Intermittent Overvoltage on a 24 V Alternator System
On a turbine-powered agricultural aircraft with a 24 V DC system, the 70 A alternator has an external voltage regulator and a separate overvoltage (OV) relay that trips at 32.0 ± 0.5 V. The pilot reports ammeter flicker and an intermittent OV warning during high-load spray runs at 1800 RPM, but no warning on the ground. A new regulator was fitted 10 flight hours ago. On ground test, the battery is 25.6 V, the alternator holds 28.6 V at 1800 RPM with no load, and 28.2 V at 40 A load. The OV relay does not trip. The battery is two years old and passes a capacity check. The alternator ground strap and engine-to-airframe bonding are painted at one fastener. You have a digital multimeter, a current clamp, and an oscilloscope with a 10× probe.
1. Describe a logical ground test sequence to separate a regulator fault from a sense-line or ground-path fault. Include what you would measure under load.
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Start at the regulator sense terminal and compare it with battery positive at the same moment, not sequentially. With the alternator at 40 A load, measure voltage from battery positive to regulator sense input; a difference above the manual limit indicates a high-resistance positive path. Repeat from regulator ground to battery negative; a ground offset can make the regulator see a low battery voltage and increase field current. Wiggle the harness while observing the meter to expose intermittent chafing. Measure AC ripple at the battery with the oscilloscope, comparing with the manual limit; excessive ripple points to a diode/rectifier fault. Measure voltage drop across the alternator ground strap and engine-to-airframe bond. Check the OV relay sense wire and its connection. Only after these pass would I condemn the regulator. Because a new regulator was fitted recently, also check its part number, mounting, and connector pin tension.
2. The OV relay trips only at high RPM and high electrical load. Explain how a high-resistance ground can cause this, and why simply fitting another regulator is risky.
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Under high load the alternator current increases, so any resistance in the ground path produces a larger voltage offset. If the regulator’s ground reference is displaced, it may interpret the battery voltage as lower than actual and drive the field harder. At high RPM the alternator is capable of more output, so the error can push system voltage toward the OV trip point. A high-resistance engine-to-airframe bond or painted fastener can produce exactly this load-dependent symptom. Fitting another regulator may temporarily change the symptom but leaves the root cause. The risk is continued overvoltage: avionics damage, battery overheating or thermal runaway, and nuisance OV trips that hide a real fault. The correct action is to repair the bonding and measure the voltage offsets under load, then repeat the high-load/high-RPM test.
3. If ground testing cannot reproduce the OV trip, what release decision and documentation are appropriate?
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I would not sign a release that says the fault is cleared when it has not been reproduced and no cause has been found. The intermittent history, ammeter flicker, and recent regulator change are enough to require further inspection of the harness, sense wires, grounds, and OV relay circuit. If the manual provides an approved troubleshooting and test procedure, complete it and record results; if it does not, raise the issue to engineering or quality. Any disturbed connector should be pull-tested and recorded. The work order should state the reported symptom, tests performed, measured values, and the unresolved status. A deferred defect or MEL may be considered only if the operator’s approved documents specifically permit it; otherwise the aircraft remains unserviceable for that system. Human factors: do not let the spray season schedule drive a premature release.