How do I know if my alternator has failed or just tripped?

Check the ammeter - if it has swung to the discharge side and the low-voltage light is on, the alternator is not charging. Attempt a reset by cycling the ALT master off and back on after verifying the circuit breaker is seated. If the ammeter returns to positive, you likely reset a tripped alternator control unit. If it stays in discharge, the alternator is not coming back.

What is the first thing to do when the low-voltage light comes on?

Confirm the failure by checking the ammeter, then attempt a reset per your aircraft's emergency checklist. If the reset fails, begin load shedding immediately to extend battery life - don't wait to see if it resolves on its own.

Do I have to squawk 7700 for an alternator failure?

Not automatically. In day VFR close to your destination with sufficient battery remaining, you can work the radio normally and request priority handling without squawking 7700. Reserve the emergency squawk for situations where you need immediate ATC attention beyond standard priority - the decision depends on your specific circumstances, proximity to a runway, time of day, and remaining battery.

What should I turn off first when load shedding after an alternator failure?

Start with high-draw items that are not essential to your immediate situation: autopilot (3-5 amps), pitot heat if icing is not a factor, strobes in clear daytime VFR, a second comm radio not in use, and interior lighting. Keep your primary comm, one nav radio if needed, transponder, position awareness, and engine instruments on.

How often should the aircraft battery be load-tested?

Every year at minimum - ideally at every annual inspection. A battery that passes a visual check can still fail to deliver adequate capacity under load. The only reliable measure is professional load testing, and battery performance degrades significantly with age, cold weather exposure, and repeated partial discharge cycles common in training aircraft.

The Alternator Failure, the Electrical Load Shedding Drill Students Never Practice, and the Battery Runtime That Is Shorter Than You Think

An alternator failure is one of the most manageable emergencies in general aviation - and one of the most mismanaged. Pilots understand the concept from ground school but rarely drill the actual procedure. At altitude with a battery draining, the difference between knowing a concept and knowing a procedure matters enormously.

What Actually Happens When Your Alternator Fails in Flight

Under normal operation, the engine drives an alternator that produces alternating current. A voltage regulator keeps output steady, a rectifier converts it to direct current, and that current flows to the aircraft’s main bus while simultaneously charging the battery. In a healthy system, the battery is essentially a backup - it handles engine start and brief demand spikes, but the alternator carries the load in cruise.

When the alternator quits, you shift entirely to battery power. No alternator means no charging. Every amp your avionics draw is coming out of a finite reserve, and nothing is putting it back.

The first indication is typically a low-voltage warning light illuminating, or the ammeter swinging from positive to the discharge side. Catch it early. Every minute you don’t notice is battery time you’ve already spent.

How Long Will Your Battery Actually Last?

A standard lead-acid aircraft battery under full electrical load - all avionics on, nav lights, strobes, transponder, pitot heat - typically delivers 30 to 45 minutes of runtime. Sometimes significantly less.

That number assumes a healthy, recently tested battery. Most training aircraft don’t have one. A battery that has been through several years of student starts, left partially discharged in cold weather, and never load-tested may deliver 20 minutes or fewer. There is no way to know without professional load testing, which should happen at every annual inspection without exception.

30 to 45 minutes is the ceiling. Plan for less.

Should You Try to Reset the Alternator First?

Yes. Before accepting the failure, attempt a reset - because it sometimes works.

Alternator control units can trip their own internal circuit breakers due to a voltage spike or momentary overload. The emergency checklist for most common trainers - the Cessna 172, Piper Cherokee, and Beechcraft Sundowner - typically calls for:

Verify the alternator circuit breaker is seated
Reduce electrical load
Cycle the alternator field switch or ALT master off, then back on
Wait a few seconds, then check the ammeter

If the ammeter swings back to positive, you may have reset a tripped alternator control unit. Continue monitoring for the remainder of the flight, and get the aircraft inspected before the next flight. A reset is not a fix.

If the ammeter stays on the discharge side, the alternator is not coming back. Accept it and shift entirely to managing what remains.

What Is Load Shedding and What Do You Turn Off First?

Load shedding is the systematic process of turning off non-essential electrical equipment to extend battery life. It is where the biggest gap between knowledge and practice shows up.

A realistic full-load draw in a piston single is 30 to 40 amps. Reducing that to 15 or even 10 amps can double or more than double usable battery time. Every extra minute buys time to think, communicate, and reach a runway.

Keep on:

Primary comm radio
One nav radio if actively in use
Transponder (squawking your assigned code)
Primary position awareness (certified GPS or backup device)
Engine instruments

Shed:

Strobes in daylight VFR with no nearby traffic
Interior lighting in daytime
Pitot heat if icing conditions are not a factor
A second comm radio not in active use
Autopilot - this alone can draw 3 to 5 amps depending on installation
Rotating beacon if strobes are the primary anti-collision light
Auxiliary fuel pump if not required for your current phase of flight

The goal is not to pull circuit breakers indiscriminately - that creates new problems. Work systematically through non-essentials and verify what you’re removing before you remove it.

When Should You Declare an Emergency?

An alternator failure is not automatically an emergency. It is a situation that may become one depending on your circumstances and how you manage it.

Day VFR, close to destination, battery healthy: Shed load, expedite to your destination, and tell the tower what is happening. A calm radio call like “Riverside Tower, Cessna 4567 Bravo, I’ve had an alternator failure and I’m inbound, requesting straight-in runway 25” is sufficient. ATC will give you priority and have equipment standing by. You do not need to squawk 7700 or broadcast emergency unless the situation escalates.

Night VFR: Materially more serious. Your position and anti-collision lights draw from the same battery. Other aircraft can see you less clearly. The urgency to get on the ground increases significantly.

IFR or IMC: This is the time for a PAN-PAN call. Spoken three times, followed by your callsign, position, nature of the problem, souls on board, fuel remaining, and intentions. PAN-PAN signals urgency without declaring a full emergency - it gets you priority handling before the battery runs out. With 20 minutes of battery and 40 miles to go, making that call early is not optional. Pilots who wait too long run out of battery before they finish the approach.

Communicate early. Options narrow as the battery drains.

What Does the ACS Expect on Your Checkride?

The Private Pilot ACS requires recognition and response to aircraft systems abnormalities, including charging system failures. The examiner is not checking checklist memorization - they are watching for logical sequence, correct prioritization, and appropriate ATC communication.

The sequence examiners want to see:

Identify the failure
Attempt the reset
Shed load methodically
Assess the situation
Communicate with ATC when appropriate

Candidates who have verbalized this procedure out loud with an instructor can execute all five steps under pressure. Candidates who have only read the checklist tend to compress or skip steps. There is no shortcut - it has to be practiced out loud.

How Do You Actually Practice This Before It Happens?

Pull out the Pilot’s Operating Handbook for your aircraft. Go to the emergency procedures section. Find the electrical system failure checklist. Read each step aloud, and as you say it, reach for the actual switch or circuit breaker.

Ask yourself:

Can you find it without looking?
Could you find it with one hand while maintaining altitude in turbulence?
Could you find it at night with a flashlight in your teeth?

Most pilots have read the checklist. Very few have put their hands on every item in it. Those are two completely different levels of preparation.

What’s the Difference Between Alternator Failure and Total Electrical Failure?

A total electrical failure means the entire bus is dead - no panel lighting, no powered instruments, nothing. It is rarer than an alternator failure but does happen: a failed main bus contactor, a wiring fault, or a shorted avionics bus pulling enough current to trip every protection device in the system.

In a total electrical failure, you’re flying on standby instruments (if non-electric), pilotage, and dead reckoning. The panel gives you nothing.

This is why a handheld aviation radio belongs in your flight bag every flight - not the car’s glove compartment, not a closet at home. Charged, in the bag, every time. It doesn’t need every feature. It needs to transmit on 121.5 MHz and your current approach frequency. That capability is the difference between landing at an airport and landing in a field.

How Do You Catch Electrical Problems Before They Become Emergencies?

Electrical failures often advertise themselves before they fully arrive. A low-voltage light that blinks and recovers. An ammeter running slightly lower than usual across several flights. An avionics bus that resets unexpectedly on the ground. These are not random gremlins - they are early warnings that the charging system is struggling.

Make the ammeter part of your cruise scan. It takes three seconds every few minutes: ammeter positive, fuel gauges checked, engine instruments green, altitude holding, heading good. An early catch means maximum time to respond. That is the difference between managing the situation and the situation managing you.

Key Takeaways

A standard aircraft battery under full load lasts 30 to 45 minutes - older or poorly maintained batteries may deliver 20 minutes or less
Always attempt an alternator reset first; a tripped alternator control unit is a documented, resettable failure on common trainers
Effective load shedding can reduce electrical draw from 30-40 amps to 10-15 amps, potentially doubling usable battery time
Day VFR near your destination does not require declaring an emergency; IFR or night VFR with a failed alternator warrants a PAN-PAN call early
Practice means physically touching every switch on the checklist - reading it is not the same as knowing it
Carry a charged handheld radio in your flight bag every flight, no exceptions