Light Aircraft Electrical Systems

Most light airplanes carry a 14-volt or 28-volt direct current (DC) electrical system. An engine-driven alternator supplies the electrical loads in flight and keeps a battery charged; the battery starts the engine and becomes the only source of power if the alternator fails. Power reaches each piece of equipment from a common bus bar through its own circuit breaker or fuse, and the pilot controls the whole system with the master switch.

The engine itself does not depend on any of this. A piston engine's ignition comes from its magnetos, which generate their own current, so the engine runs on with the electrical system dead. What the pilot loses instead is the radios and transponder, the lights, electric flaps and trim, the electric fuel pump, pitot heat, the turn coordinator and, in many trainers, the fuel gauges and stall warning. At night or in cloud that loss is serious, which is why alternator failure appears in the EASA PPL and CPL syllabus, in the FAA private and commercial pilot standards and in the emergency procedures of the flight manual.

Typical layout

The FAA's Pilot's Handbook of Aeronautical Knowledge (PHAK) lists the parts of a basic system: an alternator or generator, a battery, a master or battery switch, an alternator switch, a bus bar with fuses and circuit breakers, a voltage regulator, an ammeter or loadmeter, and the wiring that joins them. In outline:

• the alternator feeds the bus bar, with a loadmeter in its output lead in some installations;
• the battery is connected to the same bus through a heavy-duty contactor, with a center-zero ammeter in its lead in others;
• every service takes its supply from the bus through its own breaker, and many installations add a separate avionics bus for the radios;
• a short hot bus wired ahead of the contactor can keep a few items, such as a clock, powered with the master off.

The system voltage is a design choice. At 28 V a given load draws half the current it would at 14 V, so the wiring can be thinner and lighter, which is why larger and better-equipped airplanes, with ice protection and more avionics, tend to use 28 V.

Several things stay outside the electrical system by design. Besides the magnetos, the attitude and heading indicators of many trainers are driven by an engine-driven vacuum pump while the turn coordinator is electric, so that one failure cannot take out every gyro. The airspeed indicator, altimeter and vertical speed indicator need no power at all, and the emergency locator transmitter has its own battery (see vacuum and instrument air systems).

View full schematic

Battery and alternator

The battery, usually a lead-acid unit of 12 or 24 V, does two jobs: it turns the starter, and it is the reserve of power for the rest of the flight if the alternator stops. Its capacity is rated in ampere-hours, and cold and age reduce what it can deliver (see aircraft batteries). For starting, selecting the battery switch on connects the battery to the main bus through the battery contactor, or solenoid; the starter then turns only when the start position of the switch energizes a second, starter solenoid, and a clutch in the starter drive lets the engine run faster than the starter once it fires. Some airplanes have an external power receptacle for a ground power unit, which is particularly useful for starting in cold weather.

The alternator generates three-phase alternating current that diodes rectify to DC, so the airplane's system only ever sees direct current. It has replaced the older DC generator in most light airplanes because it delivers useful output across the whole engine speed range, even at idle and taxi, where a generator may not carry the load and the battery discharges, and because it is lighter for the same output. The rectifier diodes also block current from flowing back from the battery, so an alternator needs no reverse current cut-out (see DC generators, motors and starter-generators).

An alternator has one weakness that the exams like. Its rotating field is an electromagnet that must be supplied with excitation current, normally from the battery. With the battery completely flat there is no field and no output, so an engine started by other means may run perfectly with a dead electrical system. A DC generator, whose field can build up from the residual magnetism in its frame, can come on line with a flat battery.

The voltage regulator holds the bus voltage steady as engine speed and electrical load change, by adjusting the current fed to the alternator's field. It is set a little above the battery's voltage, because current flows into the battery only when the bus is at a higher potential: about 14 V for a 12 V battery and about 28 V for a 24 V battery. If the regulator fails and the voltage runs away, an over-voltage protection device opens the field circuit, at a threshold such as about 16.5 V in a 14 V system or about 32 V in a 28 V system, and the alternator drops off line.

Master and avionics master switches

Turning the master switch on supplies electrical power to every circuit except the ignition. It carries only a small control current: the battery half energizes the contactor next to the battery, which makes the heavy connection to the bus.

Many light airplanes have a split master switch, two rocker halves side by side. The BAT half controls the battery contactor; the ALT half completes the alternator's field circuit. The halves are linked so that ALT can be switched off on its own but cannot stay on without BAT, since the alternator needs the battery to excite its field. Switching ALT off alone isolates a misbehaving alternator and puts the whole electrical load on the battery, so nonessential equipment should be switched off at the same time.

The ignition does not pass through the master switch. Switched off in flight, the master removes nearly all electrical power while the engine runs on its magnetos, which makes it the first action for an electrical fire or smoke from behind the panel (see in-flight fire, smoke and fumes). Equally, a master switch that is off does nothing to make a propeller safe: the magnetos are stopped only by the ignition switch, which grounds each one through its P-lead, and a broken P-lead leaves a magneto live whatever the switches say (see piston engine ignition systems).

The avionics master switch connects or isolates the avionics bus. Radios and other solid-state equipment are sensitive to the voltage transients of engine starting and of switching the master, so the avionics are switched off for engine start and shutdown, and it is good practice to switch them off before trying to reset an alternator as well. Where no avionics master is fitted, each radio's own switch serves the same purpose.

Ammeter and loadmeter

The pilot monitors the system on one of two instruments, and they are read differently.

A center-zero ammeter is wired between the battery and the bus. It shows only the battery's current, in both directions. On the plus side it shows the rate at which the battery is being charged; on the minus side, that more current is being drawn from the battery than is being replaced. The PHAK adds two extremes: a full-scale discharge indicates a malfunction of the alternator or generator, and a full-scale charge a malfunction of the regulator.

A loadmeter is wired into the alternator's output. Its scale starts at zero and it shows the total load on the alternator, often as a percentage of its rated output, including the battery's charging current. With every electrical item switched off it shows only the current the battery is taking.

Many airplanes have a low-voltage light, alone or with a meter, which comes on when the bus voltage falls below a threshold set under the regulated voltage. At low engine speed on the ground it may glow normally and should go out when the rpm is raised; airborne and steady, it means the alternator is off line. Where a voltmeter is fitted, it should read about 14 V in a 12 V system while the alternator is charging; battery voltage alone means no charge. On a light twin, each engine drives its own alternator and the two normally share the load, so a failure shows as one loadmeter at zero while the other rises; the bus voltage hardly changes.

Bus bars and circuit breakers

The bus bar is the common terminal from which voltage is distributed to every circuit, which simplifies the wiring. Larger singles and twins divide the loads between an essential bus and a non-essential bus. The split adds no capacity, but it allows the non-essential loads to be shed in one action, leaving the essential bus to feed the radios and instruments needed to fly and land (see electrical power distribution).

Each circuit is protected by a fuse or a circuit breaker. Their ratings are matched to the wiring, so that a fault current opens the circuit before the insulation overheats and starts a fire. A placard at the panel names each circuit and gives its rating in amperes. Breakers can be reset instead of replaced; light airplanes use push-to-reset, pull-off and automatically resetting types, and most aircraft breakers are trip-free, so holding one in does not keep a faulty circuit energized. Under the FAA's 91.205(c), an airplane flown at night must carry one spare set of fuses, or three spare fuses of each kind, accessible to the pilot in flight.

The accepted practice for a tripped breaker is the same in FAA and EASA training material:

• allow it to cool for a short time;
• reset it once, and only if the service is needed for the safe continuation of the flight;
• if it trips again, leave it out and accept the loss of the service;
• never hold a breaker in, and never reset one associated with smoke or a burning smell.

Flight manuals may forbid resetting some breakers altogether (see circuit protection and switching devices).

Alternator failure and load shedding

An alternator can stop for many reasons: a broken or slipping drive belt, a failed diode, a tripped field breaker or an over-voltage trip. Whatever the cause, the signs are the same: a steady discharge on the ammeter or zero on the loadmeter, usually with the low-voltage light. The engine is unaffected.

The flight manual's procedure normally starts with a reset: check the alternator field breaker, then cycle the ALT half of the master off and on once, which clears an over-voltage trip. If the output does not return, the pilot is flying on the battery, and its endurance is set by the load that remains. Load shedding stretches it: switch off everything not needed, keeping a radio, the transponder and what is required for the approach. In a typical trainer the battery may last 30 minutes or less, so the decision is to land at the nearest suitable airport rather than continue. Telling ATC early, while the radios still work, keeps the options open, and under IFR in controlled airspace the FAA's 91.187 requires any malfunction of navigation, approach or communication equipment in flight to be reported to ATC as soon as practical; in EASA training, an alternator failure in IMC with no immediate danger is an urgency situation, announced with PAN PAN. Electrically operated flaps and landing gear will not work once the battery is exhausted, so their use is planned while power remains.

Airplanes with electronic flight displays usually have a standby battery for the essential bus or the standby instruments, with an endurance given in the flight manual (see glass cockpits in light aircraft). On a twin, a single alternator failure leaves the other carrying the whole load, which may still require shedding to stay within its rating. After an engine failure, the alternator on that engine stops with it, and load management matters most before high-current demands such as lowering the gear.

The rules expect the alternator to be adequate in the first place: under the FAA's 91.205, night flight requires an adequate source of electrical energy for all installed equipment, and IFR flight a generator or alternator of adequate capacity.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.