Aircraft Electrical Diagram

Aircraft Electrical Systems, III

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.

Normal operation

Make sure you reread the aircraft flight manual or POH to be up to speed with your aircraft. Some basic tips which apply to most aircraft are given below.

Engine start

During engine start large voltage fluctuations occur when the starter motor is rotating 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.

These devices MUST all be switched off before an engine start, do this manually via an 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 switching of the engine: first switch off all electric equipment only then shut down the engine.

Cool start 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 cranking 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

After the engine is running and verifying oil pressure is up, check the ammeter and or voltmeter to check for a charging current and 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 constant regardless of engine RPM with an alternator. A generator requires more RPM (more than 1200) to be able to generate enough electrical power.

In flight checks

During the flight pay attention to the ammeter and voltmeter (if any). Load changes do occur and this depends how many devices are switched on. It is helpful to make list of all devices in your aircraft and their current requirements so that any change in flight can be detected and is not cause for any concern.

The fuses normaly used in an aircraft are resettable. If they 'pop' wait a couple of minutes so that the fuse can cool off and try resetting. If the fuse pops again, leave it off and report the event.