One unexpected use of CBs that I have found with some integrated avionics systems is to "reboot" an individual element of the system. Sometimes, just as with your other computers, powering down and back up will restore the function of some part of the system that is connected to a common on-off switch. The CB can reboot individual functions without killing the whole system.
The bottom line on CBs is to forget what we had been taught and not reset them even once without considering why they are out in the first place.
The Avionics Master
Most airplanes have a dedicated avionics bus that is connected to one of the primary buses with a relay. The avionics master allows the pilot to depower all avionics with a single switch instead of turning off each individual item.
Earlier avionics were sensitive to voltage spikes and could be easily damaged by even a very brief surge, which can take place, for example, during the engine start sequence, so it was standard procedure to leave the avionics master off until all engines were started and the generators were on line.
Most airplanes still have an avionics master, but its function has become less important because recently designed avionics have power supplies that can handle a wide range of voltage. Most avionics will function with voltage between 9 and 33 and can withstand even higher spikes without damage, so starting the engine with the avionics powered up is not the issue it used to be. The most fundamental avionics in a glass cockpit, such as the primary display and electronic gyros, are functioning as soon as any power is applied, so you can see how well modern electronics can withstand power surges.
In many airplanes the avionics master relay is powered closed, meaning to turn off avionics power you would need electrical power to the relay. When you turn on the master switch, the avionics relay is closed, and for an instant power flows to the avionics until the relay can open. When you turn on the avionics master switch, you take power away from the relay and it closes, allowing power to flow into the avionics bus. The important issue here is that there is also an avionics master CB in this type of system. If you lose power to the avionics bus, you can pull the breaker, and that might solve the problem by removing power from the relay. It sounds backward to pull a CB to restore power, but that's how this type of system functions.
Master Switch vs. Battery Switch
Many light airplanes have a "master" electrical switch, while more complex airplanes have switches for the battery and then for each generating source. The functions are similar, but there are very important differences.
The master switch, as the name implies, connects the entire electrical system to the battery bus. It also engages the alternator that is connected to the battery bus.
A battery switch does exactly what its name suggests and connects the battery bus to the system. However, each generator or alternator has its own on-off switch so it can be controlled independent of the battery. This is important because you could have a battery or battery bus failure and you would want to isolate that from the entire system without losing the generating source. In this type of electrical system, everything operates normally with the battery turned off once the generators are on line.
It is common practice for pilots to disconnect the battery in business jets so that some forgotten light or other load that is connected to the always-on "hot" battery bus won't drain the battery as the airplane sits. It has happened that pilots forgot to reconnect the battery, had a ground power unit hooked up to the airplane for engine start, turned on the generators and took off with no battery power available. Legend has it that some pilots have even departed under the same scenario without a battery even installed in the airplane.
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Why 14 or 28 volts? |
The real threat in having no battery power available is that it is the ultimate backup source of power. There must be enough battery power available to power the critical items for IFR flight for 30 minutes in recently certified airplanes. What items are on that emergency list are different from airplane to airplane, but primary instruments, com radio, transponder and basic cockpit lighting are always available.
The battery also serves as a sort of shock absorber for the electrical system, relieving the generators during the momentary high drain of some systems. For example, the electric motors that power the landing gear in many airplanes can draw more current than the generators produce at the start of an extension or retraction cycle, and the battery supplies those extra amps. If the battery weren't there, bus voltage could fall so low that avionics would drop off line.
Some standard procedures call for starting the engine or engines with the alternators off under the theory that the alternators are adding at least a tiny amount of resistance to engine rotation during the start sequence. I can't imagine that it matters, so I leave the alternators on in my Baron during engine start, but either way works fine as long as you remember to turn them on as soon as the engines are running.
Know the Buses
The design of the electrical system is out of every pilot's hand, but it is absolutely necessary for each of us to understand how it operates and where the threats to continued function are, and to have the knowledge to isolate failed circuits or buses and restore power when possible.
In complex airplanes such as jets, many hours, and even days, are spent in the classroom and simulator learning every detail of the system so pilots can recognize failures when they occur, and prevent the failure from spreading or restore power safely to circuits that are not damaged. As in all other aspects of flying, automation has made electrical system management less critical for pilots. For example, in most newer airplanes, high draw items will be "shed" automatically if a generator is lost. But thorough knowledge of the entire system is still the only way to get through the cascade of failures that will be thrown at you in the simulator, and possibly in real life.
Even with the most basic electrical system, the pilot needs to understand how it functions and where the critical failure items are located. For example, if the starter relay sticks and the starter remains energized after start, all of the power the alternator can produce along with battery power will flow into the starter. After several minutes — usually enough time to get into the air — the starter will fry and short out the entire electrical system, leaving the pilot totally in the dark. The way to recognize a stuck starter relay is to check for an abnormally high charge rate after start. So even for the most basic electrical system, there are important ways to prevent small problems from becoming crucial threats.
