Alternator Failure Twenty Minutes Out of Nanyuki
You are ferrying a PA-28-181 Archer from Nanyuki to Wilson Airport, day VMC, one person on board, cruising at 7,500 ft. Twenty minutes after departure the alternator warning light illuminates. The ammeter reads 25 A discharge. You reset the alternator field breaker; it pops again within ten seconds. Battery voltage on the voltmeter is 12.4 V and falling slowly. The aircraft has vacuum-driven attitude and heading indicators, an electrically driven turn coordinator, electric fuel gauges and electric flaps, and an engine-driven fuel pump with an electric backup. Equipment fitted: one COM, one VOR/ILS, a GPS, a transponder, pitot heat and a landing light. The battery is 35 Ah and is four years old. Weather is CAVOK with haze, cloud base 8,000 ft. Wilson is about 25 minutes ahead; Nanyuki is 12 minutes behind. The Nairobi TMA transit requires a serviceable transponder and two-way radio.
1. Diagnose the failure and state your immediate actions, including what you will and will not reset.
Read the worked answer
Diagnosis. A warning light plus a 25 A discharge plus a field breaker that pops immediately on reset means the alternator is not producing. The aircraft is running on battery alone, which the falling voltage (12.4 V and below 12.5 V) confirms. Possible causes are a failed alternator or regulator, a broken or slipping belt, or a short in the field circuit. None of these is diagnosable in the air.
Immediate actions, in order:
- Fly the aircraft. Trim, hold altitude and heading, then deal with the electrics.
- Reset the field breaker once. It popped again within ten seconds, so leave it out. Repeatedly resetting a breaker that trips is how you start a fire in a circuit that is shorting.
- Tell ATC. A short PAN call ('alternator failure, battery only, request priority/routeing') costs nothing and buys options.
- Reduce electrical load immediately - do not wait until the battery is flat, because at that point you also lose the fuel gauges, the flaps and the radio you need for the approach.
- Open the checklist, note the time, and set your planning horizon as the first 30 minutes of battery life, not the theoretical maximum.
- Prepare for the possibility of losing comms and transponder, and pick a destination where that is survivable.
Do not climb into cloud or into the TMA to 'have a look' at anything. In a non-electrical aircraft the vacuum instruments remain valid, so you still have attitude and heading - that is the one piece of good news.
2. How will you manage the electrical load, and how long do you actually have?
Read the worked answer
Work out what the battery is running, then shed everything that is not needed for a safe landing.
Keep: COM1 (essential), the transponder if you are entering controlled airspace, one navigation source (GPS or VOR, low draw), and the fuel gauges - these are electric and cannot be shed, so accept them.
Shed: pitot heat (day VMC - but if you enter cloud it comes back on and you accept the drain), landing light, strobes, autopilot if fitted, cabin fan, and any second radio or entertainment equipment.
Accept the consequences: the turn coordinator is electric, so when the battery dies you are partial-panel. In day VMC that is manageable, but brief yourself on it now rather than at 400 ft. The flaps are electric: plan a flapless or single-stage approach, which means a longer runway and a higher approach speed, and remember you need battery left for that one flap selection.
Endurance arithmetic. A 35 Ah battery at an essential load of roughly 5 A gives about 7 hours in theory - which is meaningless. Batteries are rated at a 20-hour discharge, deliver far less at high current, lose capacity with age and cold, and cannot be run to zero without the voltage collapsing. A four-year-old battery realistically gives 45-90 minutes at these loads, and the last 15 minutes will be unreliable. Plan to be on the ground within 30-40 minutes, with the electrical system still alive at touchdown. Treat the first sign of radio weakness or gauge flutter as a hard deadline, not an inconvenience.
3. Decide your destination and describe how the electrical situation changes the approach and the RT.
Read the worked answer
Decision: divert to Nanyuki, 12 minutes behind you.
The reasoning is about what happens if the electrical endurance is shorter than you think:
- Nanyuki is 12 minutes away, so you land with most of the battery still available, full radio, transponder, flaps and gauges. The failure then becomes an engineering problem on the ground.
- Wilson is 25 minutes away but requires a transit of the Nairobi TMA, where the loss of radio and transponder at the wrong moment is a genuine traffic conflict, and where you would be flying an unfamiliar lost-communication procedure in the busiest airspace in the country.
- Pressing on because 'the engineer is at Wilson' is precisely the plan continuation bias this scenario is testing.
Approach implications:
- Expect the flaps to be slow or unavailable; plan a flapless approach, higher speed, longer landing roll. Nanyuki's runway is adequate for that; do not accept a short strip.
- If the radio dies, fly a standard overhead join, keep a good lookout, and land - you are at an uncontrolled aerodrome in VMC, and the priority is a normal landing, not a perfect radio call.
- If the flaps fail part-selected, do not fight it: land with what you have and get it fixed.
After landing: write up the defect, do not allow a 'just reset the breaker and go' departure, and have the alternator, regulator, belt and wiring checked before the aircraft flies again.