Alternator failure and battery endurance on an IFR flight
You are flying a single-engine, normally aspirated aircraft at 8,500 ft on an IFR flight plan. The aircraft has one 28 V, 60 A alternator and a 24 Ah sealed lead-acid battery. The alternator fails; the ammeter shows a discharge of about 20 A. The total non-essential load is 13 A; the essential load for radio, transponder, one GPS and basic instruments is 9 A. You are 55 minutes from destination, with 45 minutes of fuel reserve. The nearest suitable aerodrome is 20 minutes away. The weather is VMC en route, but destination has a 1,500 ft broken ceiling and 5 km visibility. The battery is two years old. The pilot has a handheld radio in the flight bag.
1. Estimate the battery endurance with and without load shedding, and state your assumptions.
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Nominal endurance at 20 A is 24 Ah / 20 A = 1.2 h. After shedding to 9 A, nominal is 24 / 9 = 2.7 h. These are laboratory figures at the 20-hour rate; a two-year-old battery, cold temperature, and high discharge rate reduce usable capacity. Use 50–60% of nominal for planning: about 1.3–1.6 h at 9 A. The battery voltage should be monitored; below about 24 V, endurance is nearly exhausted.
2. Should you continue to destination, divert, or declare an urgency condition? Justify your decision.
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Divert to the nearest suitable aerodrome, 20 minutes away, and land with the battery still able to power essential equipment. The destination is 55 minutes away plus a 45-minute fuel reserve, so the flight could require 1 h 40 min on battery. That exceeds the conservative 1.3–1.6 h estimate, especially with an IFR arrival at a 1,500 ft ceiling. Declaring a PAN PAN is appropriate because an electrical failure in IMC reduces capability, but the diversion is the safer decision. Attempt one reset per checklist.
3. Which electrical loads should be shed, and which should be retained?
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Retain radio, transponder, GPS, and the instruments needed for the approach; keep pitot heat if visible moisture or temperatures near freezing are present. Shed lights, cabin comfort, unnecessary avionics, and non-essential pumps. If the destination weather were VMC, more could be shed, but with a 1,500 ft ceiling the aircraft should be treated as IFR-capable until landing.