ALT Light and a Discharging Ammeter at Night
You are on a night VFR flight from Abuja to Kaduna in a Cessna 172S, level at 6,500 ft, 22 minutes from Kaduna. The 28-volt system has a 24-volt, 13.6 Ah battery and a 60-amp alternator. Ten minutes after take-off the ALT annunciator illuminates and the ammeter shows a 20-amp discharge. Electrical load: landing light selected on, navigation and strobe lights, pitot heat, one COM radio used occasionally, transponder, and panel lighting. The voltmeter now reads 23.5 V and is falling slowly. Kaduna has runway lighting and a manned tower; the nearest alternative is 12 minutes away with no runway lighting. Weather is CAVOK, MSA 5,000 ft, and you carry a handheld radio with fresh batteries.
1. What does the ALT light plus a 20-amp discharge mean, and what do you check first?
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It means the alternator is not supplying the bus and everything electrical is now running on the battery. The first checks are the ones in the POH: confirm the alternator circuit breaker is in, cycle the master (or ALT switch) once, and see whether the ammeter returns to a charge. A brief cycle re-excites the field. If the light stays on and the discharge continues, the fault is real - a broken or slipping belt, a failed alternator, a faulty voltage regulator/control unit, or an over-voltage trip that will not reset.
Do not keep resetting it, and do not assume the ammeter is lying. The voltmeter is the independent check: 23.5 V and falling on a nominal 24-volt battery confirms a genuine discharge. Once you accept the alternator is offline, the flight becomes a battery-management problem with a finite, non-rechargeable supply, and every decision from here is about what the battery must still do at the destination.
2. Set out a load-shedding plan and estimate your endurance.
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Budget: a 13.6 Ah battery at a 20-amp load is mathematically under 40 minutes - and in practice less, because avionics and lighting stop working properly well before the battery is flat as voltage sags under load.
Shed now, not later:
- Landing light off - save it for the final approach and landing, where you actually need it.
- Pitot heat off - CAVOK and no icing risk tonight. If there were any chance of icing, this stays on and other items go instead.
- Strobes off; navigation lights and panel lighting stay on, because you need to be seen and to read the panel.
- Transponder to standby, one COM radio on, transmit only when needed.
That takes the load from about 20 A to roughly 4–6 A, which should make the battery last an hour or more in theory. Nevertheless, plan on about 30 minutes of usable radio and navigation and do not spend it in the circuit or on the ground. Land with capacity in reserve.
3. Continue to Kaduna or divert? What factors would change your answer?
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Continue to Kaduna. The decision turns on what the battery must still do at the destination, not on the distance. Kaduna is 22 minutes ahead with runway lighting and a tower; at reduced load the battery can carry you there, keep the radio alive, and still run a landing light for the flare. That is a reasonable plan.
What would change it:
- If Kaduna were unlit, the landing light becomes essential, and 22 minutes plus a night approach leaves little margin - divert to the nearest lit field instead.
- If visibility were marginal or the flight were IFR, losing radios and panel lighting becomes a much bigger risk; declare and land at the nearest suitable airport.
- If voltage were already below about 23 V and falling quickly, the endurance estimate is unreliable; treat the aircraft as having minutes rather than tens of minutes.
The handheld radio is a genuine backup for communication, not for navigation or lighting, so it must not be counted as extending the flight. A dead battery on the runway at night is an emergency you avoid by deciding early.