Alternator failure and battery endurance
You are flying a four-seat, single-engine aeroplane at 4,500 ft on a daytime VFR sector from FIMP to a coastal strip 40 minutes ahead. The aeroplane has a 24 V battery rated 13 Ah and a 28 V alternator. Ten minutes after departure the alternator warning light illuminates and the ammeter shows a steady discharge of 15 A. The battery is two years old and was last capacity-checked 11 months ago. Current electrical load: avionics 6 A, landing light 4 A, pitot heat 3 A, fuel pump 2 A. The weather is VMC, but there is a broken cloud layer at 6,000 ft and the outside air temperature is +8°C. FIMP is 20 minutes behind; the destination has no maintenance but has a hard runway. Your passenger has a meeting and asks you to continue. You have a handheld GPS and a smartphone, but no standby alternator.
1. Calculate the approximate battery endurance with the current load and decide whether the 40-minute destination is acceptable.
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With a 13 Ah battery and a 15 A load, the theoretical endurance to complete discharge is 13 / 15 = 0.87 h, or about 52 minutes. That is not the usable endurance. A lead-acid battery loses useful voltage before it is fully flat, and this battery is two years old and has not been capacity-checked recently. Treat usable endurance as perhaps 35 to 40 minutes.
The destination is 40 minutes ahead. Even on the theoretical figure you would arrive with almost nothing left; with the battery's age and the need to keep radio and transponder alive for the landing, that is not acceptable. The passenger's meeting is irrelevant to the airworthiness decision.
Shed load first. Turning off the landing light and pitot heat in VMC reduces the load to 8 A, giving about 97 minutes theoretical and perhaps 70 to 75 minutes usable. That makes the destination possible, but only if you monitor the voltmeter and are prepared to divert to FIMP if voltage falls. If pitot heat must remain on because of icing risk, the load is 11 A and the margin becomes much thinner; divert.
2. What electrical items would you shed and why? What would you keep?
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Shed the landing light first: it is not needed in daylight and draws 4 A. Shed pitot heat only if you are certain you are in VMC and clear of visible moisture; at +8°C in cloud, pitot heat would be essential, so this decision depends on the actual icing risk. Keep the fuel pump for the approach and landing if the checklist requires it, but you may turn it off in cruise if that is normal for the type.
Keep the avionics needed for navigation and communication, the transponder, and the GPS. Do not shed the radio to save battery for a light. The aim is to preserve enough power for a controlled approach, radio calls, and the landing gear or flaps if electrically operated. Record the new load and recalculate endurance after shedding. If the aircraft has a non-electric backup for attitude or navigation, use it rather than draining the battery.
3. At 25 minutes after load shedding, the voltmeter falls from 24 V to 22 V. What does this indicate and what do you do?
Read the worked answer
A drop to 22 V under load indicates the battery is deeply discharged and the alternator is still not charging. The battery is no longer a reliable source for extended flight, and you should expect avionics to drop out without warning. This is the point to stop treating the destination as the goal.
Land at the nearest suitable aerodrome. If FIMP is within 20 minutes, turn back; if the coastal strip is closer, proceed there. Reduce electrical load to the minimum, configure early, and avoid unnecessary radio calls. If you need priority because of the electrical state, declare PAN PAN and state your position, endurance, and intention. Do not continue to the original destination merely because the passenger has a meeting; a dead battery can leave you without flaps, radio, or transponder at the most critical phase.