Aircraft Electrical System Failure

Pilots do a lot more than fly planes; they manage systems like the aircraft’s electrical system. Experience shows that when a pilot knows how the system works and what the gauges are truly telling them, managing a failure like an aircraft electrical system failure becomes easier.

Introduction

When you enter the world of aviation, you enter a contract to not only learn all about what things look like when it is normal but also when things are not normal, like aircraft electrical failure. As pilots in many ways, we only get presented with some of the story in that our instruments are our way to understanding what is going on in each system. You could say an indication of its health.

aircraft electrical system failure — Electrical equipment will still work using power from the battery, but since the alternator is not charging it, the battery will eventually run flat depending on how much electrical power is being drained.

As we cannot fix anything in flight, what we are there to do is analyze, assess, run a process, and determine the urgency of landing depending on what the gauges are saying. However, digging deeper into a system and understanding how it works means the emergency checklist makes sense and therefore your capacity (and confidence) to manage it is increased.

By seeing why an aircraft manufacturer writes a checklist a particular way also ensures you do not get slack on the order of things and inadvertently cause the failure.

Contents hide

1 Introduction

2 How does an aircraft electrical system work?

3 What is the difference between a generator and an alternator?

4 Does aircraft electrical equipment still work if the alternator or generator fails?

5 What to do if there is alternator failure

6 How does a load meter or ammeter work?

7 Aircraft electrical failure rules of thumb

8 Conclusion

cockpit, pilots — If we pay attention to the subtle signs the instruments tell us, understand what they mean in practical terms, and look after the system when it is trying to tell us it needs attention, we will be far less likely to have to deal with electrical failures.

How does an aircraft electrical system work?

In the spirit of knowing why not just what to do, it is best to understand how the electrical system works¹ rather than just learning what to do if it fails. First, the master switch in most aircraft is double-sided, one for the battery², and the other to energize the alternator field. The alternator is a bit like a generator, except that the magnet in a generator is permanent which means it will generate electricity as long as it is turning, but it can still be isolated by diverting current to ground, away from the aircraft’s electrical circuit.

Your aircraft’s electrical system has three primary components: a battery, a generator or alternator, and an electrical bus to distribute electrical power. The spark plugs in certified piston aircraft engines are powered by engine-driven magnetos, so no additional electrical power is required for the engine to run.
aopa.org/news-and-media/all-news/2020/august/flight-training-magazine/ol-how-it-works-electrical-system¹

The alternator only works when an electrical current is supplied to the electro-magnet in the alternator, enabling it to produce a magnetic field. The battery supplies the power to energize the electro-magnet, which then produces current to charge the battery and supply electricity to run all electrical equipment.

What is the difference between a generator and an alternator?

There are several differences between an alternator and a generator⁵ straight up, the alternator is effectively the upgrade of the generator. The generator is heavier consuming weight, less efficient, conserves less energy, and takes up more space. This all sounds like old technology, however on the upside, they produce energy at all times and can charge a battery from flat.

Your job is to know what you have in your aircraft, how information about the unit is presented, and what to do about it in the event of an aircraft electrical failure of some form.

Does aircraft electrical equipment still work if the alternator or generator fails?

Yes! Electrical equipment will still work using power from the battery, but since the alternator is not charging it, the battery will eventually run flat depending on how much electrical power is being drained. Therefore, if an alternator or generator fails the only initial indication will be from any warning lights or the load meter or ammeter, so it is best to know how the system works in your aircraft, so you will know what to look for.

If the alternator fails, the engine will still run because the magnetos have permanent magnets in them and provide their own power independently from the electrical system as long as the engine is turning. Of course, always refer to your aircraft flight manual³ for information on your aircraft type.

What to do if there is alternator failure

Always review the checklist of your aircraft type and follow it in the event of an alternator failure⁴. However, being confident in initial actions if an alternator failed and knowing the ‘quick first response’ demonstrates good behaviors. The first thing to do would be to turn off the alternator itself so it does not drain the battery, before turning off all non-essential electrical equipment until they are required.

“When the alternator warning light pops up on an aircraft’s dashboard, it’s important for owners and mechanics alike to remember that this warning light doesn’t always mean that the alternator is the engine part with a problem.”
airpowerinc.com/troubleshooting-ideas-aircraft-alternator-problems⁴

How does a load meter or ammeter work?

A Load meter shows load on the alternator or generator. If nothing is turned on, it would show current flow into or out of the battery, and if there is no flow indicated it would mean the battery is fully charged. The ammeter shows current flow into and out of the battery, indicating whether the battery is being charged or drained and whether the alternator is working.

Shortly after start, the ammeter should indicate a deflection toward the positive side of the instrument, indicating that current is flowing into the battery to recharge. If the battery is fully charged, the deflection will not be much, and if the battery is low on charge, the deflection will be greater. This would tell us that the battery was low on charge, which might be due to cranking the engine for too long before it started, or it could also tell us the battery is not holding charge for some reason, such as being low on electrolyte, or one cell may be failing.

aircraft electrical system — Aero commander 500U Generator gauges (note many have been converted to alternators). Picture courtesy of Keith Radke

Figure 1 Aero commander 500U Generator gauges (note many have been converted to alternators). Picture courtesy of Keith Radke

Aircraft electrical failure rules of thumb

Monitor Monitor Monitor!

If we pay attention to the subtle signs the instruments tell us, understand what they mean in practical terms, and look after the system when it is trying to tell us it needs attention, we will be far less likely to have to deal with electrical failures.

Do not reset circuit breakers in flight unless a more dire emergency exists

pilot, cockpit — When you enter the world of aviation, you enter a contract to not only learn all about what things look like when it is normal but also when things are not normal, like aircraft electrical failure.

Another possible electrical failure scenario is if a wire shorts and trips a fuse or circuit breaker or damages an electrical device. I have seen radio screens with smoke behind them because they were turned on before switching the alternator online.

This occurs because a voltage regulator is built into the electrical circuit that takes a second to work when the alternator starts producing current. As current starts flowing from the alternator after start, a voltage spike is produced until the regulator controls voltage flow.

Leave avionics off until after start

Sensitive electronics like the avionics cannot handle too much voltage, so that is why the avionics must be left off until after start, and after the generator or alternator is switched online – not before.

Check all switches in correct position PRIOR to switching battery on

An electrical fire can be ignited by an electrical short if the current from a live wire arcs onto bare metal seeking a short circuit to earth. This causes a high current flow making the wire get hot enough to burn insulation and plastic. The simplest solution is to turn OFF both sides of the master switch, isolating all equipment.

Then turn all equipment OFF and turn the battery side of the switch ON first, before switching the alternator ON and checking the ammeter to see if it is drawing too much current. If it does, leave it off and continue with minimum electrical gear turned on, only turning something like the radio on when necessary.

Go and speak to an aircraft engineer about loss of electrical power

Without electrical power the aircraft may lose flap and/or gear operation, depending on how the system works, so the more you know about aircraft systems before problems develop, the more likely you will be able to handle them without stress. Never be afraid to go old school and go to a hangar and ask an engineer, they are a world of knowledge.

Conclusion

There is a lot to learn when you are starting out in your flying career, but an experienced pilot will tell you that their day is a whole lot better when they take the time to understand the how and why of an aircraft system. We are not saying you need to know how to build an aircraft, but it is important to have a basic knowledge of what is going on behind the instrument panel.

The gauges present a lot of subtle information, and we urge you to always seek more knowledge so that in the event of a failure of a system, the pieces of the puzzle may make more sense. Systems do not always fail the same way so being able to adapt on the fly especially when it comes to aircraft electrical systems, will boost your confidence and therefore competence.