Alternator failure at night in IMC
While climbing through 6,000 ft in IMC at night, the alternator warning light illuminates and the ammeter shows a 22 A discharge. The aircraft has a 28 V system, one 60 A alternator and a 24 Ah sealed lead-acid battery. Electrical loads are pitot heat 10 A, nav/panel lighting 8 A, one com/nav 6 A, transponder 2 A; the landing light is off. Outside air temperature is -3°C and the aircraft is in visible moisture; pitot heat is required. The nearest suitable aerodrome is 25 minutes away, with an ILS and runway lighting; the planned destination is 45 minutes away. Fuel is not a problem. The Pilot's Operating Handbook gives a battery endurance of 30 minutes with the alternator failed and all loads on, but does not state the load used. The aircraft has vacuum-driven attitude and heading indicators and an electrically driven turn coordinator.
1. Estimate the usable battery endurance and explain why the simple amp-hour calculation is optimistic.
Read the worked answer
At 22 A, the nominal arithmetic is 24 Ah ÷ 22 A = 1.09 h, but that is not usable endurance. A lead-acid battery should not be run flat, and its effective capacity falls at high discharge rates. Using 80% of nominal gives 19.2 Ah, or about 52 min; with voltage collapse and the need to keep avionics alive, plan on 30-40 min.
The POH figure of 30 min with 'all loads' is a safer planning number because it already reflects a realistic end voltage. The important point is not the exact minute: the 45-minute destination is outside the safe window, while the 25-minute aerodrome leaves a margin. Do not use the nominal 24 Ah as if it were all available at 22 A.
2. What load-shedding order do you use, and which load should not be shed?
Read the worked answer
Shed in this order:
- Keep pitot heat on. At -3°C in visible moisture, losing it risks a blocked pitot and unreliable airspeed at the worst time.
- Keep one com/nav, the transponder if in controlled airspace, and the panel/nav lights required at night.
- Shed the landing light until final (it is already off), cabin/utility loads, the second radio and non-essential avionics. If an autopilot is fitted, its value depends on workload: in single-pilot IMC it may be worth keeping, but it is a discretionary load.
Do not shed pitot heat to buy time. An icing-induced instrument failure is a larger risk than 10 A of battery drain. Re-check the ammeter after each action so you know what you have actually saved.
3. Do you continue to destination or divert, and what communications and approach planning follow?
Read the worked answer
Divert to the 25-minute aerodrome. At 22 A the battery is unlikely to give 45 min plus a reserve; the destination also leaves no margin for a missed approach or a runway change. The nearest aerodrome has an ILS and lighting, so an approach can be flown without relying on battery-intensive visual manoeuvring.
Advise ATC of the alternator failure and the diversion. If the battery is already low, declare a PAN PAN rather than waiting for a Mayday. Plan to select the landing light on only for the final approach, and brief a go-around that does not depend on electrical power. Keep the turn coordinator in mind: with vacuum attitude and heading indicators, a partial-panel plan is still available if the battery dies.