Alternator Failure at Night in IMC: Battery Endurance and Load Shedding
You are flying a twin piston aeroplane at night in IMC at FL120, OAT -4 °C, on a three-hour charter. The electrical system is 28 V DC with two 70 A engine-driven alternators, separate regulators, a 24 V 34 Ah battery and a split bus with a bus tie. Connected load in cruise is 88 A: avionics 18 A, pitot heat 10 A, landing and taxi lights 14 A, anti-collision lights 6 A, propeller and airframe de-ice boots 24 A, instrument and cabin lighting 8 A, fuel boost pumps 8 A. Ten minutes after levelling off the left ALT FAIL light comes on; the right alternator ammeter reads its 70 A limit and the battery ammeter shows 18 A discharge. One reset of the left alternator gives a brief 20 A charge before the fail light returns. There is no airport within 40 minutes; the destination is 1 h 50 min ahead; an airport with an ILS is 35 minutes ahead. No icing has been encountered, but cloud tops are reported at FL150.
1. Calculate how long the battery will last and explain what determines the usable figure.
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Nominal arithmetic gives 34 Ah at 18 A, which is about 1.9 hours to complete depletion, but that number is never usable. A lead-acid battery should not be taken below about 50% state of charge, so the usable capacity is around 17 Ah, giving 17 / 18, roughly 0.95 hour. At a discharge rate near 0.5C the Peukert effect reduces the effective capacity further, maybe 15 to 20%, so plan on 45 to 50 minutes before bus voltage falls and avionics start dropping out. The battery is also the only source for the essential bus, so a margin must be kept. Treat 40 minutes as the planning figure, and remember the discharge rate will rise as you turn on lights and pitot heat for the approach.
2. Produce a load-shedding plan that stops the battery discharge, stating which loads you keep and why.
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The single alternator gives 70 A, so load must come down to 70 A or less, ideally to about 60 A so the battery can recharge. Shed: landing and taxi lights, 14 A (kept off until short final, when the flight is nearly over); fuel boost pumps, 8 A (not required in cruise with engine-driven pumps delivering and tanks not near empty); instrument and cabin lighting reduced from 8 A to about 3 A using standby panel lighting. That removes about 27 A and leaves roughly 61 A. Retained: avionics 18 A, pitot heat 10 A (required in IMC), anti-collision lights 6 A (required at night), de-ice boots 24 A (OAT is -4 °C and cloud tops are above, so icing is possible). About 9 A of surplus remains to recharge the battery and cover the field draw. Re-check the ammeter after each change, and shed nothing that is needed for the approach.
3. Decide whether to continue to destination, divert, or something else, and justify the decision.
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Divert. The battery gives roughly 40 to 45 minutes and the destination is 1 h 50 min away, so the aeroplane would go dark in IMC at night, losing gyros, radios, lighting and possibly the electric pump needed for gear and flaps. The ILS airport 35 minutes ahead is inside the calculated endurance but leaves little margin, so that is where I would go, and I would treat the electrical failure as the primary problem rather than pressing on. I would plan the approach in the minimum electrical configuration, hold the battery for gear and flaps, and brief a manual gear extension in case the electric pump is needed. In parallel I would keep looking for a gap into VMC or a closer airport, and declare an urgency condition with ATC so priority and fire cover are available if the second alternator also fails.