I sometimes chuckle when I think about the complexity of the electrical systems in new airplanes, particularly single-engine airplanes. Most current production singles have multiple electrical buses, more than one electrical power source and, often, emergency backup batteries. And that's great. But there is still only one engine, and if it quits, the airplane is not going to continue flying for long.
The level of sophistication of the electrical system in singles tells you just how important electricity is for safe flight. Pilots who are willing to fly in the clouds with only one engine demand backups and redundancy for the critical avionics and other systems powered by electricity.
Look at a new "glass" cockpit and it's easy to see why we are so dependent on electrical power. Some airplanes still use vacuum pumps to power a backup attitude gyro, but increasingly even the backup instruments are powered by electricity. Maintaining a constant flow of power after some elements of the electrical system fail is the only way a pilot can maintain control of the airplane in clouds or low visibility. In other words, you could glide to a forced landing if all engine power were lost, but without reliable power to fundamental flight instruments, it would be impossible to maintain control without visual reference, and that almost always ends in a fatal crash.
The electrical system wasn't always so important. In decades past, many thousands of light airplanes had no electrical system at all. Pilots propped the engine by hand to start it; a battery powered the navigation lights, if there were any; and mechanical energy powered everything else necessary for flight. In some airplanes, such as the two-seat Cessna 120, the electrical system was an option.
As reliable electrical power became more essential, airplane designers and pilots focused mostly on multiple charging sources. Generators and alternators are among the least reliable components in any electrical system, so it was obvious that they wanted more than one. Most pilots thought the second alternator was as important in a piston twin as the second, but minimal, source of propulsion from the engine was. To many pilots, flying in the clouds or at night with only one generator or alternator was simply foolhardy, so twin sales flourished.
This same line of reasoning led modifiers to devise methods for installing a second backup generator on the only engine in piston singles. These units were typically low in capacity and provided only enough current to power essential equipment such as a communications radio and transponder, but they were better than nothing.
Before long, the manufacturers of singles jumped on the bandwagon and offered second generators or alternators, and these grew in power capability to where most everything on the airplane would continue to operate after failure of the primary alternator. A major selling point of the twin — a second electrical power source — was eliminated.
Multiple power-generating sources are, however, only one of the desirable backups in a truly reliable electrical system. A second or even third generator is the foundation of a reliable electrical system, but there are many other failures that can leave you without power unless there is careful design of the entire system.
Isolation Is the Key All electrical systems have buses to which individual power-using items are connected. A bus is a common wire, often of high capacity, to which several power users are connected. For example, think of the circuit-breaker panel in your house. One breaker may control power to kitchen outlets. That circuit of outlets could have several items plugged in, such as a refrigerator, blender, mixer and so on. That's how a bus works. It is a common source of power for several related electrical loads.
The starting point — literally — of an airplane electrical system is a battery bus, which, as the name implies, is powered by the battery. The starter connects to the battery bus, and once the engine is running and the alternator is on line, the battery is recharged through the battery bus.
In simple electrical systems, all other buses are connected to the battery bus and then distribute power around the airplane. At first thought, that seems like a good idea. The alternator is sending power to the battery bus, and if the alternator fails, you can continue using stored energy from the battery for many minutes. But as in all things electrical, there is a trap in having the battery bus always on line.
For example, what if the big wire connecting the battery to the battery bus chafes through and shorts out to the airframe? Within seconds the alternator will be fried and the battery power will drain uselessly to ground. In reality, all that failed was the wire from the battery, while the alternator was working perfectly until it too was shorted out by that single failure.
The way to prevent this situation is to isolate the battery and its bus from the entire electrical system so that its failure can't take down the whole system. If there were isolation, only the battery and its stored power would be lost while the alternators would continue to power the rest of the system.
Isolating the battery bus is most important because it has such large capacity that is necessary for engine starting. Turbine-engine starters can draw 1,000 amps, or even more, for a brief period when we engage the starter. Even piston engine starters can draw hundreds of amps initially. That means all of the wires connected to the battery and its bus must be huge, and the failure of such large wires could short out the entire system almost instantly without proper isolation.
Every other electrical bus and component in the electrical system can also short out and drain power away from the whole system without proper isolation. A short circuit also typically creates a great deal of heat as more electrical current flows through a wire too small to carry the load. That's another reason every electrically powered item must be protected by a circuit breaker, but the threat to the entire electrical system is a more complicated issue.
Because it is not possible to fully protect every connection from a massive short, or the failure of a circuit breaker, the electrical systems in recently designed transport airplanes, and in some newer light airplanes, are physically separated during flight.
In these most redundant electrical systems, the airplane is usually split in half because there are two engines and two generating sources. The right- and left-side electrical systems operate in parallel with each other, supplying power to individual items, and the operation is transparent to the pilots. For example, the primary flight instruments on the right side of the airplane come from the buses on that side with no connection to the electrical buses on the left side. If a massive failure wipes out the buses on one side of the airplane, the other side continues to function normally.