Night Alternator Failure with a Known Battery Endurance
Night VFR in a Piper PA-28-181 at 2,500 ft, 25 nm from destination. The ammeter has shown a steady 20 A discharge since the alternator field breaker popped four minutes ago. The battery is 35 Ah and was about 80 percent charged at engine start. Electrical loads: COM1 5 A, transponder 3 A, nav lights 4 A, strobes 4 A, landing light 6 A, panel and gauge lighting 2 A. The destination has pilot-controlled lighting and a 900 m runway; the alternate is 15 nm away with full lighting and a 1,200 m runway. Ceiling 4,000 ft, visibility 10 km, no icing. ATC has cleared you for descent to 1,500 ft. The checklist says: alternator field breaker - reset once; if it pops again, leave it out. A handheld radio and a flashlight are in the flight bag.
1. Estimate the electrical endurance available after a sensible load shed, and state the order in which you would shed loads.
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Endurance estimate
Usable battery capacity ≈ 35 Ah × 0.80 = 28 Ah. It is a lead-acid battery, so only about 80 percent of that is realistically usable before voltage collapses: roughly 22 Ah of useful energy.
- Everything on (24 A): 22 / 24 ≈ 0.9 h - about 55 minutes, and the last 15 minutes would be at falling voltage with unreliable radios.
- Essential only (COM1 5 A + transponder 3 A + nav lights 4 A + panel lighting 2 A = 14 A): 22 / 14 ≈ 1.6 h - about 95 minutes.
Shed order
- Strobes (4 A) first - not required for night VFR; nav lights satisfy the anti-collision requirement in the practical sense for a light single.
- Landing light (6 A) off until short final; it is worth nothing at 2,500 ft and everything at 300 ft.
- Panel lighting down to the minimum readable, not off.
- Keep COM1 and the transponder - the transponder is needed in controlled airspace and the radio is needed for the pilot-controlled lighting and for declaring an emergency.
Tell ATC immediately: "alternator failure, battery only, request priority and pilot-controlled lighting on." That single call converts a battery problem into a handled problem.
2. Explain the reasoning behind the checklist's 'reset once only' instruction, and what you would check before touching the breaker.
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A popped field breaker means either a transient over-current or a genuine short in the alternator field circuit, the wiring, or the alternator itself. If it is a short, every reset feeds current into a fault. Behind a light-aircraft panel, an arcing fault can start a fire in wiring that you cannot see and cannot reach with an extinguisher without filling the cabin.
Before resetting:
- Smell and look. Any burning insulation smell, smoke, or a hot spot near the alternator or the breaker means do not reset - land as soon as practicable and consider the master switch off.
- Check the obvious: alternator circuit breakers and the over-voltage protection, and whether the load was momentarily excessive (for example, landing light plus gear/flap motor at low RPM).
- Confirm the ammeter is showing a discharge and not a charging indication - an over-voltage trip can look similar on some panels.
Reset once, deliberately, and watch the ammeter. If the breaker pops again, leave it out and treat the alternator as failed. Repeated resets, or holding a breaker closed, is how a $400 alternator becomes an in-flight fire. The flight then continues as a battery-only flight with the load shed already planned.
3. You are 25 nm out with a 900 m destination runway and a fully lit alternate 15 nm away. Describe how you would sequence the remaining electrical load through the approach and landing, and justify your choice of airport.
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Airport choice. Both fields are reachable inside the 95-minute endurance, so the deciding factor is lighting quality and options, not distance. The alternate has full lighting and 1,200 m; the destination depends on pilot-controlled lighting that must be keyed on the radio - and the radio is the one thing that may fail first. Prefer the alternate, and tell ATC now, while the transmitter and the battery are healthy. If the pilot-controlled lighting is confirmed already on and the destination is clearly the better routing decision (for example, weather), the destination is acceptable, but decide early rather than at 5 nm.
Load sequence
- Descent and approach: COM1, transponder, nav lights, minimum panel lighting (14 A). Strobes and landing light off.
- Before landing checks: flaps and trim are manual or mechanical on this type, so no electrical dependency. Brief a no-flap or reduced-flap landing if the panel is dim.
- At about 500 ft AGL: landing light on. If the battery is clearly dying (dim lights, radio weak), turn the landing light on earlier and accept losing the radio.
- After landing: lights off, transponder standby, minimize load until on the chocks.
Justification: the only electrical item that materially changes the outcome of a night landing is the landing light; everything else can be sacrificed to keep it alive for the last 60 seconds.