Default 737 Electrical / Avionics systems

Hi all,

I've been getting to grips with the default 737-800 of late, and with it's many flaws. Before anyone tells me, I know it's a very broken aircraft!

I've modified it with Alejandro Rojas Lucena's brilliant virtual cockpit, got the taxi lights and landing lights working independently with Aaron Parson's excellent B737-800 fixer and installed the 26 gauge fixes by several authors but packaged by Stefano Cerutti.

Persisting with the default 737 has been fun and rewarding as I've begun to uncover how FSX aircraft, panels and gauges are built.

Most of you will know that the default 737 doesn't have a master avionics switch on it's 2d panels or anywhere in the virtual cockpit. This is an annoyance, but I can live with it. I might even try to code a 2D one in one of these days.

In testing what was dependent on this and the battery however, I uncovered many more problems with the 737. So I want to see if I can't remedy these and learn a bit more about xml gauge coding as I go.

However, I want to make sure these actually are discrepancies between the default model and real life 737 workings I could use the community's knowledge around the real life 737 to confirm these or correct my understanding.

1) With the Master Battery switch off, nothing should work. However the following do work: Flaps/Leading Edge Annunciator Indicators, Cross Feed Valve Indicator, Engine Reverse Thrust Indicators, Wipers, No Smoking Sign sound and Fasten Seatbelts sound. Could you confirm for me that none of these should be working whilst the battery is off.

2) When the Battery is turned on, the following turn on: F/D Course Read Out and the EICAS displays. These should only turn on when the Master Avionics Switch is turned on, is that basically right?

3) Can anyone enlighten me as to the electrical system that drives the taxi & landing lights? Is this a separate battery to the Master Battery? Should they only work when the engine generators are on? I'm a bit clueless here.

Thank you in advance for any knowledge you can share.

Trent

3) Can anyone enlighten me as to the electrical system that drives the taxi & landing lights? Is this a separate battery to the Master Battery? Should they only work when the engine generators are on?

Well, reading the xml code for the Landing and Taxi lights, in the Overhead Pop-up gauge from Mr. Cerutti, NO, the battery or any other electrical system's status, ie: On-Off, doesn't matter. They work regardless of any electrical system being on or off.

In fact, I could be wrong, but it doesn't look like any switch or annunciator on the Overhead is dependant on any electrical system's status.

The EICAS needs the "Electrical Panel" to be ON, not the Avionics switch. I believe that means, and I'll go double check, the battery, which feeds some other systems.

Essentially, what I'm saying is that who ever coded the various systems you have concerns about didn't make them dependent on any particular system's status.

Having said that, you may be able to improve the situation without getting into the xml coding.

From the SDK:

Bus Type

Specifies which bus in the electrical system the component is connected to, according to the following bus type codes:

Bus Type Bus

0 Main Bus (most components connected here)

1 Avionics Bus

2 Battery Bus

3 Hot Battery Bus (bypasses Master switch)

4 Generator/Alternator Bus 1 (function of engine 1)

5 Generator/Alternator Bus 2 (function of engine 2)

6 Generator/Alternator Bus 3 (function of engine 3)

7 Generator/Alternator Bus 4 (function of engine 4)

Look at the [Electrical] section of the plane's aircraft.cfg.

The one for the default 737-800 is pretty limited. Everything in the plane is on the default bus, or Bus 0. See above.

IF you want to make a system dependent on a particular bus being on or off, you can add a line to the [electrical] heading, like this:

[electrical]
max_battery_voltage = 24.0
generator_alternator_voltage = 30.0
max_generator_alternator_amps = 400.0
light_landing = 0, 5 , 17.0

The first character following the equals sign ( = ) is the Bus type. If you want the landing lights dependent on having the battery Bus active, change the 0 to a 2. I believe, although I may well be wrong, that if you do, the battery switch must be on to have the landing lights work.

Note that the "light_landing =" line above is added by me to show what I am speaking of. For an example.

You can do this for any system that is listed in the SDK. Look HERE. Check the Table of Contents, near the top of the page, but down a little bit (helpful, huh? :D ), for the Electrical systems. Click on it, and read down through the section to see what systems this will work on, the adjust their Bus Type to match what you want.

Otherwise, you'll need to contact the individual gauge makers to see if you can get them to change how the gauge is coded.

Good luck!

Pat☺

Pat, that is an absolutely brilliant and helpful response to my original questions.

I've since done a bit of reading and investigating of the online SDK information.

I've got some theories in my mind about how everything works together in FSX, but I won't explain them now as it'll just confuse everyone.

Rather, I'm intrigued by one particular variable for now - the A:CIRCUIT GENERAL PANEL ON variable.

Across a number of the default 737 gauges, their ability to function depends on whether the A:CIRCUIT GENERAL PANEL ON variable contains a value of 1 (is electrical power is being supplied to this circuit. 1 = yes, 0 = no).

It's a boolean variable, and the SDK indicates that it is non-settable. That would explain I suppose why I can't find a user driven KeyEvent that toggles this variable.

So my question is: How is this variable value determined? I presume it is dependent on another variable or system but I can't find any connecting information around this.

If anyone can shed any light on this whatsoever, I'd be very appreciative.

Trent

How is this variable value determined?

I'd like to double check with the FSDev forums, but I think (very rarely indeed!) that it's essentially a "Is there electricity supplied to any electrical circuit in general, in this plane, at the moment, (Y/N)" sort of event.

So, if the battery bus is energized, the engine generator bus, whatever, it will return 1 (True).

I think. I'm not certain, though.

Having said all that, you can use a different A:Var from the list shown in the SDK to specify a more specific (how's that for over use of specif... ? :D ) circuit that must be energized to make an gauge function. Something like (A:CIRCUIT AVIONICS ON, Bool).

If the .xml file for the gauge in question is in a .CAB file, you'll need to extract all the files in the .CAB file into a folder with the same name. Edit the .xml file in question to make a more specific A:Var, then save it off.

NOW you have two choices: 1) Leave all the .bmp's and .xml's from the .CAB file in the folder you created, and use them from there, OR 2) You can re-CAB them, so to speak.

If your choice is #1, boom, you're done.

If it's #2, you need to DL and install CABPACK.exe, which will accomplish this easily and quickly. I believe v1.4 is the latest, but again, I may well be wrong.

BTW, welcome to the wonderful, headache inducing, world of .XML programming :rolleyes: :p

Pat☺

Hi Pat,

Thanks again for your help on this matter. With more reading I have some further questions around all of this. I've asked the same questions below over at the FSDeveloper forums. Casting the net wide so to speak as I suspect the questions I'm asking are a bit tricky. I'm hoping anybody can help :).

I found a useful thread from some years ago here: https://www.fsdeveloper.com/forum/threads/of-master-switches.5376/

Contained in this thread is the theory that the battery and all generators power the main bus (bus 0), and therefore every circuit connected to the main bus.

By default all circuits are connected to this main bus.

I assume these are the 13 circuits mentioned in the SDK (https://msdn.microsoft.com/en-us/library/cc526981.aspx): General Panel, Flap Motor, Gear Motor, Autopilot, Avionics etc.

I have some further questions:

1) The SDK provides for 8 Bus Types (0 - 7). See https://msdn.microsoft.com/en-us/library/cc526949.aspx#mozTocId245287. If the Battery powers Bus 0, what powers Bus 1-7?

2) Is it possible to transfer a circuit from main bus 0 to another bus? How would you do that?

With my second question, I suspect the way to do this is to through the [Electrical] section of the aircraft.cfg. The SDK (https://msdn.microsoft.com/en-us/library/cc526949.aspx#mozTocId245287) provides 38 parameters that can be added to the [Electrical] section identifying the Bus for each.

3) Are each of these 38 parameters/systems hardwired in FSX to one of the 13 circuits above?

4) If 4 is true, is a schema or reference available to review this mapping?

I'd appreciate any help with the above. After wading through many threads, I realise the electrical systems in FSX leave a lot to be desired. Nonetheless, this is a learning experience for me and I'm enjoying it.

Thanks everyone,

Trent

Contained in this thread is the theory that the battery and all generators power the main bus (bus 0), and therefore every circuit connected to the main bus.

Yes, but: the battery (any plane only has 1 in FSX) and the Generators are parallel, not series. The only time the Main Bus has power is if 1) the battery still has a charge on it OR 2) a generator is operating. Actually, since they're parallel, it's more of an AND/OR situation. Batteries in FSX discharge unrealistically fast, BTW. The generators will recharge the battery as defined in the [Electrical] Section. That's why the line generator_alternator_voltage= is always higher than the line max_battery_voltage=, so the generator can charge the battery and run the systems connect to the bus(s) indicated. To run the systems connected to the Main Bus, the line max_generator_alternator_amps= needs to be higher than the sum of the loads on it, PLUS an undefined amount used to charge the battery. It's actually a decreasing curve, with the current needed to charge the battery tapering off as the battery "fills up".

If the current draw on the all the buses is higher than the generators provide, the battery will try to make up the difference, until it runs dry. Things like starters can put a huge drain on a Bus, but it's (hopefully) relatively shortlived. Once the engine fires off, whether Jet, Turbine, or Piston, it's generator, as defined in the engine_map= line, will then provide the power, running whatever circuits are defined and charging the battery back up. Of course, once the battery is fully charged, the generator's entire output is available for the various circuits to use.

1) The SDK provides for 8 Bus Types (0 - 7). See https://msdn.microsoft.com/en-us/lib...mozTocId245287. If the Battery powers Bus 0, what powers Bus 1-7?

So, you have a battery, AND a generator (or 2, or 4), feeding onto the Main Bus, and all other buses drawing from the Main Bus. You can provide switches to permit the other 7 buses to be controlled individually. You can then attach an individual circuit to an individual Bus, and the individual circuits may also be switched ON/OFF.

2) Is it possible to transfer a circuit from main bus 0 to another bus? How would you do that?

 

With my second question, I suspect the way to do this is to through the [Electrical] section of the aircraft.cfg. The SDK (https://msdn.microsoft.com/en-us/lib...mozTocId245287) provides 38 parameters that can be added to the [Electrical] section identifying the Bus for each.

You are correct. See my answer to your Question #1 :)

3) Are each of these 38 parameters/systems hardwired in FSX to one of the 13 circuits above?

By default, they all run off the Main Bus, but that may be altered by changing the first parameter following the equals sign. For example: autopilot = 0, 5 , 17.0 shows the autopilot run off the Main Bus. Change the 0 to a 5, thus: autopilot = 5, 5 , 17.0 and the autopilot won't have power available until Engine #2 is operational. This is just an example. I personally would run it off the Avionics Bus.

The main reason, in a real airplane, you want certain circuits individually capable of being switched off, or a group of similar circuits on a specific Bus, is in case you get an engine failure, forcing one engine's generator to power the entire plane. You want to get rid of superfluous loads, so as not to over-draw the remaining generator's capabilities. Things like Radar, Weapons Control (military planes, obviously),

Cabin Lights, and Logo Lights. Just for example. If you're running only on Engine 1 of 2, it may not provide enough juice to run IT'S circuits, AND those, so you switch them off the Bus quickly.

Generally, most planes with multiple engines will have a "Bus Tie" system, so if one engine crumps, the other picks up the load. Usually automatic, but not always.

4) If 3 is true, is a schema or reference available to review this mapping?

Yes, but strictly on a per-plane basis. Every plane will have different circuits on different buses, and different crosstie set-ups.

SO: A universal schematic? No. You can try Google-ing an individual plane's electrical schematic, though.

Does this all help any?

Pat☺

Hi Pat,

Thanks for taking the time to explain all of that. I've nearly got it all!

There is just one point of confusion I'm left with.

I understand there are 38 'systems'. Each of these systems can be tied to a Bus of my choosing through the Electrical section of the Aircraft.cfg file. I get that.

Isn't each of those 38 systems also mapped to a circuit though? I believe (though I could be wrong) that these are hard coded.

I would have thought it would make more sense to re-map a circuit to an individual Bus, rather than the individual components/systems of that circuit.

Because it appears to me now that it would be possible (maybe silly, but possible if you wanted to) to map two (of the 38 listed) different systems that are hard-coded on the same circuit, to two different Buses. That seems illogical to me, but full disclosure, I'm not an electrician.

So, by way of an example:

The Panel Lights and the Cabin Lights are 2 systems of the 38 that can be configured in the Electrical section of the aircraft.cfg.

Let's assume that Microsoft hard coded these to both belong on the General Panel circuit (one of 13 available circuits). We'll not know for sure without a documented schema, but let's just run with that assumption.

Now I have the ability through the Aircraft.cfg file to tie the Panel Lights to Bus 4 (I realise this would only work with Engine 1 operating). Now, two systems on the same circuit are mapped to 2 different buses. Panel Lights on Bus 4 and Cabin Lights on Bus 0.

Should that be possible?

I assume not. If so, I suppose it's up to me to know which systems are mapped to which circuits, and ensure that I transfer all systems of a circuit over to the new Bus should I choose to do so?

Hopefully I've just got this all wrong, and there is no problem having individual components of a circuit powered by different Buses but I don't think so...

Trent

Actually, you'd want Cabin Lights mapped to a switchable Bus, like Bus 4. That way, if Engine 1 dies for whatever reason, the Main Bus (Bus 0) has that much less current draw. Panel Lights are basically an "essential for flight" sort of thing, especially at night. The passengers can live with (separate) battery operated Emergency lights. No such thing in FSX, but in the real world...

I realize, that was just used for example purposes.

The reason they made the 38 circuits mappable to a specific bus is for "realism". Since "real" aircraft have specific circuits on specific buses, the programmers made that all available on FSX.

Most Devs leave most circuits set to default, or Bus 0. They aren't Electricians.

BUT: To make a plane's electrical systems more realistic, they made it so that different circuits can be assigned to different buses. That way a dev, seeking systems realism, can achieve it. At least as far as the FSX software permits.

Hopefully I've just got this all wrong, and there is no problem having individual components of a circuit powered by different Buses but I don't think so...

SO: There's no problem having different circuits assigned to different buses. If you want. You can't have bulb 1 of the Panel Lights on Bus 0, and bulb 2 on Bus 4. Both will be operated by Bus 0. Unless you assign the Panel Lights to Bus 4, or something. You get the idea.

Does this make things as clear as mud for you? :rolleyes: :confused: :D

By the way, just so you know, I was a Radar Tech my entire working life. All this stuff is Basic Electricity. Pretty simple stuff for a Radar Tech, really.

Pat☺

Thanks again Pat,

You've really helped me to understand this all far better.

In the final analysis, I think my error was assuming that the 38 circuits weren't actually circuits.

It sounds like the problem is the way these 38 circuits are grouped into 13 groups which the FSX internals describe as circuits.

Circuits within circuits. Is that a thing? I think this is where FSX is broken.

Another example to confirm my understanding:

Currently the PFD & MFD displays in my default 737-800 aircraft require power to be provided to the Avionics Circuit before they turn on. This is as per visibility sections of the pfd.xml and mfd.xml files that refer to (A:CIRCUIT AVIONICS ON, bool). That circuit is one of the 13 internal FSX variables that I can't change. Let's assume that FSX has it hard-coded that this circuit gets its power from Bus 1 (the Avionics Bus).

Now, if I remap the 2x Avionics circuits (avionics_bus & avionics) in the aircraft.cfg file to Bus 4, if the battery were on and engine 1 was disabled, then I would expect none of my avionics to work.

However, the avionics will work will because the battery is on, feeding Bus 1 which feeds the Avionics Circuit that the pfd.xml and mfd.xml files refer to.

So, despite my best efforts to remap the avionics circuit within the aircraft.cfg file, because FSX has hard-coded the 13 circuits to Buses my efforts are in vain.

I suppose the only recourse I have after remapping circuits in the aircraft.cfg file to my taste is to amend the pfd.xml and mfd.xml in order to change the variables it uses to determine whether power is being fed to that particular circuit. I imagine I would use one of the 38 customisable circuits that I defined in the aircraft.cfg file and not the hard-coded 13.

Does all that sound about right?

Thanks again Pat for all your help, I think I get it now!

I suppose the only recourse I have after remapping circuits in the aircraft.cfg file to my taste is to amend the pfd.xml and mfd.xml in order to change the variables it uses to determine whether power is being fed to that particular circuit. I imagine I would use one of the 38 customisable circuits that I defined in the aircraft.cfg file and not the hard-coded 13.

 

Does all that sound about right?

Pretty much, yes.

You need to think of the whole setup like a tree. You have some roots, say the Battery and Bus 0, that feed the various buses, think of them as stems, or small branches. No real main trunk, so maybe a bush is a better analogy. The Buses are the various stems coming up out of the roots, ie: they feed power to the various circuits, and the 38 circuits are leaves, growing off the stems. Only difference is that you can move them around. So you can have the Panel Lights leaf on whichever stem (Bus) you like.

The A:Vars in the SDK are there so that failures and switchology can be properly simulated. Don't confuse the A:Vars with the aircraft.cfg circuit listings. Not the same thing. The A:Vars are for doing things in the .xml files, like saying: For this device to work, you need to have power on Bus X, but if Bus X OR circuit Y fail, it will no longer function. Or you can have it set up so that if circuit Y fails, this PART of the device won't work any longer.

The aircraft.cfg entries are for assigning the various circuits, not devices, to specific buses, and telling to sim how much current they pull, and the minimum voltage they require to be active. Thus, you can simulate a Voltage and an Amperage gauge, so you can SEE what these parameters are doing. Can I have this curcuit AND that circuit operating at the same time? Nope, together they both pull more current than the Engine 1 Generator can provide, so they'll be draining the battery while they are both on, and when the battery drains far enough, they both will drop off-line. SO: I need to put them on different generators (buses), so Engine 1 (Bus 4) can operate one of them, and Engine 2 (Bus 5), the other. If Engine 2 fails, I'll have to shut one of them off (now steps in the A:Vars), or they both will die in a little while, when the battery drains. Presuming you have a proper cross-tie system in the plane for the electrics, as you should.

OR: I need to make sure one of the two will only operate for a short length of time, then shut it's self off (like a starter for Engine 2 that shuts down once the engine starts), so I don't drain the battery dead, and loose them both. That way I can run them both off Engine 1 Generator (Bus 4), and save Engine 2's Generator (Bus 5) for other circuits.

Keep at it! Electrics aren't really all that complex, they just LOOK that way. Once you really begin to understand them, all of a sudden you will look at them and think "Heck, this is EASY!" :cool:

Have fun!

Pat☺