Alternator Overvoltage at Night in IMC
Night, IMC, single-engine aircraft on a 28 V DC system with a 70 A alternator, a 24 V 33 Ah sealed lead-acid battery, and an overvoltage protection module that trips the alternator field at about 32 V. In level flight the overvoltage warning illuminates, the ammeter swings to −22 A and the battery bus sits at 23.6 V and falls about 0.2 V every 10 minutes. A single reset attempt reproduces the trip within 20 seconds. The aircraft has electrically operated flaps (about 15 A per cycle through the motor) and fixed gear. Current loads: landing light 10 A, pitot heat 10 A, nav and panel lights 6 A, radios and transponder 10 A, autopilot 4 A, strobes 3 A. Destination is 45 minutes ahead; a suitable alternate with an ILS is 18 minutes behind. Both have 600 ft ceiling and 2 km visibility.
1. Explain the overvoltage trip and whether a further reset is acceptable.
Read the worked answer
The overvoltage module trips the field when bus voltage exceeds about 32 V, protecting avionics from a runaway alternator. The trip is a symptom, not the fault: the usual cause is a failed voltage regulator or a field-circuit fault. One reset is reasonable and normally appears in the checklist - if the alternator returns and holds about 27.5–28.5 V for a few minutes, monitor it. If it trips again within seconds, stop: a second reset on a genuinely overvoltage alternator can push the bus high enough to damage radios and the transponder, and you are no better off. Accept the loss and treat the aircraft as battery-only.
2. Make a load-shed plan with numbers and estimate your realistic endurance.
Read the worked answer
At −22 A the battery will not reliably last 45 minutes. A 33 Ah sealed lead-acid gives roughly 60% usable capacity down to 20 V, and at a 20+ A discharge the Peukert effect cuts usable capacity further - plan on about 35–40 minutes at that load, not 90. Shed the landing light (10 A), autopilot (4 A) and strobes (3 A). Pitot heat stays on because it is night IMC at 600 ft, and you count its 10 A. Keep nav/panel lights (6 A), one com and the transponder (about 8–10 A). That brings the load to roughly 12–14 A and gives about 70–80 minutes. Record the time of failure, note the numbers, and re-check the voltmeter every 10 minutes; below 22 V assume the battery will not carry the flap motor.
3. Decide the destination and brief the landing.
Read the worked answer
Go the 18 minutes back to the alternate, not 45 minutes forward. At 12–14 A the battery covers the diversion with margin; forward at 22 A it does not. Brief the landing before starting down: expect to land without the landing light and with limited flap, plan a longer, flatter approach, use one flap setting rather than cycling the motor, and keep a no-flap landing in mind if the voltmeter is below 22 V on downwind. Advise ATC of the electrical failure and the diversion, and ask for a straight-in if it saves time. Night, IMC, no alternator and a limited battery is not the time to protect the schedule.