The electrical system
6 min readLesson 19 of 59
- Describe the main parts of the electrical system
- Read the ammeter or loadmeter
- Respond to an alternator failure
Most trainers have a 14- or 28-volt direct-current electrical system powered by an engine-driven alternator, with a battery for starting and backup.
What it powers
The starter, radios, transponder, lights, electric fuel gauges, flaps (in many Cessnas), the turn coordinator and sometimes other instruments. It does not power the magnetos.
Key parts
- Master switch: usually a split switch. The BAT side connects the battery; the ALT side turns on the alternator's field.
- Circuit breakers or fuses: protect each circuit from overload. A popped breaker can be reset once; if it pops again, leave it out.
- Ammeter or loadmeter: an ammeter shows whether the battery is charging (+) or discharging (-). A loadmeter shows the load on the alternator.
- Low-voltage light: warns that the alternator isn't keeping up.
Alternator failure
If the alternator fails, the battery takes over, but it won't last long. Follow the POH checklist, then turn off everything you don't need (load shedding) to save the battery for the radio and anything else you'll need to land. Land as soon as practical.
Key takeaways
- The electrical system does not power the engine's ignition.
- Split master: BAT and ALT sides.
- Alternator failure: follow the checklist, shed electrical load, land soon.
What the examiner may ask
Practice answering out loud before you open each answer.
What's the voltage of your airplane's electrical system?
What happens if the alternator fails?
What runs on electricity in your airplane?
Common mistakes
- Resetting a popped circuit breaker repeatedly.
- Not noticing an ammeter discharge until the radios die.
- Forgetting which instruments are electric versus vacuum.
Read it in the FAA sources
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Lesson check
Get every question right to complete the lesson. Take your time; wrong answers explain why.
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