Question

While in flight in a single engine piston powered airplane that is equipped with an alternator, you notice that the ammeter is showing a discharge condition. On further investigation, you note that the battery voltage is below 12 volts. You start eliminating loads, so you turn off all of the light that aren't absolutely necessary.

As you shed loads, you notice that the volt meter shows an increase in voltage and the ammeter is starting to show a charge. You suspect that the alternator is not functioning properly. What is the most likely problem with the alternator?

a). A failed set of brushes
b). A failing bearing
c). A failed diode in the rectifier unit
d). No answer text provided

Answer 1

The most likely problem with the alternator in a single-engine airplane is a failed diode in the rectifier unit, as shedding electrical loads leading to an increase in voltage and a charging ammeter suggests the alternator is not fully functional but still operating at a limited capacity.

Step-by-step explanation:

When an airplane is showing a discharge condition on the ammeter and the battery voltage is below 12 volts, and after eliminating some electrical loads, you notice the ammeter shows a charge and the voltage increases, it indicates that the alternator is still functioning but not at full capacity. The most likely problem with the alternator could be a failed diode in the rectifier unit. This is because if the brushes were completely failed, shedding electrical loads would not result in increasing voltage. A failing bearing usually presents with noise or vibration and would not typically cause this issue. However, when a diode fails in the rectifier unit, the alternator’s output can be diminished, which would be consistent with the observed symptoms during load shedding that leads to increased voltage and ammeter indicating a charge.