Electrical system

A light aircraft runs on an alternator plus a battery (TP 1102). An alternator is used rather than a generator because it still gives useful output at low RPM (taxi and idle), where a generator needs high RPM to charge. The master switch connects the battery, and a separate alternator switch controls the alternator (TP 1102); on many trainers the two are combined in one split rocker, a battery (BAT) side and an alternator (ALT) side (POH/AFM).

Ammeter (reads current to/from the battery):

• Needle on the + side (charge) = the alternator is charging the battery. Normal.
• Needle on the - side (discharge) = the battery is supplying the load, so the alternator is not producing (likely failed).

Distribution and protection:

• The bus bar distributes power to the circuits.
• A circuit breaker (or fuse) protects each circuit by opening on an overload. A breaker can be reset or pulled.

Alternator failure: you are now on battery only, and it just drains. Shed non-essential load (extra lights and radios) to stretch it, and land while you still have power.

Trap: a discharge on the ammeter is not a charging battery, it means the battery is being drained because nothing is charging it.

Source note: exact numbers and system details are aircraft-specific. These are typical light-trainer values; your own aircraft's POH/AFM is the authority.

Practice