October 1, 2004
Marc E. Cook
A Nathaniel Hawthorne character in "The House of the Seven Gables" exclaimed: "Is it a fact — or have I dreamt it — that, by means of electricity, the world of matter has become a great nerve, vibrating thousands of miles in a breathless point of time?" Indeed, it's not a dream, and by the same hands of nature and science, we have airplanes that we need not start by hand and that may carry communication and navigation aids that would have boggled Hawthorne's mind.
And yet what pilot hasn't stared out over a forest of circuit breakers, almost surely labeled in enigmatic abbreviations, and wondered aloud, "What in Ohm's law is going on behind the panel?" Though the idea seems straightforward enough — flip a switch and something happens — when something conks out or, worse, fails intermittently, the pilot's work load can take a turn for the worse.
Fortunately, the most common electrical system maladies are just that, common. In the never-ending quest to simplify airplanes and reduce production costs, our steeds receive some fairly rudimentary electrical technologies.
Simplicity doesn't necessarily portend reliability. So it's worthwhile to become familiar with your airplane's electrical diagram, found in the pilot's operating handbook. (If you're flying an older airplane whose manual lacks such information, look in the service manual for the diagrams.) Take note of whether you are running around with a generator or alternator up front — knowing which type will help you diagnose charging problems later — and its type of drive, gear or belt.
Also, pay attention to which circuits are clustered together, sharing circuit breakers. Some Skyhawk pilots are amazed to find that a shorted oil temperature gauge is capable of taking down the fuel quantity indicators as well. Another example: After a pitot-static problem rendered the landing-gear airspeed switch inoperative in the Mooney I fly, I discovered in the next few minutes that the warning horn isn't connected to the circuit breaker labeled GEAR WARN (warning) or even GEAR ACT (actuator). Instead, it was hooked to the GEAR CONT (control) breaker, and I enjoyed a good minute or two of beeping before I stumbled upon the answer. Had I studied the manual beforehand, I probably would have remembered this tidbit and could have quickly shushed the squawking.
Once you've boned up, you'll be ready for the most common of electrical system failures, which concern the charging system. That system, of course, starts at the battery. It's unfortunate but true that aircraft batteries live unappreciated lives, usually only getting attention when they give some trouble. Once a month or so, you as conscientious owner/pilot ought to check its electrolyte level and specific gravity. Go out and get a small hydrometer (you can use an automotive variety as long as it has a scale showing specific gravity). A fully charged battery's electrolyte will show a specific gravity of about 1.30; a reading below 1.16 indicates a dead dog. While you're nosing about, make sure the battery contacts are free of corrosion and that the battery or box is vented correctly. Any sign of corrosion or spilled electrolyte should be attended to immediately; a solution of warm water and baking powder will stop the corrosive effects of the acid. Don't be fooled by the presence of petroleum jelly or grease on the terminals — these are common ways of trying to prevent corrosion.
Most airplanes in service today use an alternator to provide ship's power and to charge the battery. (The age-old generator has been on the way out for years, and rightly so. Though it can be somewhat more reliable than the industry-standard alternator, it has poor low-speed output, too much heft, and parts are getting scarcer every day.)
How do you know your alternator lives a happy, healthy life? Look first for poor charging characteristics right after engine start. Normally, there will be a large surge of power sent back to the battery in thanks for cranking the engine over. In a few seconds, though, that demand will subside, and the alternator will be left to running the airplane's systems. Most alternator systems should be able to sustain normal current drain at or near idle speed. (That is with the exception of the truly high-current items like the landing light and pitot heat.) A discharge on the ammeter or a zero indication on the load meter, together with illumination of a low-voltage light, are your obvious clues of lacking alternator performance. Mostly, just be aware of what's normal for your airplane.
(Ammeters and load meters are essentially the same instrument indicating fundamentally different things. The ammeter shows the relationship between the battery and the rest of the electrical system. A positive indication means the battery is being charged, while a negative needle movement signals that the system cannot support the load without the battery's help. A load meter, on the other hand, simply shows the current being supplied by the alternator and indicates the total system load on it.)
The modern alternator is simply a three-circuit alternating current (AC) generator; it is light and compact for the power output and fairly reliable if kept cool and tightly screwed together. (Split case halves and heat damage are the two most common mechanical failures for alternators.) The alternator requires an excitation voltage, applied to the field coil, to induce production of electricity; this field voltage is also varied or toggled by the voltage regulator to control alternator output. In simple terms, the voltage regulator watches main-bus voltage. When it drops below a preset value, it sends more voltage to the field coil, which in turn causes the alternator to increase output. When the overall voltage reaches a preset limit, the field power is tailed off to maintain this equilibrium.
Generally, if the alternator suddenly goes off line — as indicated by the appropriate gauge indications or a warning light — it can be brought back on easily. Modern voltage regulators contain over-voltage controllers or relays, and a sudden spike in the system voltage could cause the device to shut down alternator output. Powering down the alternator field circuit — either through the combined master/alternator switch or a pullable field breaker — and then reinstating field voltage will usually bring the alternator back. If there's a persistent problem, be ready for it to go off line again, and begin making plans to get on the ground before the battery gasps its last breath. Make sure you switch off the avionics master before attempting to regain alternator output; it's possible there will be a large voltage increase when it comes back to life, and you don't want to take the chance that it's more than the expensive radios can handle.
If you can't revive the alternator with this reset method — or a more specific one mentioned in the POH — land, and look it over on the ground. A good rule of thumb is to never reset any circuit breaker more than once. The first time we can believe it's a fluke, but if it trips a second time, it's safe to assume something's really broken.
Don't be too eager to consider the alternator itself shot, either. It needs that field voltage, and a broken wire or bad circuit breaker will deprive it of that current. Also, voltage regulators have been known to just give up; check it by the book first. Finally, it's possible to have a poor connection at the master/alternator switch wreak havoc with the alternator or voltage regulator. Check the connections, and substitute a known good switch before you go replacing the most expensive part of the system.
When alternators fail, it's usually the diodes that go. Consider that there are at least six of the hummers in the aft portion of the alternator case (more if you've got a high-current alternator). They are exposed to heat and vibration all their lives. Often, a diode will fail open, which reduces the current capacity of the alternator and will send a nasty whine into the rest of the electrical system. If you get this whine — and note that its pitch will rise and fall with engine speed — it's a good chance you've got a bad diode. (It's worth checking, too, that you aren't chasing magneto noise. In flight — and over the airport if you're squeamish about such things — shut down each mag individually to make certain that you aren't also hearing ignition noise.)
Mechanical disorders can prevent the charging system from doing its job, too. With belt-drive alternators or generators, frequently inspect belt tension and condition. Look for signs of pulley misalignment (one edge of the belt worn more than the other, for example) or advanced wear (such as cords showing through or serious cracking). Gear-driven alternators are a different breed and difficult to inspect. A precautionary measure worth the effort is to remove the alternator at the annual and inspect the drive coupling. These systems are designed with a frangible coupling between the engine and the alternator, in the belief that a seized alternator should not be able to damage the powerplant. Still, it's not uncommon for a major blowup to send pulverized alternator bits through the most expensive mechanism in your airplane. In the event of an in-flight alternator failure, it's probably wise to consider landing quickly to make sure it's something simple like a broken wire, rather than some under-bonnet apocalypse.
What about some of the lesser electrical system maladies? One of the more common concerns erratic ammeter indications — that is usually a sign of resistance built up in the field coil circuit, frequently in the master switch or field circuit breaker.
Many electrical engine instruments will show erroneous readings if the bus voltage is out of spec or if they have a bad ground. This is a persistent problem in the Mooney, for example. Poor ground connections between the shock-mounted panel and the airframe often cause the oil pressure and temperature gauges, fuel indicators, and cylinder-head temperature gauge to reach for the margins. Ironically, as the temps go up, the pressure goes down, just as it would in the case of a real problem. If you see this, have your mechanic double-check the ground or add additional grounding points.
Ineffective ground connections are at the heart of many electrical problems in aircraft. Noisy intercoms are often caused by either poor shielding or by insufficient or incorrect shielding. Microphone circuits, working at low voltage, are susceptible to stray electrical noise.
Finally, understand that wires have a finite life. If your bird is more than a couple of decades old, be prepared for random circuit outages caused by broken, loose, or corroded wires. (This goes double for those airplanes with aluminum wire.) Don't be too surprised if you find a wiring loom or two that goes nowhere and is connected to little more than air. It's a fact of life that many avionics replacements get installed without a thorough removal of the old bits and pieces. Many owners report that it takes substantial effort to strip the old wire from the airframe, but the results — a reliable electrical system — are usually well worth the trouble.
Safety and Education
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