Night Alternator Failure and Electrical Load Shedding
You are flying a single-engine piston aircraft at night, VFR, 45 minutes from your destination. The ammeter suddenly shows a 20 A discharge and the alternator warning light is on. The battery is a 24 Ah lead-acid unit and was fully charged at departure. Your electrical load is: landing light 10 A, navigation lights 5 A, strobe 3 A, radio 5 A, transponder 2 A, pitot heat 5 A, panel lighting 2 A. The sky is clear, visibility 20 km, and you are in controlled airspace. There is an airport 15 minutes behind you and another 25 minutes ahead. The aircraft has no standby alternator. The engine runs normally. You have a handheld radio in the flight bag but no spare battery.
1. What does the ammeter reading mean, and what is the approximate battery endurance with the present load?
Read the worked answer
A 20 A discharge with the warning light means the alternator is not supplying current; the battery is carrying the whole load. The ammeter is showing current out of the battery. With a 24 Ah battery, the theoretical endurance is 24 Ah divided by total load. The listed loads total 32 A, so 24/32 = 0.75 h, or about 45 minutes, before the battery is flat. In practice voltage falls before that, and radio or transponder may drop out earlier. The destination is 45 minutes away, so continuing is marginal at best.
2. Which loads should you shed and in what order? What must you keep for a night flight in controlled airspace?
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Shed load in order of least safety benefit: landing light 10 A, strobe 3 A, pitot heat 5 A if there is no icing risk, and nonessential panel lighting. Keep navigation lights 5 A because they are required at night, radio 5 A, transponder 2 A and panel lighting necessary for instruments: about 14 A. That gives 24/14 = 1.7 h theoretical, but treat it as less. If you need the landing light for the approach, switch it on late and brief the approach. Do not shed navigation lights at night while airborne.
3. What diversion decision follows, and why is the destination not the safe choice?
Read the worked answer
Divert to the airport 15 minutes behind. The 25-minute airport is also better than the 45-minute destination, but the nearest suitable airport is the safest. The 45-minute destination is beyond the calculated 45-minute theoretical endurance and leaves no margin for a go-around, radio calls or a missed approach. Use the handheld radio if the main radio fails, and tell ATC. Land and have the alternator checked. Do not continue just because the engine runs normally; the engine is independent of the electrical failure.