Electrical Systems for interviews

[tcco94]

So I've been looking and trying to get a jump start on studying for my interviews...mainly this is for skywest.

In doing my research I've learned something and confused my brain a lot. I've read on various websites stating that the battery supplies all the electrical power and the alternator just charges the battery in the process. Also, the alternator amps are just a projection of how powerful the alternator is...but in fact, the alternator will only supply amps to the battery for charging and system that is needed at that exact moment. The alternator just has the ability to supply that many amps, if needed.

Then I read my company AOM and it clearly states (which is what I have to teach students) that the alternator supplies output and the alternator just charges the battery...as if the battery has no use other than to start the engine and supply electrical power in the event of an alternator failure?

So which one is it? The first one makes sense to me that the battery would supply the electrical power while the alternator supplies the battery juice to keep it fully charged and if the alternator fails the electrical load would be the same instead of jumping ship? Then I read someone state that the alternator holds the electrical load and when you switch something on to add to that load, it will start it's supply from the battery and then transition to the alternator? Maybe I've confused myself more in this process but so many links and articles are posting two completely different answers.

Also, I know interviewers like to ask about AC to DC and DC to AC....which I know about rectifiers and inverters ...but why? AC power is more useful to have than DC because it supplies the more precise amount of electrical load instead of DC always supplying a certain load that could strain the loads over time. In longer fuselage aircraft such as regional jets, narrow and wide bodies though, I read that DC is better? I'm not sure why but I would assume because it has a longer distance to travel, you wouldn't want AC traveling over all of that?

Last question, when drawing the system you explain the volts your alternator has and battery has (which alternator is more because it will charge the battery) but then why does it matter about stating the amps on the battery and alternator. Again, my AOM only states how many amps the alternator has but not the battery. The alternator would have more amps because it would need to be able to take the load of the entire system and charge the battery....correct?

I might be overthinking how difficult these questions are but I don't want to know the answer...I really want to understand the why so I can confidently explain and understand it.

My AOM's are for PA28 and PA44

 

[Roger Roger]

The battery does not supply diddly squat during normal ops. If the alternator is working the battery is a load not a supplier.

 

[Roger Roger]

Also the rest of that post made my head hurt. I don't know part 25 stuff very well but I consider my self pretty dang good at electrical on small aircraft so when I'm not at work I'll see if I can write up something to help set you straight.

 

[Cessnaflyer]

All I remember about the ATR-72 is the AC Wild it has.

 

[RedBaron2000]

An alternator has a certain amount of amps that it can put out continuously, and will do that forever, assuming it is still turning and working fine. A battery typically has an amp-hour capacity rating for one hour and also maybe for 30 minutes, meaning it should be able to provide that many amps for that amount of time. So, it may be rated to provide 17A for 30 minutes or 10A for one hour. That is its capacity rating and most batteries must be checked periodically to assure it can provide that (or at least 80-85%) to remain airworthy. That doesn't mean it can't put out more though. Aircraft batteries are starter batteries and are easily capable of putting out several hundred amps for starting. The larger the draw, the shorter the time it can supply that current. An alternator cannot put out that many amps even for a short period. Most will be limited to somewhere between 40-65A or so.

 

[Richman]

I found this video very helpful:

Richman

 

[Dan208B]

Are regional interviewers really asking this stuff? If you haven't flown an airplane with a DC and AC electrical system before are they expecting you to study up on it? I guess things have changed since I went through regionals but wow that seems overboard for interview questions.

 

[thevideographer]

Are regional interviewers really asking this stuff? If you haven't flown an airplane with a DC and AC electrical system before are they expecting you to study up on it? I guess things have changed since I went through regionals but wow that seems overboard for interview questions.

No, no they definitely aren't.

I did get the question "what is the difference between AC and DC power" on one but they didn't seem to care about the answer.

 

[Dan208B]

I might be overthinking how difficult these questions are

I think you are. Based on what @thevideographer said and what I've heard from friends going through interviews you should not focus that heavily on aircraft systems. Have a basic understanding of the systems of the airplane you fly and maybe be able to draw a basic diagram. After that put more time into regulations and HR questions/Tell me about a time questions.

 

[z987k]

I think you are. Based on what @thevideographer said and what I've heard from friends going through interviews you should not focus that heavily on aircraft systems. Have a basic understanding of the systems of the airplane you fly and maybe be able to draw a basic diagram. After that put more time into regulations and HR questions/Tell me about a time questions.

A basic understanding yes, but I don't think the op has any idea how electricity works based on his post. Which makes a basic understanding of an electrical system very hard.

 

[ComplexHiAv8r]

No, no they definitely aren't. I did get the question "what is the difference between AC and DC power" on one but they didn't seem to care about the answer.

SKW has you draw the electrical of last airplane, a turbine engines be, and describe CAT II runway lighting. Then a CRM event.

 

[tcco94]

A basic understanding yes, but I don't think the op has any idea how electricity works based on his post. Which makes a basic understanding of an electrical system very hard.

I've been doing my research up through some of the advanced systems books by Jeppesen about the basic understandings of electrical systems and then I start trying to read up on AC/DC systems and I get lost in translation through various links on the Internet. I was never taught about the alternator having all the electrical load but then CFIs out there post this stuff on the Internet just confusing me. I teach basic electrical system like alternator supplying the electrical load and charging the battery etc....but I have yet to have a checkride go beyond that.

These questions might seem out of this world for someone with no 121 experience but why is it on almost every interview gouge I read? I don't wanna just assume they just want the answer and no explanation and botch the interview. That's kind of why I'm here asking for experienced help. Systems (like many colleagues I have) is not my most powerful strength at all...not afraid to admit it.

What is a basic understanding? The drawings on YouTube/PHAK or the full blown drawings from a POH? Forgive me for asking a foolish question in your eyes but I'd rather ask than just assume and be wrong. Sorry dude.

 

[tcco94]

Also the rest of that post made my head hurt. I don't know part 25 stuff very well but I consider my self pretty dang good at electrical on small aircraft so when I'm not at work I'll see if I can write up something to help set you straight.

Thanks. Sorry my mind got running and started confusing me. I probably shouldn't have confused myself in the process either...

Figured here might help me figure out the weird questions I had to get more confident for the electrical systems part on the interview.

 

[CoffeeIcePapers]

So I've been looking and trying to get a jump start on studying for my interviews...mainly this is for skywest.

In doing my research I've learned something and confused my brain a lot. I've read on various websites stating that the battery supplies all the electrical power and the alternator just charges the battery in the process. Also, the alternator amps are just a projection of how powerful the alternator is...but in fact, the alternator will only supply amps to the battery for charging and system that is needed at that exact moment. The alternator just has the ability to supply that many amps, if needed.

Then I read my company AOM and it clearly states (which is what I have to teach students) that the alternator supplies output and the alternator just charges the battery...as if the battery has no use other than to start the engine and supply electrical power in the event of an alternator failure?

So which one is it? The first one makes sense to me that the battery would supply the electrical power while the alternator supplies the battery juice to keep it fully charged and if the alternator fails the electrical load would be the same instead of jumping ship? Then I read someone state that the alternator holds the electrical load and when you switch something on to add to that load, it will start it's supply from the battery and then transition to the alternator? Maybe I've confused myself more in this process but so many links and articles are posting two completely different answers.

Also, I know interviewers like to ask about AC to DC and DC to AC....which I know about rectifiers and inverters ...but why? AC power is more useful to have than DC because it supplies the more precise amount of electrical load instead of DC always supplying a certain load that could strain the loads over time. In longer fuselage aircraft such as regional jets, narrow and wide bodies though, I read that DC is better? I'm not sure why but I would assume because it has a longer distance to travel, you wouldn't want AC traveling over all of that?

Last question, when drawing the system you explain the volts your alternator has and battery has (which alternator is more because it will charge the battery) but then why does it matter about stating the amps on the battery and alternator. Again, my AOM only states how many amps the alternator has but not the battery. The alternator would have more amps because it would need to be able to take the load of the entire system and charge the battery....correct?

I might be overthinking how difficult these questions are but I don't want to know the answer...I really want to understand the why so I can confidently explain and understand it.

My AOM's are for PA28 and PA44

I will try to help you out. The battery is there to power stuff when the alternator isn't running in your basic Piper piston aircraft. The battery does offer some surge protection, but that isn't a primary function. Electrical components draw a certain amount of current, measured in amps. The generator on a Seminole is 70 amps, if I remember correctly. It is capable of supplying this much current, but generally, it will never get that high of a load. If you look at your circuit breakers, they have numbers on them. Those numbers represent the maximum amount of current those components can supply before blowing the breaker. So if you see a 5, you can assume under normal operation, that component should draw less than 5 amps of current.

AC or DC isn't inherently good or bad. Most larger aircraft use a combination of both for different components.

 

[inigo88]

I know people have already said "who cares, this won't be an interview question," but I happen to enjoy this stuff (engineering) so let's talk about it.

Here's a forum with a bunch of EE types debating the reasoning behind generating AC power and then rectifying it to DC on large transport category aircraft:
http://aviation.stackexchange.com/questions/3103/why-do-airplanes-use-ac-electric-power

- Check out the electrical system schematic for the Boeing 737-300/400 (source):

- And the electrical system schematic for the Airbus A320 (source):

Notice that the 737-3/400 Constant Speed Drives (CSDs, Gen 1 and Gen 2) and APU Generator supply 115 V 400 Hz 3-phase AC power to the electrical system. The A320 Integrated Drive Generators (IDGs, Gen 1 and Gen 2) and APU Gen also supply 115/200 V 400 Hz 3-phase AC power to its electrical system. Both aircraft use Transformer Rectifiers (TRs) along with a backup Inverter to convert the supplied 3-phase AC power to DC.

- Why might they do this?

While for one thing, the 737-3/400 has a ton more electrical components on the AC buses (Main, Gen, Transfer, TF) using several provided voltages (115 VAC, 28 VAC). Also notice that the AC systems tend to be large components like valves, fuel boost pumps and hydraulic pumps, while the TRs downstream mainly provide 28V DC to avionics systems.

Alternating current has (at least historically) been preferred in power transmission due to the ease in which you may step up or down voltage using transformers. You'll notice the lack of any special equipment between the 115 VAC and 28 VAC buses on the 737 classic, because there are simple transformers changing the voltage between them. The A320 schematic skips transformers entirely, and many of the destination components may require voltage step downs performed by transformers.

Or maybe a lot of these important components were simply more available (or cheaper to produce) in 115VAC, since that's already the standard in the US?

- What are some advantages of 3-phase AC over single phase, and why such high frequency generators (400 Hz vs 60 Hz residential)?
http://electrical-engineering-portal.com/single-phase-power-vs-three-phase-power
http://www.electrotechnik.net/2009/11/why-are-advantages-of-three-phase.html

There are some obvious advantages of 3-phase AC vs single-phase AC that would be applicable to an aircraft electrical system. They are:

1. Three phase alternators have a higher power/weight ratio than single phase. They deliver more power per a given size and weight, so they can be lighter than their single phase counterparts. (I imagine the higher generator frequency is also advantageous to this efficiency argument and cuts down on weight.)

2. A 3-phase AC system requires 25% less wiring to transmit the same amount of power over a given distance than a single phase system. This further cuts down on weight!

3. 3-phase AC motors are self-starting while single phase AC motors are not. This may be advantageous to some of the pumps and valves which run on AC power, along with the Integrated Drive Generators (IDGs) on newer aircraft which work double duty as both starter motors and AC generators.

A disadvantage of 3-phase AC is the greater complexity, and for AC in general, that frequency and phase must be synchronized between all AC generators in order to feed a common bus.

One need only look at the electrical panel on the Boeing 727 flight engineer's station and see three individual "FREQ" knobs where you literally have to tune the frequency of each AC generator before tieing the AC buses together to realize that this is indeed a complicated system. Fortunately for everyone (except @Adler) this process became automated on later aircraft.

Admittedly I'm not an Electrical Engineer so some of this might be bogus, but I figure it's a good start. I know somebody will argue that High Voltage DC transmission is a thing and that DC-DC converters can transform voltages as efficiently as AC transformers can... But I believe this relatively recent revival in DC is based on semiconductor technology, so given the historical perspective of the big aerospace companies designing the first jet airliners in the 1950s and 60s I believe my reasoning is sound.

Also, I know at least some of you do have EE degrees (but probably don't like to type a lot), so I hope you guys weigh in if you feel the need... And @Roger Roger, because your input is ALWAYS good.

 

[ppragman]

stuff

Quality content as usual, my friend. Keep it up!

Not an engineer, but I've always heard that AC is useful on large airplanes for the simple fact that that the wiring and generating equipment is lighter. From what I gather, power transmission is easier over longer distances, and what you get "bang for your buck" wise, is better with AC than DC for certain applications (things that like voltage like CRTs, and Radars, etc. come to mind). On light airplanes, DC is prevalent because of the engineering simplicity and negligent weight savings. Again, not an engineer, but this is what I've been told when I asked the question, "uhh why come not DC everywhere?"

 

[tcco94]

Let me make this easier....my OP was just confusing questions my head was sending me. Let me just show you where I am at and give me some help or advice from this onto a more complex electrical system knowledge that is expected on the interviews. All the comments so far have been great help so I really appreciate it. Maybe I was reading too many of those debate forums between EE that just confused me all together.

All of this information that I have in my notes is from the Jepp Advanced Electrical System which is what I have been using to get an upper hand knowledge on the basics...I guess...

Please correct me for what I'm wrong with or if I explain it weird and you have something easier ...even better...

This would be a basic 12-volt system for a GA airplane. It's a 6 cell battery that produces 2 volts per cell. This would be a 60 amp alternator (assuming around a 14 volt so that it could effectively charge the battery then) Starting from the top, you have the battery connecting to the Master Solenoid...giving it up to 400 amps. The solenoid is just a moveable-core electromagnet. The electromagnet is a magnet in which its magnetic field is produced by an electrical current, which would disappear once the field is turned off. To explain this function: the electrical current would flow from the positive terminal of the battery to the coil, which would extend the contactor and provide the amps to the master switch. The starter solenoid would operate the same way, except it gets the amps from the master solenoid and then puts its electrical current into the main busbar.

Busbar would be a simple way of terminating a lot of wires and is connected to the positive terminal of the battery (I assume from the starter solenoid...but not sure because the drawing from the book doesn't have a line straight from the positive terminal of the battery)....anyways, it's just a place that holds together the electrical current in a simple way. The avionics busbar has a switch that will send amps from the main busbar which powers all the avionics/radios.

The only other thing on this drawing that I have is the master switch and alternator switch are connected (which would have to fit a Cessna, not my Piper's that have two individual switches). The alternator just has a regulator that would regulate the amount of output the alternator is sending the system (preventing overloading), and you have circuit breakers are connected to the busbars that will prevent that certain equipment from getting too much electrical current or if it's not working properly it will pop the circuit breaker and prevent it from continuing to get current. A fuse would act in the same manner but it will melt the link and open the circuit...which I assume would not be recoverable like a circuit breaker that you just pop in.

Some other key terms I have in my notes that might be useful for questions that I'm not 100% on:
Relay is an electrically operated switch to control a circuit by a low-power signal or several circuits must be controlled by one circuit (not really sure where I would see this though).....Switches, relays, and solenoids stop or permit the flow of electrons.
AC (Alternating Current) is current that changes its direction and continually changes its values of voltage and current (I don't get how?)...with ease can get the most effective use of electrical energy
Insulator material of atoms that holds their outer electrons tight so they can't flow in response to an electromotive force (why do I need to remember this?)
Resistors increase the resistance of a circuit, decreasing the current flow....converting electrical energy into heat to drop voltage (kind of like a regulator???)
Capacitors stop DC flow and permit AC current flow...stores electrical load in an electric field (why does it stop DC flow?)
Inductors (coils) are use to pass DC and stop AC flow and to produce magnetic fields...such as in a solenoid (opposite of the capacitor for solenoids then?) and these also oppose a change in current by the generation of back voltage??
Diodes are a one-way "valve" that allows electrons to move in only one direction through the conductor (assuming that this makes AC flow into DC essentially because it can only flow one direction)
Transformer permits voltage of AC current to be increased or decreased (why?)
Rectifier is AC to DC
Inverter is DC to AC
Solenoid (sort of like a relay??) permits the flow of electrons...used when necessary to open/close a circuit carrying a large amount of electrical current to one remote location

Anyone can help me on this and comment on what I could add to my electrical drawing. I'm starting small because I need to move it up to a more complex drawing for the interview but is this drawing too much for the interview? Besides wanting to draw a good picture for the interview, I'd really like to start understanding this a lot better just for my knowledge. Sorry if anything sounds stupid!

 

[ppragman]

For questions like this, refer to this:

https://www.faa.gov/regulations_pol...raft/amt_airframe_handbook/media/ama_Ch09.pdf

 

[tcco94]

I feel like there is this awkward line that I keep feeling like I need to know all this when I really don't....

 

[Roger Roger]

Ok a few things to start off. First, some people never develop a real understanding of electricity. I don't know why, but some folks just have a mental block when it comes to understanding electricity. Second, it can be helpful to get hands on to really really understand because the difference in a schematic vs what that physically looks like can be a little mind blowing. Third, to understand DC you absolutely must grasp ohm's law in its various forms. It's the foundation of everything else.

Let's start with circuit breakers because everything prior to that is more or less correct. Circuit breakers are not there to protect the component or the generation system, they are to protect the wire going to the component in case the wire shorts to ground, and therefore circuit breaker size typically corresponds to wire size (for example a 20 gauge wire typically has a 5 amp breaker) though occasionally a smaller breaker will be specified by a component manufacturer. Wire that carries too much current has a tendency to get hot and catch on fire. Which for some reason is bad.

A fuse is simply replaced with new if it melts.

A relay or solenoid (same thing for all intents and purposes) is simply a method of using a small amount of current to control a large amount of current. This is handy because, for example, you don't want to have an electric-chair style switch on the panel turn your starter on and off. Relays and solenoids are used all over the place. For example, the Navajo uses a relay to turn the landing light off when the gear is up. This relay is controlled by the gear down limit switch, and you wouldn't want to run landing light size current through the gear down switch, either you'd smoke it or the switch would have to be impractically large. Another example, the Caravan uses several relays controlled by outputs from the SRL computer to activate different engine functions in the start cycle.

AC....I'm not up to typing a description of how AC voltage varies right now. Animations are better for that sort of thing anyway.

Insulators are, I suppose, important because they're everywhere in your electrical system. A couple basic examples are the insulation on your wiring and the material in your cannon plugs that keeps the pins separate from each other. Without insulators, electricity goes where it wants! And that's generally a bad thing.

Resistors can be anything! For example, a light bulb is a resistor, and so is your pitot heater, or electric windshield heater. Shoot, even all your wires are resisitors to a degree.

Capacitors and inductors I'm not going to dive into now either. But they're used all over the place for various purposes.

Diodes get used a few ways, probably the one you need to know about from a system design standpoint is isolating buses so that current can only come to or from the bus. Also of course light emitting diodes (LEDs) and in alternators for converting AC to DC.

Transforming AC voltage up and down is done simply because different components (or even different parts of different components) have different voltage requirements.

I'd say your drawing is great for an interview. Like I said, some people just get this stuff (I do) and some go years working on airplanes and still just don't get it. If I ever meet @inigo88 in person I've got some great stories for him about some of the stuff I've seen over the years.