FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
During cruise flight, a pilot notices the center-zero ammeter needle resting exactly on zero with the alternator online and all electrical equipment operating normally. What does this indication mean?
- AThe alternator has failed and the battery is supplying all loads
- BThe battery is neither charging nor discharging because the alternator is supplying all electrical loads
- CThe ammeter is malfunctioning and should read a positive value
- DThe battery is being charged at a low rate
Show answer & explanationAnswer & explanation
Correct answer: B. The battery is neither charging nor discharging because the alternator is supplying all electrical loads
A center-zero ammeter shows current flow to or from the battery, not total electrical load. A reading of zero indicates the battery is fully charged and the alternator is supplying exactly the current needed by the electrical system, so no current is flowing into or out of the battery.
Why the other options are wrong
- A. If the alternator failed, the ammeter would show a discharge (negative) reading as the battery supplies all loads.
- C. This is normal operation, not a malfunction.
- D. A positive (right of center) deflection would indicate charging, not zero.
Center-Zero Ammeter
An ammeter that indicates the rate of charge or discharge of the battery, with zero indicating the battery is neither charging nor discharging.
- Different from a loadmeter, which always reads alternator output
- Positive deflection = battery charging
- Negative deflection = battery discharging, often alternator failure
Memory trick: 'Zero means the battery is happy—no gain, no loss'