FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
A pilot notices the alternator's low-voltage annunciator light illuminate in cruise flight, and the ammeter shows a discharge. What is the most likely cause?
- AThe battery has reached a full charge and the alternator is reducing output as designed
- BThe alternator field circuit has failed or the alternator drive belt has broken, so the battery is now supplying all electrical loads
- CThe pitot heat system has drawn excessive current, tripping a circuit breaker
- DThe overvoltage relay has correctly disconnected the alternator due to excessive voltage output
Show answer & explanationAnswer & explanation
Correct answer: B. The alternator field circuit has failed or the alternator drive belt has broken, so the battery is now supplying all electrical loads
A low-voltage light combined with an ammeter discharge indication means the alternator is no longer supplying the electrical bus, so the battery alone is powering all loads. This is typically caused by a failed alternator field circuit, a blown alternator circuit breaker, or a broken drive belt.
Why the other options are wrong
- A. A fully charged battery does not cause a low-voltage warning light or ammeter discharge.
- C. A pitot heat circuit breaker trip would not disable the whole alternator system.
- D. An overvoltage trip would also show a discharge, but this describes correct protective action, not a fault causing the low-voltage light without operator awareness of overvoltage.
Low-Voltage Warning Light
Illuminates when bus voltage drops below a preset value, typically indicating alternator failure and battery-only operation.
- Paired with ammeter discharge reading in most light aircraft
- Common causes: failed alternator field circuit, tripped breaker, broken belt
- Pilot response: reduce electrical load, consider precautionary landing
Memory trick: Light on, ammeter down, alternator's out of town