16.electrical-failures. Electrical System Failures
An electrical system failure in flight is rarely an immediate threat to controlled flight in a typical single-engine airplane, because the engine continues to run on its magnetos independent of the electrical system. However, electrical failures can cascade into the loss of avionics, navigation equipment, transponder, pitot heat, electric flaps and trim, landing gear (in some retractables), interior and exterior lighting, and stall warning — any of which can compromise safety, especially at night or in instrument meteorological conditions (IMC).
The airplane's electrical system typically consists of an engine-driven alternator (or generator), a battery, a voltage regulator, a master switch (often a split rocker for BAT and ALT), bus bars, circuit breakers or fuses, and an ammeter or loadmeter. The alternator supplies current during normal operation and keeps the battery charged. If the alternator fails, the battery alone supplies the bus, and total electrical endurance is limited — typically 30 minutes or less depending on load and battery condition.
Indications of Electrical Malfunction
The pilot should recognize an electrical problem promptly by cross-checking instruments and warning lights:
- Low-voltage annunciator illuminated (typically below 13.0 V in a 14-volt system or 26.0 V in a 28-volt system).
- Ammeter showing a steady discharge (negative deflection) or a loadmeter reading zero.
- Avionics dimming, navigation flags appearing, GPS or radios resetting.
- Smell of hot insulation, smoke, or visible arcing — indicating an electrical fire.
- Tripped circuit breaker that will not reset.
Alternator/Generator Failure Procedure
When a low-voltage light illuminates or the ammeter shows a discharge:
- Verify the failure — confirm with the ammeter/voltmeter and check that the alternator field circuit breaker is in.
- Reduce electrical load — turn off non-essential equipment (landing light, taxi light, pitot heat if not needed, autopilot, secondary radios, cabin lighting).
- Cycle the ALT half of the master switch OFF for a few seconds, then ON. This may reset the over-voltage relay and restore the alternator.
- If the alternator does not come back on line, leave it OFF, conserve battery, and land as soon as practical at a suitable airport.
- Squawk and contact ATC while the radios are still usable; advise of the failure and intentions.
Manage the remaining battery by prioritizing equipment needed for the phase of flight: one communication radio, one navigation source, transponder, and (at night) position lights and one source of cockpit lighting.
Electrical Fire or Smoke
An electrical fire is identified by acrid smoke and the smell of burning insulation. The immediate action is to remove the source of electricity:
- Master switch — OFF.
- All individual electrical switches — OFF.
- Cabin vents/heat — closed initially to deny oxygen to the fire; open vents and side windows once the fire is out to clear smoke.
- Fire extinguisher — use a Halon or other approved extinguisher rated for Class C (electrical) fires.
- After the fire is extinguished, if continued electrical power is essential (e.g., night/IFR), turn on the master and individual switches one at a time, allowing time between each to identify the faulted circuit. Leave the offending circuit OFF.
- Land as soon as possible — not merely as soon as practical.
Complete Electrical Failure
If both the alternator and battery are lost, the airplane becomes a NORDO (no-radio) aircraft with no transponder. The engine will continue to operate normally on its magnetos. The pilot should:
- Maintain aircraft control and navigate visually using pilotage and dead reckoning, or with handheld GPS/EFB if available.
- Comply with 14 CFR 91.185 if in IMC (continue per clearance, expected route, and filed altitudes).
- Follow 14 CFR 91.126/91.127 lost-communication procedures at non-towered and Class E surface area airports.
- At a towered airport, follow light-gun signals from the tower (steady green = cleared to land, flashing green = return for landing, steady red = give way, etc.) per AIM 4-3-13.
- Enter the traffic pattern at a normal altitude, rock wings to acknowledge signals, and remain alert for other traffic.
Preflight and Prevention
During the runup, verify alternator output by momentarily turning on a heavy load (pitot heat, landing light) and confirming a positive ammeter indication. A weak or discharging battery on the ground is a warning to discontinue the flight. Carrying a handheld transceiver, handheld GPS, and a flashlight (especially at night) provides important redundancy when the ship's electrical system fails.