Alternator failure at night: what the battery can still buy you
You are pilot-in-command of a single-engine piston aeroplane on a night VFR flight at 6,500 ft, clear of cloud, visibility 25 km, OAT +1 °C, no visible moisture. Fuel gives 55 minutes to destination plus a 15-minute night reserve. Forty minutes after departure the ammeter swings from +8 A to −12 A and the low-voltage warning light illuminates. The battery is 24 Ah, of which about 16 Ah is usable before the avionics fall over. Current loads are: avionics (COM, transponder, GPS, intercom) 5 A; pitot heat 8 A; landing light 10 A; navigation and strobe lights 4 A. The destination is a lit aerodrome with maintenance and a long runway; the only nearer aerodrome is unlit, 20 minutes away, and has no maintenance. The engine is magneto-ignited with a mechanical fuel pump; the flaps are electric.
1. Work out the electrical endurance and set out your load-shedding plan in order.
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With everything on, the load is 5 + 8 + 10 + 4 = 27 A. At 27 A the 16 Ah usable capacity lasts 16/27 h, about 36 minutes - less than the 55 minutes still to run, so the avionics would be dead before arrival.
Shed in this order:
- Landing light (10 A). Not required by night rules for cruise, only useful for the flare. Shed it now: load 17 A, endurance 16/17 h = about 56 minutes. Still too tight.
- Pitot heat (8 A). You are VFR, clear of cloud, with no visible moisture, and the OAT is above freezing. Shed it: load 9 A, endurance about 107 minutes, against 70 minutes of flight plus reserve. Comfortable.
Keep the avionics and keep the navigation and strobe lights - those are legally required at night, and the strobes are your main see-and-avoid defence. Turn the landing light back on only for the last two minutes; 10 A for two minutes costs about 0.3 Ah.
If at any point you enter visible moisture or the temperature falls toward zero, pitot heat goes back on and you accept a shortened endurance - but then you must also shorten the flight, not just the endurance.
2. Before you commit to treating this as an alternator failure, what would you check, and what would you reset?
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First confirm what the instruments are actually telling you. A discharge on the ammeter, with the low-voltage lamp on and the lights dimming or brightening with RPM, is the signature of an alternator that has stopped producing, with the battery carrying the whole load.
Cross-check with the voltmeter: a healthy charging system reads around 14 V; a battery-only system slides below 12.5 V and keeps falling. A steady 0 A on the ammeter with a normal 14 V reading is more likely an ammeter fault than an alternator fault, and it changes nothing about your electrical plan but does change what you believe.
Then run the short list: master switch fully on and not half-detented; alternator field or over-voltage breaker - if it has popped, reset it once. A single reset that holds is a genuine recovery; a breaker that pops again is telling you there is a real fault and must be left alone. Cycling a breaker repeatedly is how you turn an electrical failure into a fire.
If the reset does not restore a positive charge within a few seconds, accept the diagnosis and execute the load-shedding plan - do not spend endurance on hope.
3. Given the diagnosis, do you continue to the destination or divert, and how does the electrical loss change your arrival?
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Continue to the lit destination. The 20-minute diversion saves 35 minutes of flight but delivers you to an unlit runway with no maintenance, at night, with a landing light you can barely afford to burn and no ability to fix anything. The destination gives you runway lighting, a visual approach slope and help on the ground from the moment you stop.
The engine keeps running: magneto ignition and a mechanical fuel pump need no electricity, so the aeroplane is not about to stop. What you lose is information and convenience, and you plan around each loss now rather than discovering it on final:
- Tell ATC before the radio dies. State the failure, your endurance and your intentions, and ask for any assistance you may need on arrival.
- Flaps. Electric flaps may not survive; plan a flapless approach - higher approach speed, longer landing distance, a flatter path. Pick a long runway and add a speed margin for the gust or the misjudged flare.
- Landing light. Switch it on at short final only, or rely on runway lighting and the approach slope.
- Transponder. If it survives, squawk as instructed; if the radio fails too, squawk 7600 and fly the published arrival, keeping the light signals in mind.
- No pitot heat means no flight into visible moisture; you are VFR and should stay that way.
Write the failure up and have the charging system inspected before the next flight.