Alternator failure after departure in a Cessna 172S
You are pilot-in-command of a Cessna 172S (Lycoming IO-360, 28 V electrical system) on a day VFR flight from Nelson (NZNS) to Wellington (NZWN), planned 35 minutes. Cruising at 2,500 ft, the alternator warning light illuminates and the ammeter shows a steady 10 A discharge. You reset the alternator circuit breaker; the light stays on and the discharge remains. The battery is placarded 24 V, 11 Ah. Current electrical load is about 9 A (avionics 5 A, landing light 4 A; pitot heat is off). Weather is VFR, visibility 30 km, cloud base 6,000 ft. Wellington is 25 minutes ahead; Nelson is 10 minutes behind. There is a suitable grass strip 5 minutes ahead. The aircraft has a current maintenance release. You have one passenger and no maintenance tools. Consider battery endurance, controlled airspace, and radio/transponder requirements.
1. Explain what the 10 A discharge indicates and list your immediate in-flight actions.
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The 10 A discharge means the alternator is not supplying the bus; the battery is carrying the electrical load. A steady discharge with the warning light on after one reset indicates a failed alternator, broken belt, or wiring/regulator fault, not a nuisance trip.
Immediate actions:
- Maintain aircraft control and trim; do not troubleshoot at low level.
- Confirm master switch on, alternator switch on, and check the alternator circuit breaker once. Do not repeatedly reset a tripped breaker.
- Reduce electrical load: landing light off, pitot heat off unless required, non-essential avionics off. Keep one com radio and transponder as needed.
- Advise ATC or other traffic of the electrical failure and your intentions.
- Plan to land at the nearest suitable aerodrome before the battery is depleted; do not continue over terrain where a forced landing would be high risk.
- If smoke or burning smell appears, turn the master switch off and land immediately.
The key judgement is that the battery is a limited source; the flight should be shortened, not continued to destination by default.
2. Using the stated 11 Ah battery and 9 A load, estimate endurance. Decide whether to continue to Wellington, divert to Nelson, or land at the grass strip, and justify.
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Nominal endurance = 11 Ah / 9 A = 1.22 h, about 73 minutes. That is an upper bound: usable capacity is less, voltage falls, and radios/transponder may drop out before the battery is flat. At 9 A, 25 minutes to Wellington uses about 3.75 Ah, nominally 34% of capacity. But there is no alternator restart, no reserve, and Wellington is controlled airspace where loss of radio/transponder creates a clearance and traffic risk.
Nelson is 10 minutes back; the grass strip is 5 minutes ahead. The safest decision is to land at the nearest suitable landing area, provided the strip is long and firm enough for the aircraft, and certainly not to continue to Wellington. If the strip is unsuitable, divert to Nelson. Continuing to Wellington would leave little battery margin and no alternative if the electrical system fails further. The pilot should also consider that a dead battery may prevent a restart after landing.
3. After landing, what maintenance and airworthiness actions are required before the aircraft flies again?
Read the worked answer
After landing, the aircraft should be considered unserviceable until the alternator system is inspected and the defect cleared. Actions:
- Enter the defect and the in-flight symptoms in the aircraft tech log, including the 10 A discharge, warning light, and any radio/transponder failures.
- Do not sign a maintenance release or fly the aircraft until a licensed engineer has inspected the alternator drive belt, wiring, connectors, voltage regulator, and battery condition.
- The battery may be deeply discharged or damaged; it should be capacity-checked or replaced as required.
- After repair, run the engine and check alternator output under load; confirm the ammeter shows charge and the warning light is out.
- Make the required certification entry in the maintenance release/tech log before further flight.
If the defect cannot be rectified, the aircraft may only fly under an approved ferry/special flight permit, not under normal PPL privileges.