Your Cockpit Power Plant: A Pilot's Guide to the Aircraft Alternator

As pilots, we train relentlessly for engine failures, but what about the smaller, quieter failure that can just as easily ruin your day? Imagine you’re flying in solid cloud, your GPS is guiding you, and suddenly... the screen flickers and dies. Your radio goes silent. Your instruments dim. This is the reality of an alternator failure, and for any General Aviation pilot in the UK, understanding how to spot and manage it is just as critical as any "Aviate, Navigate, Communicate" drill.

Your aircraft's alternator isn't just an accessory; it's the beating heart of your electrical system. It powers your avionics, your lights, and your instruments. Without it, you’re flying on borrowed time, with only your battery to keep you safe.

This guide isn't for engineers. It's for you, the pilot. We’ll skip the deep-level electronics and focus on what you need to know in the cockpit: What is it, what clues does it give you when it's failing, and what are the immediate actions you must take to get on the ground safely?

Content

1. What is an Alternator? (The Pilot's Need-to-Know)
   

2. How to Spot a Failing Alternator (Your Cockpit Clues)

   • The Big Red Light (Annunciator)

   • Your Ammeter: The Telltale Discharge

   • Your Voltmeter: The Drooping Needle

   • Other Subtle Signs
     

3. Alternator Failure In-Flight: Your Immediate Action Checklist

   • Step 1: Aviate (Fly the Aircraft!)

   • Step 2: Verify the Failure

   • Step 3: Conserve Your Battery (Shed Load)

   • Step 4: Communicate

   • Step 5: Navigate and Land
     

4. Conclusion: It's a Manageable Emergency
   

5. Frequently Asked Questions (FAQ) for Pilots

1. What is an Aircraft Alternator? (The Pilot's Need-to-Know)

Think of your aircraft's electrical system in two parts: the battery and the alternator.

• The battery is like a large, rechargeable torch. It's used to start the engine and provides a limited supply of emergency power. On its own, it might only last 20-30 minutes.

  
  

• The alternator is the main power plant. It's a small device, usually driven by a belt connected to your engine. As long as the engine is spinning, the alternator is generating a continuous supply of electricity.

  
  

This electricity does two vital jobs:

1. It powers all your "kit" – radios, GPS, autopilot, lights, flaps, and electric instruments.

   
   

2. It constantly recharges the battery, keeping it topped up for when you next need it.

   
   

There's also a partner component called a voltage regulator. Its simple job is to act as a bouncer, making sure the alternator doesn't send out too much power (an "over-voltage"), which could fry your expensive avionics.

In short: Engine running = Alternator on = Power for everything + Battery charging.

2. How to Spot a Failing Alternator (Your Cockpit Clues)

An alternator rarely fails with a bang. It usually gives you warnings. Your job as a pilot is to be scanning your instruments, ready to catch these clues early.

GA cockpit instruments showing alternator failure signs. Licence: CC BY-ND (PPL Club)

The Big Red Light (Annunciator)

This is your most obvious warning. Most aircraft have an annunciator light (often red or amber) labelled "ALT", "ALTN", or "LOW VOLTS". If this light illuminates in flight, your first thought must be "alternator failure." It's telling you the alternator is no longer powering the main electrical system, and you are now running on battery alone.

Your Ammeter: The Telltale Discharge

The ammeter is your power accountant. It shows the flow of electricity to or from the battery.

• Normal (Charging): After start-up, the needle will show a positive charge as the alternator refills the battery. In normal flight, it should sit at or just slightly positive (showing a "trickle charge").
  

• Failure (Discharging): If the alternator fails, the system will instantly draw all its power from the battery. Your ammeter will swing to the negative (discharge) side. A big negative reading means you are draining your battery fast. This is a primary, undeniable sign of failure.

Your Voltmeter: The Drooping Needle

The voltmeter shows the "pressure" of your electrical system.

• Normal: In a typical 14-volt system, the voltmeter should read between 13.8 and 14.2 volts. In a 28-volt system, it'll be around 28 volts.
  

• Failure: When the alternator fails, the voltmeter will drop from its normal 14-volt (or 28-volt) reading to the battery's resting voltage. This will be noticeably lower, perhaps 12-13 volts (or 24-26 volts), and will steadily decrease as the battery's power is consumed. A high reading (e.g., 16+ volts) is also dangerous and points to a voltage regulator failure (an "over-voltage" condition), which can damage your equipment.

Other Subtle Signs

• Dimming Lights: Your instrument backlights, landing light, or cabin lights may start to dim.
  

• Radio Static/Weakness: You might hear a new "whine" in your headset (alternator noise) or find your radio transmissions are weak.
  

• Erratic Avionics: GPS units or autopilots may begin to reset or behave strangely as the voltage drops.

3. Alternator Failure In-Flight: Your Immediate Action Checklist

You've spotted the signs. Your ammeter is discharging, and the "ALT" light is on. Don't panic. This is a manageable emergency if you act decisively.

Step 1: Aviate (Fly the Aircraft!)

Before you touch a single switch, fly the aircraft. Maintain your heading, altitude, and airspeed. If you're in cloud, stay focused on your instruments.

Step 2: Verify the Failure

• Check your instruments: Confirm the ammeter discharge and low voltage.
  

• Check circuit breakers: Check that the "ALT" circuit breaker hasn't popped. If it has, you can try resetting it once. If it pops again, leave it.
  

• Attempt a Reset (POH Permitting): Your Pilot's Operating Handbook (POH) is the law. Most POHs will have a checklist that involves:
  

  1. Turning the Alternator (ALT) side of the master switch OFF for a few seconds.
     

  2. Turning the ALT switch back ON. This can sometimes reset the voltage regulator and bring the alternator back online. If it works, great – but be very suspicious of it for the rest of the flight. If it doesn't, proceed to the next step.

Step 3: Conserve Your Battery (Shed Load)

You are now on battery power, and the clock is ticking. You must reduce your electrical load to the bare minimum to extend your battery life. Turn OFF everything non-essential.

• Non-Essential:

  • All lights (landing, taxi, strobe, navigation – unless essential for night/IMC)

  • Autopilot

  • GPS (use your compass, map, or a tablet with its own battery)

  • Secondary radio (Comm 2 / Nav 2)

  • Cabin fans or air conditioning
    

• Essential:

  • Your primary radio (Comm 1)

  • Engine instruments

  • Pitot heat (if you are in icing conditions)

Step 4: Communicate

Do this before your radio dies. Get on the radio to the nearest Air Traffic Control (ATC) unit.

• Use a "Pan-Pan, Pan-Pan, Pan-Pan" call. This signals an urgency, but not an immediate life-threatening distress.
  

• Tell them clearly: "G-ABCD, Pan-Pan, Pan-Pan, Pan-Pan, experiencing an alternator failure, on battery power only, request vectors to [nearest suitable airfield] for a precautionary landing."
  

If your transponder is still on, you can squawk 7700 (emergency) if you feel the situation is deteriorating or you lose communications. Step 5: Navigate and Land

Get the aircraft on the ground at the nearest suitable airfield. This is not the time to try and make it back to your home base 50 miles away.

• Plan for a simple approach.
  

• Be aware: If your flaps or landing gear are electric, you may have issues.
  

• Flaps: You might only have one shot at deploying them, so do it at the appropriate time in the circuit. Plan for a flapless landing if you're unsure how much power they draw.
  

• Gear: If you have electric gear, extend it early whilst you still have battery power. Be prepared to use the emergency manual gear extension procedure.

4. Conclusion: It's a Manageable Emergency

An alternator failure can be a frightening event, especially in poor weather. But it is almost always manageable.

By knowing your aircraft, constantly scanning your instruments, and having a clear plan, you can turn a major panic into a controlled, precautionary landing. Your alternator is vital, but your knowledge as a pilot is what will ultimately get you safely on the ground. 5. Frequently Asked Questions (FAQ) for Pilots

Q1: What's the difference between an alternator and a generator? A1: They both do the same job: make electricity. Alternators are more modern, lighter, and more efficient, especially at low engine RPMs (like when taxiing). Most GA aircraft you'll fly (like a PA-28 or C172) use an alternator. Older, classic aircraft might have a generator.

Q2: How long will my battery last if the alternator fails? A2: This is the "how long is a piece of string" question. It depends on your electrical load and your battery's health. A brand-new, fully charged battery might give you 30-40 minutes with a high load (all radios, lights, etc.). If you immediately shed load and only keep one radio and your engine instruments, you could extend that to an hour or more. But you should always assume you have less time than you think and land as soon as practicable.

Q3: My "LOW VOLTS" light is on, but my ammeter is at zero (not discharging). What's happening? A3: This is the classic sign of an alternator failure in an aircraft equipped with a loadmeter (which is common in many Cessnas).

The confusion comes from two types of "ammeters":

• Loadmeter (Your Scenario): This instrument measures the total electrical output from the alternator. When the alternator fails, its output drops to zero, so the loadmeter correctly reads zero. The battery instantly takes over, and the bus voltage drops from ~14V to the battery's ~12.6V, triggering the LOW VOLTS light.
  

• Center-Zero Ammeter: This instrument measures the flow of electricity into or out of the battery. In an alternator failure, this type of ammeter would show a negative (discharging) reading because the battery is draining to power the aircraft.
  

So, your symptoms (LOW VOLTS light on, ammeter at zero) are a clear-cut alternator failure in an aircraft with a loadmeter. Your battery is now your only source of power and is not being recharged.

Q4: Can I take off if I know my alternator is broken? A4: No. This would be illegal and extremely unsafe. In the UK, this would make the aircraft un-airworthy (and similarly in most other countries). You are intentionally taking off with a major system failure, no way to recharge your battery, and will almost certainly lose electrical power in flight.

Q5: What's an "over-voltage" failure? A5: This is when the voltage regulator fails, causing the alternator to send too much power (e.g., 17+ volts) into the system. This is very dangerous as it can "fry" your avionics, boil your battery, and cause a fire. The signs are a very high voltmeter reading and often a strong smell of sulphur (from the battery acid boiling). The immediate action is to turn the Alternator (ALT) switch OFF to stop the surge and then follow the same "land as soon as possible" procedure.