at 400 Hz plus or minus 10 percent. If the frequency strays more than 10 percent from 400 Hz, the electrical systems don't operate correctly.
To hold the frequency, a constant-speed drive sits between the engine and the alternator. The handbook compares this hydraulic unit to an automatic transmission: the engine rpm changes while the output speed stays constant. A drive built into the alternator housing is an integrated drive generator (IDG). Two types of control unit manage the system: typically one generator control unit for each generator, which regulates voltage and frequency, and at least one bus power control unit that connects the buses.
The two-engine airliners the handbook names, including the Boeing 737, 757 and 777 and the Airbus A300, A310 and A320, use a split-bus system. Each generator normally powers only its own bus, and two generators can never power the same bus at the same time, because their outputs aren't phase regulated and joining out-of-phase generators would damage the system. If one generator fails, a bus tie breaker lets the other feed both buses until the APU generator is started. Older designs such as the Boeing 727, MD-11 and early 747 use a parallel system, where the generators connect together and must be phase regulated. The DC side comes from transformer rectifiers fed from the AC buses.
The layers of backup are the point. The handbook says that with four AC generators available it's highly unlikely a complete power failure occurs, and that if all of them are lost the battery continues to supply DC power to vital systems, with a static inverter making essential AC from the battery. The certification rule says something similar in plain terms. 14 CFR 25.1351(d) requires it be shown that a transport category airplane can be operated safely in VFR conditions for not less than five minutes with the normal electrical power sources inoperative, battery excluded, starting at its maximum certificated altitude. That's a design test, and the flight manual covers what to do in real life.
Frequently Asked Questions
What voltage do aircraft electrical systems use?
The FAA says most aircraft have either a 14-volt or a 28-volt DC system. The battery is nominally 12 or 24 volts, and the alternator or generator runs a little higher, about 14 or 28 volts, to keep it charged. Large transport aircraft also use AC power, typically 115 volts at 400 Hz.
What's the difference between a generator and an alternator?
Both are engine-driven and charge the battery. A DC generator uses a commutator to turn its internal AC into DC and often can't power the whole system at low engine rpm. A DC alternator makes three-phase AC and rectifies it, and it supplies enough current for the whole system even at slow engine speeds.
Do airplanes use AC or DC?
Both. Light aircraft run mostly on DC, with a battery as backup. Transport category aircraft run mostly on AC, usually three-phase 115 volts at 400 Hz, and make DC from it with transformer rectifiers.
Why do aircraft use 400 Hz instead of 60 Hz?
The FAA's maintenance handbook explains why aircraft use AC at all, but it doesn't give a reason for the 400 Hz figure. It does say the systems stop operating correctly if the frequency strays more than 10 percent from 400 Hz, which is why a constant-speed drive holds it.
What does the master switch do?
In most light aircraft it controls the battery solenoid, so it turns on virtually all electrical power. The FAA says that in the ON position it powers all the electrical circuits except the ignition system.
Will the engine keep running if the alternator fails?
On an engine with magnetos, the ignition continues, because a magneto makes its own current independent of the aircraft's electrical system. FADEC engines are different, and the FAA says they need a backup electrical source.
What does a full-scale minus reading on the ammeter mean?
The FAA says it indicates a malfunction of the alternator or generator. A full-scale plus reading indicates a regulator malfunction. Consult the flight manual for the action.
What should a pilot do if the alternator fails?
The battery then carries the entire load, so the FAA says all nonessential electrical equipment should be turned off to conserve battery power. The flight manual gives the procedure for the specific aircraft.
What's a bus bar on an aircraft?
A common terminal that connects the main electrical system to the equipment using the power. The FAA says it simplifies the wiring and gives a common point from which voltage is distributed.
Which battery do small airplanes use?
The FAA's maintenance handbook says most small private aircraft use lead-acid batteries, and most commercial and corporate aircraft use nickel-cadmium. Valve-regulated lead-acid batteries are also used in general aviation and turbine aircraft.
