/Aircraft Electrical Systems
   /Electrical Systems

Aircraft Electrical DiagramAircraft Electrical Diagram

Aircraft Electrical Systems, Handling

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 advanced electronic system of your choice. The electrical system consist of a battery and an alternator or generator on older aircraft. All of this is connected through several meters (kilometers/ miles 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.

The aircraft manual should contain a block schematic of the aircraft electrical system this gives you a grand overview of which item is connected through what switch and fuse, refer to it often.

The electrical system is one of the most reliably you can find in an aircraft, but it deserves a watchful eye from the pilot as most issues have a tendency to announce themselves before they become a real problem.

Normal operation

Make sure you do reread the aircraft flight manual or POH to be up to speed with your aircraft system. If you do this every now and then you will keep that knowledge fresh. Some basic tips which apply to most aircraft are given below.

Engine start

During startup, large voltage fluctuations occur when the starter motor is cranking the engine, the amount of current flowing can run into hundreds of amps. These fluctuations can severely damage the sensitive electronic radio's and EFIS systems, the risk of any damage is just not worth it.

Avionics OFF

These devices MUST all be switched off before the engine starts, do this manually via the avionics master switch or have a relay installed which is activated by the starter switch and switches off all bus bars except the engine instruments (these should come online automatically the moment the engine is running). The same should be done when shutting the engine down: switch off all electronic equipment before you shut down the engine.

Cool starter motor

Engage the starter motor only for short periods (less than 30 seconds) and let it cool before attempting a second start, they get really hot (and might burn out) when the cranking periods are too long. Release the start switch as soon as the engines fires to prevent damage to the bendix and gears. During starter cool down, the battery is able to recover to a certain extend for the next cranking attempt, this process can not be repeated indefinitely however. The battery will need a proper recharge if its drained too far.

Post-start checks

Aircraft Generator

After the engine has started and verifying that oil pressure is up, look at the ammeter and or voltmeter to check for a charging current / voltage. The battery will need an extra charge, indicated by a higher than normal current but this should be back to normal within thirty minutes. The voltage should be around 13,8 (28) volts and be more or less constant regardless of engine RPM if you have an alternator. A generator requires more RPM (more than 1200) to be able to generate enough electrical power and you will not see a charge to the battery when the engine runs at idle.

Rotax Voltage Regulator

Rotax regulator

With a Rotax engine the regulator needs over 3000 RPM to properly maintain the output voltage. When increasing RPM for the run-up check that you will see the voltage rise until it reaches around 13,8 V. It should then stabilize at that point, make sure it does that. Any higher or lower voltage indicates a problem with the generator and/or its regulator. As the generator are made of fixed coils and rotating magnets on the crankshaft, its usually the regulator which is at fault (although not very often). The capacitor is the weak point here as it can dry out over the years.

In-flight checks

During the flight do regularly pay attention to the ammeter and voltmeter (if any). Load changes will occur, think of landing lights and other high power devices, and this also depends on how many devices are used at the same time. It is helpful to make list of all devices in your aircraft and their current requirements so that any change during the flight can be detected and is not cause for any undo concern.


The fuses that are normally installed in an aircraft are of a resettable type. If they 'pop', wait a couple of minutes so that the fuse can cool off and then try resetting. Should the fuse pops again, leave it that way and report the event to your A&P mechanic. It depends on which fuse/ or device that failed that will determine your next action.

Sometimes homebuilt aircraft are equipped with car-like or even glass fuses. This is not a problem but you will need to carry enough spare fuses, just in case one blows. Remember that when a fuse blows, something is usually wrong with the device or wiring connected to the bus and this will need to be checked.

Written by EAI.

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