Night IMC Alternator Failure
You are flying a retractable single-engine aircraft at night in IMC, 35 min from destination, at 9,000 ft. The 28 V electrical system has a 24 V lead-acid battery rated 20 Ah and a 60 A alternator. The alternator warning light illuminates; the ammeter falls from +18 A to 0; the voltmeter reads 24.5 V and is decreasing about 0.2 V per minute. There is no smoke or burning smell. Current loads: pitot heat 7.5 A, landing light 10 A, nav lights 3 A, anti-collision/strobes 2 A, transponder 2 A, comm 5 A, GPS 1.5 A, autopilot 2 A, panel lighting 2 A, cabin light 1 A. Outside air temperature is −5°C and you are in cloud. Destination weather is forecast 600 ft overcast, 2 km visibility, and the alternate is 15 min further with 1,000 ft broken and 8 km. You have 45 min of fuel reserve. The autopilot is engaged.
1. Interpret the electrical indications. What is the most likely condition, and how much useful battery time do the indications suggest?
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The alternator is not charging. A zero ammeter with falling voltage means the battery is supplying all loads; the warning light confirms an alternator or control failure. No smoke or smell makes a short circuit less likely.
Do not assume the battery will last. At 0.2 V per minute from 24.5 V, reaching about 20 V takes roughly (24.5 − 20) / 0.2 = 22.5 min. That is shorter than the 35 min to destination and much shorter than the 50 min to the alternate. A checklist-approved reset may be attempted once, but if voltage does not rise, treat the situation as an emergency and plan to land within the remaining battery endurance.
2. Build a load-shedding plan for night IMC. Which loads do you keep and which do you shed, and approximately what load remains?
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Keep equipment needed for safe night IMC flight: pitot heat 7.5 A, nav lights 3 A, anti-collision 2 A, transponder 2 A, one comm 5 A, GPS 1.5 A, panel lighting 2 A. Total = 23 A.
Shed the landing light 10 A, autopilot 2 A, and cabin light 1 A. That removes 13 A and slows voltage decay. Hand-fly the aircraft to reduce autopilot load. If voltage keeps falling, further options are dimming panel lighting, dropping one comm after advising ATC, and diverting to the nearest airport with a short approach. Do not shed pitot heat in −5°C cloud unless absolutely necessary; without it, airspeed and altitude indications may become unreliable.
3. What would make you declare an emergency rather than continue to the destination or the planned alternate?
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Declare an emergency when voltage approaches the equipment minimum - commonly around 20–22 V, but use the aircraft flight manual - or when radio/navigation capability is about to be lost. The destination is 35 min away and the alternate 50 min away, while the voltage trend suggests about 22 min before 20 V. Neither planned option fits the electrical endurance.
Request priority and the nearest suitable airport, even if it has a shorter runway or a less convenient approach. A night IMC arrival without reliable electric instruments or radios is higher risk than an early diversion. Plan to be on the ground before battery exhaustion, not after it.