What does a left-hand zero ammeter reading indicate?

A left-hand zero ammeter reading typically indicates that the alternator has failed. In an aircraft's electrical system, an ammeter is used to display the electrical current in and out of the battery and alternator. A reading at the zero mark on the left side suggests that there is no charge being generated by the alternator, meaning it is not supplying any current to the electrical system or charging the battery.

In normal circumstances, when the alternator is functioning correctly, the ammeter would show a positive reading, indicating that it is charging the battery. Conversely, if the alternator were completely inoperative, the system might rely solely on battery power, but without any incoming charge, the battery would gradually deplete.

Thus, the specific context of a left-hand zero reading signifies an alternator failure, which is critical for pilots and maintenance personnel to recognize in order to ensure the aircraft remains safely operational.