>One expression I came across that I've never heard of prior to>the 737 is the "Switched Hot Battery Bus". My understanding of>HOT is that it is always live. The Switched Hot Battery Bus>seems to need the Battery to be ON? If it has relays, I assume>it's a plain ol' Battery Bus. Is there a distinction?>Main source for switched Hot B.B. is the TRU3 - powered by either AC transfer bus 1 - or AC transfer bus 2. In other word: TRU3 is always powered with any AC source connected. TRU3 supplies Stby DC bus 1 and Stby DC bus 2.And -FINALLY - Those two Standby DC buses are connected to the Switced Hot Battery Buses!>When you say there is only a CB between the unit and the>indicator, do you mean there are no Switched Hot Battery>relays involved? I have some diagrams in front of me for the>Standby Power Control Unit, but none for the Switched Hot and>the (real) Hot Battery Busses (24-61-11).Switched Hot Bat relays are located inside the Standby Power Contrl Unit. Guess the unit monitors the Switched Hot Bat Bus supply and kicks in the (real) Hot Bus if it fails.The 737NG manuals are split in a way we are not used to. Check Maint Manual part 1 for (a lousy) description. Part 2 covers only maintenance tasks. eker>

"Actually I'm trying to simulate a generator failure."Hi, Leo.I understand this, but what I wasn't sure about was whether the Bus Transfer Switch was in Auto....If you have a generator failure, with the Bus Transfer Switch in Auto, the bus ties close, allowing the other generator to power the stuff the previously undead generator was powering.However, depending on what electrical load you have the time, the single generator may not be able to handle the entire electrical system. In this case, certain parts of the electrical distribution system will be shut down. This is called loadshedding.There are three types of loadshedding (which results in different bit and pieces of the electrical system to shutdown), but for a single engine generator in flight, on our -800's, the following stuff (for lack of a better word) will be shut down in sequence (in response to a 274 amp load(nominal) for 300seconds... or a 340amp load for 5 seconds)....Galley Bus Load Shed Relays: All the stuff you expect to be in the galleys (ovens, coffee makers, water boilers, chillers (food coolers), wine coolers, etc).Main Bus Load Shed Relays: Cabin Aircon Recirc Fans, Logo Lights, Drain Mast Heaters, Lavatory water heaters, Hose(?) heaters.If this fails to reduce the electrical load, then the Bus Ties will trip, leaving you with half a dead airplane. It always seems to be a case of the straw breaking the camels back... so let's hope that flight attendant wasn't using his/her electric can opener at the time the generator died ;-)If the generator can't handle even half the airplane's elec system, then I believe it, too, trips off.... In this case, you would be left with only Stby Power and other Battery powered stuff."In both my Lufthansa and Boeing manuals there is a list of "inoperative components - loss of 1 generator". Since both lists are similar and the Boeing manual is not company specific I should think there must be some common ground here."Loadshedding depends on the actual load to which the single remaining generator is subjected. I can't tell you if your manuals are giving you a worst case scenario, where you have, say, all the ovens in the galleys operating, the electrical hydraulic pumps being used at the time, etc.... or whether this is an average expected scenario. Even within a fleet, some aircraft have greater loads than others (Some Inflight Entertainment Systems have different demands).The other types of loadshedding revolve around:1. The APU being the only source of power in flight2. The APU starter generator (APU parameters looked at.... e.g. high EGT)."But as I mentioned, there is also the thing with the annunciator panel lighting up with items that have no warning lights on. No matter how the electrical system is modeled, that can't be right. So I was curious if it's just me or of other people get the same."Sorry, I've been a little busy this week. I'll try to set this scenario tonight when I have a bit more free time ;-)Hope this helps.Cheers.Ian.

Thanks, Eker."The 737NG manuals are split in a way we are not used to. Check Maint Manual part 1 for (a lousy) description. Part 2 covers only maintenance tasks. "I think Boeing is going that way with all their newer aircraft. The 777 is like this also. Unforunately, some airlines use the "lousy description" as training manuals. However, I do find some of the information more easy to understand in this format. The step by step maintenance manual and the schematics can fill in some of the knowledge gaps, but it's a nuisance having to jump from one to the other. If they fail, there's always the AOMs and the real aircraft to play with ;-)Cheers.Ian.P.S. (Edit) I was observing the behaviour of the Standby Att Indic on a Classic, last night. All three flags dropped into view as soon as I switched off the battery (leaving a dead ship). Two disappeared almost immediately when I switched the Battery back on... The gyro flag took about a minute to disappear.... i.e. until the gyro came back up to speed.

Hi Ian,Sorry about not being specific enough about what I was trying to do. For the moment I was trying to keep it simple and so yes, I meant a generator failure with the bus transfer switch in auto.Of course, I'm no expert, but my understanding is this: in both my manuals there are electrical diagrams which are more or less the same. They show, among others, generator 1, and then first the generator bus 1 which connects to the main bus 1 and the 28V bus 1, and only after that we get to the transfer relay 1, below which is transfer bus 1 and a whole lot more. The way I read this is that if generator 1 quits and you haven't got the APU fired up yet to take over, everything between the generator and the transfer relay loses power, meaning the generator bus 1, the main bus 1 and the 28V bus 1. (Sorry for the un-technical way to describe what I see). In my Lufthansa manual there are diagrams for the CB-panels, indicating the power source for each CB. For instance, the forward fuel pump of tank 1 is powered by the main bus 1. The elec hyd pump for sys B is powered by the generator bus 1. So, from my reading of the electrical diagram I would predict that these are unpowered after generator 1 fails. And indeed, both are mentioned in both my manuals in the list "Inoperative components - loss of generator 1. Failure in flight, transfer busses normal".So, can you see why I would assume that there is a very well defined list of inoperative components? In my manuals loadshedding is mentioned only for the galleys. And indeed, again in both of the above-mentioned lists, for generator 1 and 2 it says that the galleys will be unpowered.Now, I know this is getting rather detailed, but like I said, a generator failure is one the failure options. I do not expect this to be implemented in the patch, but since failure training is mentioned as one of the attractions of the 800/900 upgrade I do think this whole issue will come up some day. I want to be well-prepared :-)Leo BakkerOops, Ian, almost forgot my new standard phrase: My manuals are for the 3/4/5. Any resemblance to actual NG aircraft is pure coincidence :-)

Here is what the Continental 737 Flight Manual says for the -700/800/900:"For single generator operation, the system is designed to shed electrical load incrementally based on actual load sensing. The galley busses A and B on transfer bus 2 are shed first; if an overload is still sensed, the galley busses C and D on transfer bus 1 are shed; if overload still exists, main bus 1 and main bus 2 will be shed. When configuration changes to more source capacity (two generator operation), automatic load restoration of the main busses and galley busses occurs; manual restoration of galley power can be attempted by reset of the Galley Power Switch to OFF, then back ON.In flight, if the APU is the only source of electrical power, all galley busses are automatically shed. If electrical load still exceeds design limits, both main busses are also automatically shed. On the ground, the APU attempts to carry a full electrical load. If an overload condition is sensed, the APU sheds galley busses first and then main busses until the load is within limits. Manual restoration of galley power can be attempted by moving the Galley Power Switch to OFF, then back ON."Lee Hetherington (KBED)

"in both my manuals there are electrical diagrams which are more or less the same. They show, among others, generator 1, and then first the generator bus 1... "Hmmm... Lost me already, Leo.The #1 Generator is connected, firstly, to the "115Vac XFR Bus 1" via a relay (Gen Breaker). On the 737's, these are the core busses, unlike a lot of other Boeings, which use the terminology "main" bus. The #1 and #2 115ac XFR Busses are connected to each other via two Bus Tie Breakers in series (These are controlled by the Auto switch). ".... which connects to the main bus 1 and the 28V bus 1, and only after that we get to the transfer relay 1, below which is transfer bus 1 and a whole lot more."I think I see where you are going wrong. You are assuming that the Auto switch is controlling relays further down the "food chain". This is most likely not the case... at least not directly.In your diagrams, are the busses connected directly to the Gen's shown larger than the others? In the Boeing MM, this is something which helps you to identify the most important AC busses. These two busses, as I say, should be connected to each other via BTB's (two inline circuit breakers). Perhaps if you could send me a copy of your diagram, I could highlight the path taken by the electrons, allowing the #2 gen to feed the #1 busses... and vice versa.... and send it back to you. I will destroy my copy if copyrights are an issue. I think I already know more than what they are telling me, so there should be no worries about company/Boeing secrets being distributed on the internet ;-) " The way I read this is that if generator 1 quits and you haven't got the APU fired up yet to take over, everything between the generator and the transfer relay loses power, meaning the generator bus 1, the main bus 1 and the 28V bus 1. (Sorry for the un-technical way to describe what I see). "I think you are looking at some relatively minor transfer relays here, perhaps controlled by other means, automatically."In my Lufthansa manual there are diagrams for the CB-panels, indicating the power source for each CB." Unfortunately, so far, I haven't been able to find such a handy list. I've had to follow a paper trail of extremely detailed wiring diagrams. It takes me about 15 minutes just to find where one system is getting its power from 8-)Anyway, I think you'll find that there is nothing wrong with the basics of the PMDG electrical system. All the 737 types seem to have roughly the same layout ;-)Cheers.Ian.

Hi guys,I really appreciate you taking the time to enlighten me on the subject. And I think it has helped me actually see the light :-)From your descriptions I think the reason why we have a hard time understanding each other is quite simple: I'm looking at a different diagram than you guys and I think that means the NG has a more sophisticated electrical system than the classics in my manuals. I'm not so technically advanced that I can send you a copy, Ian, but I really am convinced I'm reading the diagram right: the generator first leads to what is called a generator bus and only THEN to THE transfer relais that is controlled by the transfer bus switch. There is no relais between the two generator busses. So in my case there is no load shedding based on actual load, those things that are powered by the generator bus lose power with one generator off, and those things that are "downstream" of the transfer relais will be powered by the other generator.From what you guys tell me, I now understand that in the NG there is no part in the system that is linked ONLY to its own generator, since the "first" bus is already a XFR bus. Hence the need for loadshedding if the one generator is unable to cope with all the demand. I assume that the first stages of load shedding would probably involve the same things as are connected to the generator bus in "my" Boeings: galleys you already mentioned, one fuel pump, some window heat, one elec hyd pump and some exterior lighting.Unfortunately, what all this tells me is that my handy Lufthansa CB list isn't handy at all when you're trying to find your way around the NG circuitry :-(Thanks!Leo Bakker

Hi, again, Leo...I found the following diagrams at one of the 737 websites which shows a 737 Classic elec bus schematic, complete with "Gen" Busses.It may or may not be like the one in the LH manual(???)http://www.b737.org.uk/schemeelectxt.gifI'm not 100% familiar with the transfer switch operation, but I've taken a guess at how it would operate with the #1 generator offline.http://members.ozemail.com.au/~b744er/737/OneGen.gifThe added red lines show how the power gets to both Gen busses and XFR Busses.Hope this helps.Cheers.Ian.

Hi Ian,Well, that's a lot more colourful than the things you usually get in the Boeing manuals. Very nice, thanks a lot for digging that up! I think I'm getting the idea of what you're saying now :-)Inspired by your example I did some digging myself as well and found the electrical diagram for both the Classic and the NG at the following site:www.smartcockpit.com/b737.htmlThere are a number of systems covered by that site. Especially helpful for guys like me who do have more information about the Classic than the NG is that the differences between the two are mentioned as well.Leo Bakker

Here a little one for the NG series ;)http://forums.avsim.com/user_files/26343.jpg[h5]Best Wishes,Randy J. Smith[/h5][h6]The Next Great one? PMDG's 737NG is here![/h6][h3]" A little learning is a dangerous thing"[/h3] [h5]AMD XP 2200 |MUNCHKIN 512 DDR RAM |ECS[/b ][i] K7S5A MB[/i] |GF3 64 MEG @ 215/545|WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"[/h5]

Interesting use of colour there, Randy :-)The Red looks really dangerous.... but, in reality, a lot less dangerous than the blue stuff :(Note that the Ext Power configuration is shown. Both BTB's are closed allowing Ext Power to power both AC Transfer Busses. Normally, with two engines running, both Bus Ties will be open (with the Bus Transfer Switch (On the overhead panel) in AUTO). LIke the 767, the 737 electrical system is designed to be (normally) split into two halves.Thanks.Cheers.Ian.

>"Had to consult the system schematic for this one:">>Only one, Eker? :-)Dont we all have these books? oh maybe it is only us who are tech and trainned on the aircraft hehe (min eis on the shelf next to girlfriends novels hehe)>One expression I came across that I've never heard of prior to>the 737 is the "Switched Hot Battery Bus". My understanding of>HOT is that it is always live. The Switched Hot Battery Bus>seems to need the Battery to be ON? If it has relays, I assume>it's a plain ol' Battery Bus. Is there a distinction?hot bat bus is an allways alive bus, but only when some sort of power is supplied to it, that is normally from the battery, but allso TR can help you do this, so no matter what you should have power on it, but to get it from a black aircraft, it arrives at Batt on (same on A319-321 that im allso licenced on)>Wasn't sure if the -600/-700 series had MMR's, but realised>that they'd probably be needed for the TERRain System? (GPS>signals)The whole NG series have that as standard, but you can choose not to get it like this with the gps capable MMR, then it is like the A319-321 basic MMR that dont have the GPS in it.>You have our sympathies.... You're missing out on all the fun.>This post-release beta phase brings out the best in all us>"closet" bug bounty hunters :)hehe Real Aircrafts have bugs to hehe, in the airline i work for we had to disconnect some stby instruments because we found out that they could send data back through the ARINC busses and ake a huge amount of trouble, now we got new ones and it is all good again (we hope hehe)>>"Anyway - what really matters: The 767PIC is still running ->reliable as the old P&W JTD8 fuel to noise converters."JT8D nice engine, untill the day a teacher tell you to learn everything inside of its fuel control, talk about to many parts to do such a little job heheDennis

Hi guys,I also have an electrical question.When i start the flight at the gate, engines are running etc. So I start the APU select the APU buses, and shut dowun the engine. Shut off all fuel pumps except L?H I/B for the APU. Not it is in the A/C is in the state it would be at the gate before the pilot gets to the A/c. But after about 5 mins the ship goes dead, like its run out of power from the batts. Also cannot get a/c to stay powered on GND power for more than 5 mins either.Am AV TECH and work on 700s and 800s so i konw what i am doing. anyone else have this prob?

"But after about 5 mins the ship goes dead, like its run out of power from the batts."This is a basic MSFS programming problem, Mr...er.. Kiwi. By the way, this forum insists that you sign your messages with a "regular" name (otherwise the forum may not respond). The only workaround is to install the FSUIPC program. For FS2002, the program is free. For FS2004, to get all of FSUIPC's functions operating, you will have to pay extra $$$. FSUIPC will give you as much battery time as you want. The FSUIPC program should be relatively easy to find... basically everyone who has an add-on program (like this) uses it.Hope this helps.Cheers.Ian.