Alternator-Out and Open Field Circuit
After takeoff in a 28-V DC twin, the left alternator-out light illuminates and the bus-tie relay opens. Battery voltage is 24.5 V and falling; the battery ammeter shows 25 A discharge. On the ground with the master switch ON, you measure 24 V at the voltage regulator field output terminal but 0 V at the alternator field terminal. The alternator case ground strap is secure, and the field wire shows no obvious chafing at the alternator end. The aircraft has a 60-A alternator and a 24-V battery. The flight was planned for 1.5 hours; current load is 20 A.
1. Use the measurements to isolate the fault. What is the most likely cause?
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The regulator is supplying field voltage, but the alternator is not receiving it. With 24 V at the regulator field output and 0 V at the alternator field terminal, the fault is an open circuit between those two points. Likely causes are a broken field wire, a disconnected or corroded connector, an open wire in the harness, or a faulty terminal at the alternator. The secure ground strap and lack of visible chafing at the alternator end do not rule out an open connector or internal harness break.
2. Can the flight continue with the current load? Explain the electrical reasoning.
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Continuing the planned 1.5-hour flight is not advisable. The battery is at 24.5 V and falling while supplying a 25-A discharge. A 24-V battery under that load will continue to discharge and may not support essential loads for the planned duration, especially if additional load is added. The bus-tie opening also indicates the electrical system has already reconfigured.
The appropriate action is to reduce electrical load, land as soon as practicable, and troubleshoot on the ground. Do not treat the alternator-out condition as a nuisance light.
3. What checks should be performed before replacing the alternator?
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Do not replace the alternator until the open field circuit is ruled out. Check:
- Continuity and resistance of the field wire from the regulator to the alternator.
- Connector pins for corrosion, pushback, or poor crimp.
- Field terminal voltage with the master on and the alternator control circuit energized.
- The regulator output under load and the alternator ground circuit.
If the field circuit is repaired and field voltage reaches the alternator but output remains zero, then test or replace the alternator per the manufacturer's procedure.