Alternator Failure at Night in IMC
During a night IFR flight in a Piper PA-28R-201 Arrow at 6000 ft, the alternator warning light illuminates and the ammeter shows a 20 A discharge. The aircraft has a 12 V 35 Ah battery and no standby alternator. The pilot is 35 min from destination in IMC; a suitable airport with better weather is 12 min away. Current electrical load is 24 A: pitot heat 7.5 A, nav/position lights 5 A, transponder 3 A, one COM 4 A, one NAV 3 A, panel lights 1.5 A. The battery was fully charged at departure. The pilot has a handheld COM and a standby attitude indicator with its own battery. Forecast destination weather is at minima; the nearer airport is VFR. The pilot must decide whether to continue or divert and what to shed.
1. Explain the likely cause of the ammeter indication and estimate the battery endurance at the current load and after a realistic load shed.
Read the worked answer
A 20 A discharge with the alternator warning light indicates the alternator is offline and the battery is carrying the electrical load. At 24 A, the theoretical endurance of a 35 Ah battery is 35/24 = about 1.46 h, but that is optimistic: capacity falls with high discharge rate, voltage drops before full depletion, and the battery may not deliver usable avionics power for the whole period. After shedding non-essential loads to roughly 8.5 A, the theoretical endurance is about 4.1 h, but a realistic planning figure is lower. The pilot should preserve battery for the landing gear, flaps and one COM/NAV, because the Arrow has an electric landing gear. The key point is not the exact hour figure but that continuing 35 min into IMC at destination minima leaves little protection if a second failure occurs.
2. Which electrical loads would you shed, which would you keep, and why?
Read the worked answer
Keep one COM, the transponder, panel lighting, and one NAV if needed for the approach; keep pitot heat if icing is possible, because loss of airspeed indication is more dangerous than battery depletion. Keep nav/position lights at night for collision avoidance. Shed non-essential items: cabin floodlights, landing light until final, autopilot if fitted, second COM/NAV, unnecessary avionics and pitot heat only when not required. Preserve battery for the electric landing gear and flaps. Use the handheld COM as a backup, not as the primary, and tell ATC about the electrical failure so they can provide separation and expect a possible loss of transponder. The load shed should be deliberate, not random.
3. Make the operational decision and state how you would communicate it.
Read the worked answer
Divert to the nearer VFR airport. Continuing 35 min into IMC with a failed alternator and destination weather at minima concentrates risk: a vacuum or instrument failure, a missed approach, or a gear problem could leave the aircraft without electrical power at the worst time. The nearer airport is only 12 min away and is VFR, giving a much better chance of a visual landing. Advise ATC of the alternator failure, the diversion, and the possibility of loss of transponder or radio; request vectors and traffic information. Use the handheld if the main radio fails. Do not declare an emergency unless safety is actually threatened, but do not understate the urgency either.