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Please begin with an informative title:

Time to look at another major aircraft system. This time it's the electrics. The trend in newer generation airliners is to power more systems by electrics rather than hydraulics.

This has some advantages in weight plus I would assume ease of maintenance. Hydraulic pumps and seals wear out and hydraulic lines can spring leaks. Hydraulic fluid turns to molasses in very cold temperatures. Electrically powered systems are less troublesome for the most part.

As we come to rely more and more on the electric system the more important it becomes. So let's dig in.


You must enter an Intro for your Diary Entry between 300 and 1150 characters long (that's approximately 50-175 words without any html or formatting markup).

First the basics. We actually have two electrical systems on an airliner AC and DC (no, not those guys). Alternating Current and Direct Current. Since I've forgotten almost everything I learned when I got my Electrical Engineering degree years ago I won't go into too much theory or I'll embarrass myself. Generally speaking, stuff with motors (like your washer and dryer) run on AC and electronics (like your iPhone) run on DC. The two systems are, like everything else on the jet, interconnected and have redundancy built in.

Let's start with what we call the "cold dark cockpit" scenario. Since we're in the freight business it's probably dark out and maybe the jet's probably been sitting on the ramp all day with no power.

So we climb up into the cockpit with our flashlight and start looking for a light switch.

Emergency Lighting
Emergency lighting is on what's usually called the "Hot Battery Bus". "Hot" because it's always powered. It's hot-wired to the battery.

Except for 787s, which have been in the news a lot lately, an aircraft battery is just a bigger version of the one in your car. It's a lead acid battery that puts out around 28 volts.

Now you may be wondering what I mean by the term "Bus". An electrical bus is just a bunch of electrical stuff on the same circuit. Let's say you have a power strip for your computer, monitor and printer. Turn the power strip off and you lose everything on the strip. An electrical bus works the same way.

Things that have to be powered all the time, like the clock, are on the hot battery bus. So the emergency lighting will work even if the battery switch is turned off. Leave them on long enough and you'll drain the battery.

Now that we can see what we're doing, we can bring the battery on line.

757 DC Control Panel
By turning the battery switch to on we connect the battery to the battery bus and the standby bus.

So what's on the standby bus? Important stuff. The guy who taught me 727 systems was from Mississippi and he pronounced it "bat-tree". He'd say "If you gotta have it, it's on the bat-tree". I can't tell you exactly what's on the standby bus but you mostly get one radio and a few basic flight instruments. If you're down to standby power in flight you're in a world of hurt.

So what will we see in the cockpit? Some lights should come on, some instruments should power up and the battery discharge (DISCH) light should come on. That tells us we're draining the battery. If you see that in flight you've got maybe 20 minutes of standby power left.

So to summarize the DC power system we've got:

Hot Battery Bus (Always on)
Battery Bus
Standby Bus (Stuff you gotta have)

In addition we have the main DC buses which don't get powered until later.

Left Main DC Bus (Captain's DC powered equipment)
Right Main DC Bus (First Officer's DC powered equipment)

Obviously we don't want to sit here all night draining the battery, so let's get some AC power on the jet. On the ground we run our APU, which has a generator, or we can get "external power" either from a generator cart or municipal power. The ground crew plugs a great big extension cord into the plane and we should get a light in the cockpit that tells us external power is available.

External Power
Now the jet is fussy about wanting "clean" electrical power. If the voltage isn't in the acceptable range or the frequency of the AC power isn't correct it won't close the circuit. This is to protect the electronics from damage.

If we have good clean power the circuit should close and our main electrical buses should be powered.

Aircraft on External Power
Once we get everything powered up we can run our pre-flight checks. Just prior to pushback we'll start our APU and switch from external power to the APU's generator. We don't want to drag the power cart along with us when we push back (it's happened).

The electrical panel will look exactly the same except the APU generator is now standing in for the ground power cart.

We have 3 total generators on (most) 757s. The APU has one, which is currently powering the plane and then each engine has one. The engine driven generators are connected to the engine by what's called a constant speed drive. It works like your car's transmission. As the engine RPM varies, it keeps the generator turning at the correct speed. A piece of electronics called a Generator Control monitors the "health" of the generator and will kick it off the circuit if it has a problem.

Back in the 727 days, we had to manually parallel the generators because they all fed into a common bus. That wasn't a great setup, so newer jets like the 757 just have each generator power its own bus. If a generator fails, the APU generator or the other engine-driven generator can pick up the load.

So once the engine-driven generators are on line this is what we'll see. The left generator (in blue) powering its respective buses and the right (in green) powering its buses. Most of the Captain's stuff is on the left bus and most of the First Officer's stuff is on the right bus. The utility buses power less essential things like the galleys, which we don't even have on a freighter.

757 Engines Running
The main DC buses are now also being powered by the generators. Remember how I said the systems were interconnected? We have things called Transformer/Rectifiers (TR) that convert 115 volt AC power to 28 volt DC power. We have a left and a right TR that power the left and right DC buses respectively. This also powers the battery charger that keeps the battery topped off.

It all sounds pretty overwhelming, but in basic terms everything runs on its own power source. If that fails, it tries to switch to a different power source.

It's pretty simple and pretty reliable. Much better than the 727 panel, which looked liked something out of Mystery Science Theater.

727 Electrical Panel
Look upon me and despair!
So what can go wrong with it?

1. Generator drops off line.

Usually a temporary glitch. Maybe the voltage spiked and the Generator Control kicked it off the circuit to protect the electronics. No big deal. Push the button to reset it. If it doesn't reset, you're done. Start the APU and let its generator pick up the load. If the APU isn't working, the one remaining generator will power most everything.

2. Generator overheats.

This is a little more serious. In this case we actually push a button to physically disconnect the generator from the engine so it stops spinning. This is a one-time good deal. It can only be reconnected on the ground.

3. Circuit breaker pops.

Each piece of equipment has a circuit breaker. They're a little round button that pops out when the breaker trips. You can try to reset it once. If it doesn't work, you've lost whatever was on that breaker. Note that we never reset any circuit breaker associated with the fuel system - things might get explody.

4. Bus fault.

It's possible for one of the buses to short out and then you lose everything on that bus. Important flight instruments can be switched over to the other bus. So the Captain's instruments on the left bus can be switched over to the right bus.

5. TR failure.

No big deal. We've got two. One can do the job. If both fail the battery charger can still power some of the DC buses.

6. Loss of all generators.

We're having a bad day. Or worse if it's at night. Assuming we can't get at least one back on line we're down to standby power and we've only got it for maybe 20 minutes. We need to find a place to land now.

This is pretty unlikely. I've heard of people losing all the generators but I don't know of any instance where they couldn't get at least one back on line.

7. Electrical fire.

This is pretty bad as well. We have to deal with smoke in the cockpit as well as trying to find the source of the problem. If the source is obvious, let's say one of the radios, we can pull the circuit breaker for that item. Most likely we're going to find a place to land quickly.

I had this happen to me in a KC-135. We smelled burning insulation in the cockpit. Fortunately we were practicing landings so we just landed. Usually your best bet.

Now there are some 757s that are specially modified for flying long oceanic flights. These are called ETOPS (Extended Two-engine Over-water Operations). They have a whole extra generator that can be driven off the plane's hydraulic system. We don't have any of these so I've only read about them. It's to give you that extra measure of redundancy when you're way out over the North Atlantic.

Airbus does it about the same way. The electrics on the A300 worked very similarly to the 757. The generators on the Airbus put out 90,000 watts each so I think we could have powered a small town with that.

That's the electrical system in a nutshell. It's more complicated than this but now you know what I see from the cockpit.

It's a pretty reliable system. In my 30 years of flying I can count the number of generator failures on one hand.

Extended (Optional)

Originally posted to Major Kong on Tue May 21, 2013 at 06:26 PM PDT.

Also republished by Aviation & Pilots, SciTech, Kossack Air Force, and Central Ohio Kossacks.

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