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Aircraft Electrical DiagramAircraft Large Electrical Diagram
Aircraft Electrical Diagram

Aircraft Electrical Systems, VI

Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other electronic systems. The electrical system consist of a battery and an alternator or generator on older aircraft. All of this is connected through several meters (kilometers in large aircraft) of wire.

Even for the private pilot it pays to have some basic knowledge of the electrical systems of his or her aircraft, which could be a life saver in case of an emergency.

Failure of the electrical system is not very common but does happen every now and then. Usually a blown fuse or circuit breaker popping indicating an momentarily overload on that device or circuit.

Failures are rare but usually its a fuse popping due to overload or a short circuit in a device. Other issues you will see most often are flat batteries on the most impossible moments.

Electrical failures

If a bus bar circuit breaker pops, switch off any device attached on that bus before resetting the circuit breaker. The initial inrush current with every thing still switched on could be enough to pop the circuit breaker again.

When it happens, wait a minute before resetting the circuit breaker, if it pops again you can be sure that there IS a problem. Do NOT reset it again. Keep your nose ready for any burning smell or smoke indicating an electrical fire in the cockpit. Removing electrical power should normally put out such a fire. And a precautionary landing would be the best action in this case.


Never, ever replace a fuse with one of a higher rating, an electrical fire could be the result of this action. Fuses and circuit breakers are installed to protect the aircraft wiring in case of a short circuit in the attached equipment and they are rated to the length and thickness of the wires. By using a higher rating fuse or circuit breaker the wiring will become the weakest point and will burn out before the fuse or circuit breaker can do its job.

Aircraft BatteryBattery Internals
Battery Internals

Battery problems

Lead acid or gel batteries either last years or die very soon. With good service, regular cleaning of the connections and just keeping them charged and the electrolyte topped up they will serve you well. Normally batteries die when they are needed the most: during winter time. In low ambient temperatures batteries tend to loose much of their power even up to 50% of their rated capacity, making a winter start a more than interesting event.

To keep them active and alive, batteries need regular exercise, as in charge/discharge cycles. But do not discharge them too far, they might fail. Usually one of the six (12 volt) fails and the voltage gets no higher than about 10 volt. Which is a good indication that it needs replacement. Overcharging with too much current is also a recipe to kill them prematurely.

Low charging current

Insufficient current from the alternator (indicated by a zero current on a left-zero ammeter and by a negative current on a center-zero ammeter) will result in all electrical power being supplied by the battery. Switch off all non-essential equipment and prepare for an early landing as the capacity of the battery will not be enough for a very long period (hours), and sometimes not even for an hour.

Aircraft which are fully electric with systems as EFIS, flaps, propeller, fuel pumps, engine FADEC and maybe an AOA could run into serious trouble if provisions are not made for redundant electric systems as dual alternators, batteries and bus bars.

High charging current

This will show up on the ammeter as a higher than normal current. This could indicate a battery problem and an overheated battery (explosion danger) is a real possibility. Check the voltmeter to check for a faulty volt regulator. Momentarily switching off and on off the alternator could reset the problem for both a low and high current situation but do anticipate an early landing or a diversion to an alternate airport.

Rotax four stroke engines use a separate regulator which converts the alternating current from the stator to a direct current at around 13.8 volts. This device is usually very reliable but does fail every now and then. I have seen it fail twice in the last 20 years. Symptoms you might see are a high output voltage (>16 V) with a very high (>10 A) charge current to the battery.

Aircraft Alternator

Alternator / Generator failure

This should show up on the ammeter as a low or no charging current maybe accompanied with a warning light on your panel. But how may pilots really check the ammeter during flight? The first indication usually shows in the radio where the transmitter uses the most power. By that time the battery is almost empty and the pilot is practically without options.

In-flight checks

Do regularly check (use the SADIE checks every 15 minutes during cruise flight) the electrical system (ammeter and or volts). If a fail is detected try to reset by switching the ALT switch off and on again, that might activate the alternator again. If not: reduce the electrical load, prepare and make a precautionary landing on your alternate airport or one close-by.


When working on and with electrical systems, wiring, batteries and the like: disconnect the battery and remove any watches and rings from your hands. A possible short circuit with a fully charged battery will result in some nasty burns or fires, you might loose a finger there. Be very careful as some batteries are capable of discharging with several hundred amps without any problem at all.

Written by EAI.

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