Aircraft alternator failure manifests itself in a falling amperage gauge, a LOW VOLTS light, and, as battery discharge continues, radio & transponder failure. Pilots first notice the symptoms when the low-voltage light comes on; The panel shows a decreasing voltage, and the amperage gauge shows a discharge indication; shortly afterward, the alternator-inop light confirms that the unit has gone off-line.
The failure could be due to a broken alternator belt, a broken wire somewhere in the harness, or a voltage regulator failure. An internal fault inside the alternator itself is another cause of failure. The immediate response requires turning off non-essential electrical loads and following the checklist's troubleshooting steps to preserve the remaining battery power while the flight continues under sole battery supply.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What causes alternator failure in aviation?
A broken drive belt is a common cause of alternator failure. Internal faults in the alternator, like a short or open circuit, de-energize the unit. Problems with the voltage regulator - whether from internal faults or outputting a voltage higher than the nominal battery voltage - lead to system-wide electrical failure. In many cases, the radio and transponder fail first because those components draw more amps than most other systems.
Incorrect wiring is another cause for alternator problems. If the airplane's wiring is not hooked up correctly to the alternator, it causes the alternator to overload. Incorrect wiring overloads the alternator, while wiring not hooked up correctly de-energizes it. Chafing wiring - especially in the engine compartment - causes alternator problems and creates dangerous conditions. Loose connections at the battery, at the airframe ground, or elsewhere in the system cause alternator problems. Corroded or burned contacts inside the electrical master switch contribute to charging system failure.
What are the signs of alternator failure in aviation?
In flight, the first sign of alternator failure is often the low-volts light illuminating, a clear sign the alternator is not producing enough power. At the same moment, the voltmeter will drop from the normal 14-volt reading to the battery's resting voltage of about 12 V, confirming that the system has moved from generating to consuming. A loadmeter's declining or zero indication is another sign of alternator failure. If the unit is fitted with an ammeter instead, the needle swings to the negative side and shows a discharge, meaning the battery is running down. When these indications appear together - low volts, voltmeter at ~12 V, and either loadmeter zero or ammeter discharge - the pilot can be certain the alternator has stopped working and the aircraft is now flying on battery power alone.
Is an alternator failure an emergency in aviation?
Whether an alternator failure is an emergency in aviation depends on conditions and aircraft configuration. When your only alternator fails at night, it is an emergency. By day in VFR conditions, electrical failure is not really an emergency: the engine keeps running because it has several redundancies, and the pilot still has a wet compass. Yet the same failure is a serious emergency if the airplane has only one generating system and the pilot has very little time available from the battery.
Declaring an emergency buys extra ATC attention and priority handling, so pilots declare an emergency to ATC as soon as the alternator-out situation is recognized. Once the failure is confirmed, fly the airplane first, then load-shed non-essential loads. Pilots turn off the transponder if it is not required. Voltage drop dictates land as soon as practical.
When the alternator stops working, the aircraft must depend exclusively on the battery. I handle electrical load by removing non-essential electrical gear and preserve battery ability for vital subsystems. I convey my condition to ATC, examine trouble, and deviate to the closest airfield.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What are the alternator failure procedures in aviation?
The immediate procedures dictate you to turn off non-essential electrical loads and to turn the Alternator (ALT) switch OFF, which stops the surge. A pilot conserves battery by shedding load, because the pilot has very little time available from the battery. Typical advice is turn off what you can to conserve power Beacon lights can be turned off as strobes are non-necessary electrical loads.
The pilot will attempt a reset if POH permits. If the reset fails, the pilot navigates to the nearest suitable airfield and requests for a precautionary landing. The airplane's magnetos provide the electricity that powers engine ignition so the engine keeps running.
How to troubleshoot an aircraft alternator?
To troubleshoot an aircraft alternator, begin every investigation with a known healthy battery: inspect connections, confirm posts are perfectly clean, bright and tight, and verify battery voltage and condition. While the master is still OFF, open the wiring diagram for your model. Confirm the ground strap is properly connected at Alternator F2 and that shielded wiring ends are properly grounded and have good continuity to the airframe. Ground resistance less than 0.2 ohms requires inspection, and any reading higher obliges repairs. Energise the system: close the master and connect a VOM to the B+ terminal, measuring voltage with respect to ground. Then run the engine with typical load and again measure the voltage with respect to the ground; a good alternator will raise system voltage to the prescribed regulated value.
If bus voltage is absent or low at the B+ stud, move step-wise along the control circuit. Check for Bus voltage at Regulator ENABLE; if there is none, replace wire or connectors from Regulator ENABLE to point A. Continue downstream: Bus voltage at point B must be checked; if absent, trace and replace wire or connectors from point B to point C. Bus voltage at point C must be checked; if absent, repair the harness section just tested. Bus voltage at point D must be checked to prove the output lead. When all control and power paths show correct bus potential yet the alternator still fails to charge, the unit itself is suspect. Remove and bench-check.
How to test an aircraft alternator?
To test an aircraft alternator, begin with the engine shut down. Turn the electrical system on without starting the engine and measure voltage coming off the output terminal of the alternator - a healthy alternator will reveal system potential at B+. Carry out the full field test. Jumper connected from B+ alternator output to the field terminal applies full bus voltage directly to the field. Clamp around inductance DC meter used to check output for full field shows a steep rise in charge current. Field voltage measured with VOM reads 75% bus during the test, confirming the rotor is capable of full excitation. Finally, engine run with typical load proves real-world performance. After start-up needle shows positive charge as alternator refills battery and steady bus voltage and cool operation signal a serviceable unit.
