Intermittent Generator Drop-Off in Flight
A twin-engine piston aircraft has two 28 V DC alternators with a split-bus system. The pilot reports that during climb, the right alternator warning light flickers and the right bus voltage drops from 28.2 V to about 26.5 V for a few seconds, then recovers. It happens more often when the landing light and pitot heat are on. On the ground at idle, everything checks normal: both alternators produce 28.1 V and the warning lights are out. The aircraft is due for a 100-hour inspection. The operator asks whether it is safe to fly a 2-hour training sortie while you investigate.
1. The fault is not reproducible on the ground at idle. What does this tell you about the likely cause, and what tests would you run?
Read the worked answer
A fault that appears only under load and in flight and disappears at idle points to a load- or vibration-dependent cause, not a dead component. Candidates:
- Alternator drive belt slipping under high electrical load (more torque demand) or at high RPM.
- Loose or corroded connections in the field/regulator circuit or the main output cable, where resistance rises with heat/current.
- Voltage regulator marginal behaviour under load or temperature.
- Overvoltage/undervoltage trip logic reacting to transient load steps (landing light + pitot heat is a large step).
Tests:
- Load test on the ground with a ground power cart or by switching on all loads (landing light, pitot heat, flaps) and measuring voltage at the alternator output and at the bus, plus voltage drop across cables and connections.
- Belt tension and condition check per the maintenance manual.
- Wiggle/continuity and resistance checks on field and output wiring while loaded.
- Regulator function check per the manual; monitor for trip behaviour during load steps.
2. The load test shows a 0.9 V drop across a connector in the right alternator output circuit when the landing light is switched on. What is the significance and your rectification?
Read the worked answer
A 0.9 V drop across a single connector under load is excessive - good practice is a few tens of millivolts. At, say, 40 A that is ~36 W of heat at the joint, and it explains the in-flight voltage sag: as current rises with the landing light and pitot heat, the drop increases and the bus voltage falls, tripping the warning.
Rectification:
- Isolate the circuit, inspect the connector for corrosion, fretting, loose crimp or heat damage.
- Repair or replace the connector/terminal per the standard practices manual; re-terminate with the correct crimp tool and inspect.
- Re-test under full load and confirm the drop is within limits (typically <0.1–0.2 V for a power feed).
- Check the mating half and adjacent connectors - heat damage often spreads.
- Record in the tech log.
3. Can the aircraft be released for the training sortie before the connector is replaced? Justify.
Read the worked answer
No. The fault is an intermittent electrical supply degradation under normal operating loads - exactly the load profile of a training sortie. Consequences:
- Loss of the right alternator in IMC or at night reduces redundancy and can lead to bus undervoltage and loss of essential equipment.
- A high-resistance joint is a fire risk (localised heating).
- An intermittent fault that is not rectified cannot be signed off as 'no fault found' - it is a known defect.
The aircraft should be grounded until the connector is rectified and the load test passes. If the operator insists, the only defensible position is that the defect is confirmed and must be corrected before flight; a deferred defect is not appropriate for a confirmed power-supply fault affecting a required system.