In-Flight Emergencies

Zap, Crackle, Stop

March 1, 1999

When the juice stops flowing

It's been very quiet on the frequency for the past half-hour or so, and try as you might, no one seems to be answering your calls. You couldn't even pick up the automated weather observation system (AWOS) broadcast from the last airport you flew by. What's going on?

Immediate Action Items

  • Reset alternator/generator by turning their switches or circuit breakers off, then on again
  • If problem persists, turn alternator/generator off
  • Electrical fire: turn off battery and alternator; extinguish fire; vent cabin, then close vents; land as soon as possible
  • If electrical power is needed and system voltage is restored, attempt to identify faulty component (turn all components off, then turn each one back on until source of bad circuit, smoke, or odor is found), then land as soon as practical
  • Conserve battery power for the landing

Most likely, one of four things. In order of seriousness, these are: a) The radio's volume control is set way too low; b) The audio panel is misset; c) The radio you've been using has given up the ghost; or d) The electrical system itself — or part of it — has died. Obviously, that last situation is by far the most serious. You'll have to make some big decisions when experiencing an electrical failure and, depending on the situation, those decisions may be critical to the safe conclusion of the flight.

System basics

Aircraft electrical systems can be broken down into several basic elements. First, there's a primary source of electrical power. Typically, this is an engine-driven alternator or generator. There's also a means of regulating voltage output — either an alternator control unit or voltage regulator. An ammeter or loadmeter indicates the system's charging status (ammeter) or electrical load (loadmeter). There are the electrical components themselves — flap, landing gear, fuel pump and other motors or subsystems that do the work we command. These components are arranged on buses — the branches of the electrical system's tree-like schematic. There are switches and relays to turn these components on or off, or regulate the way they work. Circuit breakers (or fuses, in older airplanes) serve as watchdogs that prevent faulty components from dangerously overheating. A battery or batteries provides power to start the engine(s), absorb voltage surges and compensate for voltage drops, and serve as an emergency source of system power should the alternator or generator fail.

With this as background, let's turn to some failure modes and look at how best to deal with them.

Alternator/generator out

This is a big problem. With a dead alternator or generator (we'll refer to both components as alternators from here on, given their prevalence in the current fleet of aircraft), the battery is the airplane's only source of electrical power. How long until the battery goes dead? That depends on the condition of the battery and how you manage the airplane's electrical load after you've noticed the failure.

That's why it's important to include the ammeter/loadmeter in your instrument scan. Too often, it's not in our scan at all.

One indication of an alternator failure would be a discharge indication on an ammeter. Ammeters should indicate a zero charge in normal operations. A discharging condition, indicated by a minus indication on these gauges, means that the battery is running down. Why? The alternator is no longer charging it, and the battery's power is being robbed by the electrical system's components.

A loadmeter's declining — or zero — indication could be another sign of alternator failure. There's no load on the system because perhaps the alternator isn't putting out enough electricity to meet the components' demands. Other indications could be alternator-out or low-bus-voltage annunciators. It all depends on the gauges and warning systems on your airplane.

When an alternator conks out, you may have only one way to try to bring it back to life: resetting it. If your alternator is protected by an alternator circuit breaker, and it has popped, you could try pushing it back in to resume the flow of power. Resetting can also be accomplished by turning off, then on again, the alternator's on-off switch, usually a panel-type switch paired with the battery switch. If power is restored, fine. Just keep a sharp eye on the ammeter, loadmeter, or electrical annunciators afterwards. Why? The alternator may have gone off-line because of an internal fault, such as a short- or open circuit or other dangerous condition, that could crop up again. If voltage drops once more, turn the alternator off, turn off all non-essential radios, and land as soon as practical.

Runaway alternator

This is a problem where the alternator produces too much voltage, and the alternator control unit or voltage regulator cannot stem the flow of electricity. The danger here is that the extra current will fry all the components currently in use and progress into a full-blown electrical fire. The tactic in this situation is similar to that used to deal with an alternator-out condition: reset the alternator and hope that voltages returns to normal. If it doesn't, the only option is to turn off the alternator switch, and leave it off. As always, follow the advice in your airplane's pilot operating handbook (POH) for specific procedures for dealing with any electrical or other emergencies.

Popped circuit breakers

If an electrical component's circuit breaker pops, that's another warning of electrical trouble. It means that the component is either receiving too much current, is overheating, or maybe just received an inconsequential, transient, random shot of voltage. Try pushing the circuit breaker back in to see if things return to normal. If the breaker pops again, leave it popped. Something bad is at work, and you don't want to keep aggravating the fault.

Electrical fire

Specific advice, tailored to your airplane, for handling an electrical fire can be found in your POH. The drill is usually somewhat similar to that required for an overvoltage. Turn off the battery and alternator switches, turn off all radios and other electrical components, use a fire extinguisher to put out the fire, and land as soon as practicable. Some manuals don't specifically recommend landing ASAP if the rest of the flight doesn't require electrical power, but that advice sounds questionable. The fire could have caused hidden damage that could later cause another fire.

Some POHs suggest that you troubleshoot to determine the source of any fire, smoke, or odor of electrical origin. Here, the drill often calls for you to turn off everything on the panel and extinguish the fire. After the fire appears out, reset the battery and alternator switches and then turn on one radio or other component at a time until the bad circuit is identified or the odor or smoke is duplicated. In this way, you pinpoint the trouble spot, shut down the offending component, turn on the rest, and proceed to a nearby airport.

Again, this sounds like dubious advice. If you've had fire, smoke, or a burning odor, the risk of reinitiating the problem may be too great. It's one thing if the problem happened over mountains or on an overwater leg. It's quite another if you're over friendlier terrain where airports are more numerous.

By the way, don't forget to vent the cabin with fresh air after using a fire extinguisher. This should help eliminate any remaining toxic fumes from the extinguishing agents. Most manuals say to close any air vents after purging fumes to prevent reigniting any fire.

VFR or IFR?

A complete electrical failure in VFR weather conditions isn't nearly as problematic as one that happens in instrument meteorological conditions. In good day-VFR conditions, you shouldn't need any navaids or communications radios to fly to and land at a nearby uncontrolled airport. Besides, you've still got your hand-held GPS aboard, right? As for landing at an airport with a control tower, well, you do remember the light gun signals, don't you?

Night VFR is a little bit worse. With a complete electrical failure there'll be no way to turn on the runway lights at uncontrolled fields, no landing lights, no position lights to help other airplanes see you and, of course, no way to communicate your position to other airplanes — unless you've been smart enough to pack a handheld transceiver in your flight bag.

On IFR flights, pilots experiencing an alternator-out situation should consider making one final broadcast to ATC before powering down. Tell ATC that you're having an electrical failure, declare an emergency, ask for vectors to the nearest suitable airport (or use your handheld GPS), and then continue the flight using a single radio and battery power. Leave the transponder on so that controllers can issue you heading and altitude information.

Hopefully, you'll be landing soon. But in the meantime, powered-down flight in IMC can be as nerve-wracking as any other emergency. It's even worse at night. Oh, you've got several flashlights on board, right? They all have fresh batteries, right?

Battery considerations

By now you've deduced that load-shedding is a central part of all prime directives addressing electrical failures. It's essential in order to conserve battery power — and you'll need as much as possible. You may need to send a critical message. You'll certainly want to be able to listen to controllers, and to leave your transponder on — especially in controlled airspace or when flying under IFR — so your progress can be monitored by ATC. You may need pitot heat if flying in or near visible moisture when freezing OATs prevail. And in the case of some retractable-gear airplanes, you'll need enough electrical power to lower the gear before landing. If your airplane's landing gear free-falls after a loss of electrical power you won't have this worry — but you may have another one: an increase in drag and decrease in airspeed as the gear automatically extend.

The biggest electrical loads are generated by voice transmissions; heating elements in pitot tubes and windshields; pulse equipment such as radar, transponders, and DME; and transient loads caused by landing gear and flap extensions and retractions. So to spare the battery, fly with one radio, keep your voice transmissions to an absolute minimum, and run the transponder only if necessary.

You often hear that a battery will last 45 minutes after it's deprived of alternator energy. But that would be for a new battery that's in tip-top condition. An older, poorly maintained battery won't last nearly that long. Put a big electrical load on an older battery and you may only have 15 minutes of electrical power. That's one more reason why landing as soon as possible is the safest recourse after a total electrical failure. You don't want to wait for the sounds of silence to learn that your airplane is voltless.


E-mail the author at tom.horne@aopa.org.