Alternator Failure at Night in IMC
At 21:30 local a 28 V single-engine aircraft is in IMC at 7,000 ft, 35 NM from destination. The ammeter suddenly shows -40 A and the alternator field circuit breaker has popped. The battery is 24 V, 20 Ah, and the operator's minimum usable allowance before avionics dropout is 12 Ah. Current load is pitot heat 10 A, nav/position lights 6 A, landing light 10 A, avionics 8 A, autopilot 5 A and instrument lighting 2 A. The pilot has a handheld radio and a tablet with GPS. Destination has a 1,800 m runway and an ILS; weather is 600 ft ceiling and 2 km visibility. The nearest airport is 20 NM away with a 900 m runway and no instrument approach; the alternate is 45 NM away with an ILS. The POH permits one alternator reset attempt after shedding load; if the breaker pops again, the alternator is considered failed.
1. Calculate the current electrical load and battery endurance using the 12 Ah usable allowance. Is the present load acceptable?
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Add the loads: pitot heat 10 A + nav/position lights 6 A + landing light 10 A + avionics 8 A + autopilot 5 A + instrument lighting 2 A = 41 A.
With 12 Ah usable, endurance = 12 Ah ÷ 41 A = 0.293 h, or about 17.6 min. That is less than the time needed to reach destination, alternate or even a safe diversion with an approach. The present load is not acceptable. The battery would be depleted while still in IMC, with loss of pitot heat, radios and instrument lighting. Load shedding is required before any reset attempt, and the pilot should be prepared to land at the first suitable airport.
2. Describe a load-shedding sequence and decide whether to attempt the reset. What load remains, and what endurance does it give?
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Shed the landing light (10 A) and autopilot (5 A) first; they are not required for control in IMC. That leaves pitot heat 10 A + nav lights 6 A + avionics 8 A + instrument lighting 2 A = 26 A. Endurance = 12 ÷ 26 = 0.462 h, or about 27.7 min. If instrument lighting can be replaced by a flashlight, shedding it as well leaves 24 A and about 30 min.
With the load reduced, make the single permitted reset attempt. If the alternator comes online and the ammeter shows positive charge, treat it as a temporary reprieve: the breaker popped for a reason, so plan to land at destination rather than continue normally. If the breaker pops again, do not reset a second time. Continue on battery at 26 A. Destination is 35 NM away; at 120 kt that is about 17.5 min plus an approach, leaving a small margin inside 27.7 min. The 45 NM alternate is about 22.5 min plus approach, which is too tight. The 20 NM airport has no instrument approach in 600 ft ceiling, so it is not a suitable IMC destination. Proceed to destination, advise ATC and request priority.
3. If the reset succeeds but the ammeter later shows -5 A with the breaker still in, what does this indicate and what should you do?
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A discharge with the field breaker still closed means the alternator is not supplying the bus, even though the field circuit is energised. Possible causes include a failed alternator, a broken drive coupling, a wiring fault or a load that exceeds the alternator's output at low RPM. Treat the alternator as failed. Do not assume the small -5 A is harmless; the battery is still being drained and may fail suddenly.
Shed non-essential load again, keep pitot heat, nav lights and essential avionics, and land at the nearest suitable airport. Do not continue to a more distant destination merely because the breaker has not popped. Use the handheld radio if the panel radio fails, and tell ATC that you are on battery with limited endurance. The priority is to land before electrical failure removes lighting, pitot heat or communications in IMC.