Night Alternator Failure and Battery Endurance
During a night VFR flight in a PA-28-181, the alternator warning lamp illuminates. The ammeter shows a 20 A discharge. The battery is a 24 Ah, 12 V unit, about three years old. Electrical load is: nav lights 6 A, strobe 4 A, transponder 2 A, COM 4 A, GPS 2 A, and pitot heat 2 A. The aircraft is 35 NM from the destination with 1 h 30 min fuel remaining. The pilot's handheld radio is in the flight bag but not connected. Cloud base is 4,000 ft, visibility 10 km, and the destination has a 2,000 m asphalt runway with pilot-controlled lighting. The pilot's personal minimum is 45 min of battery endurance at night. The pilot has not attempted the alternator reset checklist. Battery voltage is 12.4 V and falling slowly. The pilot is at 6,500 ft AMSL; terrain along the route requires 7,500 ft MSA for the next 10 NM before descending.
1. Estimate the battery endurance with the current load and decide whether the pilot can continue to the destination. Show your reasoning.
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With a 20 A load, the theoretical endurance is 24 Ah ÷ 20 A = 1.2 h, or about 72 min. That is already close to the pilot's 45 min personal minimum once an aged battery, voltage decay, and the need for approach lighting and radio calls are considered. A realistic usable endurance is perhaps 50–60% of theoretical, or 36–43 min, which is below the personal minimum. However, the pilot can shed load: strobes 4 A and pitot heat 2 A can be switched off if icing is not expected, and the GPS 2 A may be expendable if the route is familiar. That reduces the load to about 12 A, giving a theoretical 2 h and a more usable 60–70 min. At 35 NM and about 110 kt, the destination is roughly 19 min away, so continuing after load shedding is reasonable, provided the pilot attempts the reset, advises ATC, and identifies a closer landing option if voltage falls further.
2. What checklist actions and load-shedding sequence should the pilot follow, and what should be reported to ATC?
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The pilot should first attempt the alternator reset: check the alternator switch is on, cycle the alternator field or reset as prescribed, check the alternator circuit breaker, and confirm the master switch is on. If the discharge persists, the pilot should reduce electrical load in a planned sequence: switch off strobes, pitot heat if no icing conditions, and non-essential GPS or interior lighting, while retaining nav lights, transponder, and COM. The pilot should notify ATC of the alternator failure, the battery endurance, and the intention to continue to destination or divert. ATC should be asked for the destination lighting, any available vectors, and a phone number for the operator. The pilot should also connect the handheld radio as a backup and monitor battery voltage. If voltage drops below about 11.5 V, or radio/nav failure becomes likely, the pilot should land at the nearest suitable aerodrome rather than press on.
3. If the battery voltage falls below 11.5 V and the COM fails, what should the pilot do?
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The pilot should treat this as an imminent loss of electrical navigation and communication. Immediate actions are to select the handheld radio, squawk 7600 on the transponder if it still has power, and proceed visually to the nearest suitable aerodrome. At a controlled aerodrome, the pilot should expect light signals and must remain clear of the runway until a steady green is received. At the destination, pilot-controlled lighting may be activated by clicking the microphone, but if the radio is unserviceable the pilot should use any published alternate method or divert to an aerodrome with known lighting. The pilot should not continue over high terrain with a failing electrical system at night; a precautionary landing or diversion is preferable. After landing, the pilot should report the failure and have the charging system inspected.