Can someone please remind me what happens to the Electrical Bus system on a 767 (real) during engine start when transferring from Ext Power (not APU) to Generator Power. Haven't done it for a while.i.e. If starting the Right Engine first, does the Right Engine Generator take over the Right side of the Bus only (Leaving Ext power on the Left side). And when starting the second engine (Left), does the Left Gen take over the left side of the Bus... with the the ON light in the EXT power switch extinguishing?Thanks.Cheers.Ian.P.S. My apologies to the person who answered this question a year or so ago (I've misplaced my notes).

Ian,I take it this would be done using an airstart unit to provide Engine Bleed Air, followed by X-BLEED start for second engine.

each engine has an IDG, so LEFT and RIGHT IDG's while on external power, the buses are directly powered through the bus tie.when engine generator comes alive, the RESPECTIVE IDG will take over that MAIN BUSLEFT IDG LEFT Main busRIGHT IDG RIGHT Main busso i guess you're right in saying that with the right engine running and an external power coupled up you'll have external power through the bus tie on the left bus and right IDG power on the right bus.Greetz(source: www.smartcockpit.com document 'electrical' page 7/7 show a clear schematic)

to add to this i've taken the scheme's in that file and added a bit of clarification of what I think is the way it happensthis first one is with only EXT PWR appliedhttp://forums.avsim.net/user_files/58812.jpgon second thought, i just read abit more into the file and noticed they say the EXT PWR can electrically disconnect the IDG's and that it has priority over all other sources, could it be then, that the IDG's remain disconnected though the engine is running?, but then again pressing the GEN ON button 'disconnects the previous source'so...

perhaps the following is a good scheme?http://forums.avsim.net/user_files/58821.jpg

Hi guys!Although I shouldn't be posting on this subject ;-), let me just post what I know (or at least what I think I know) and then you guys can come up with the answer :-)Anyway, according to my notes, if both Bus Tie Breakers are in the AUTO position, switching on external power will close both BTBs and both generators will stop supplying their respective busses. External power will power both Busses.Thus you cannot power one bus from external power and the other bus from an IDG (at least according to smartcockpit, as I believe in one of the quizzes this exact question is asked and the answer is false.)Or am I wrong here?Regards,Mark

it was either one or the other...i agree with you on this that the EXT PWR has priority nonetheless and that the activation of an IDG can only disconnect power from the APU or the other engine (through the Ties)

Regarding priority, this isn't 100% correct either.If the aircraft is being supplied by Ext Pwr, and you start an engine (or recycle a gen pushbutton), ext pwr will be switched off, both BTBs open and each gen supplied its own Bus (or, if only one gen is available, that one generator will supply both busses (for that both BTBs have to close))Mark

this should be it then:http://forums.avsim.net/user_files/58831.jpg

Thanks for your efforts, gentlemen!My notes say:"Upon engine start-up, the IDG has priority over external or APU generator power. The GCB REQUEST enabling signal causes the Bus Tie Breaker to trip open isolating the main ac load bus from the external or APU generator power."My notes don't say "Bus Tie Breakers" (plural), but it does say main ac load bus. Hence the confusion. It also says elsewhere in my notes that...."The External Power Contactor trips after both IDG's are brought on line and the corresponding GCB's are closed."Still not 100% sure what they are trying to say.Cheers.Ian.

Hey guyz!!! The power sources operate in a hierarchy of priority. If the BTB system set in auto (both switches flush with panel) and an engine running. The IDG has priority over all other sources. The Left main ac bus and the Right main ac bus will take power from these sources in the order as follows: Associated generator, opposite generator, ext pwr and apu gen. If starting an engine with external power only (assuming you have an air source) once the first is started the associated gen (left IDG) will power both buses if both bus tie switches are in auto. With both engines running power will be received from the associated generators. Anytime power is lost from the associated gen ac power is diverted from the opposite gen (IDG). NOTE: EXT PWR has priority over the apu gen.

"The Left main ac bus and the Right main ac bus will take power from these sources in the order as follows: Associated generator, opposite generator, ext pwr and apu gen. "Thanks guys, but I'm now at the point where I think I'd have to see it to believe it.I'd have to look at things on a case by case basis. What were the start preconditions? (APU only, Ext Only or APU AND Ext Power?). Are you talking about the latter? With no Ext Power, isn't the priority associated gen, APU, opposite gen? (Or is this only applicable in air mode?).Cheers.Ian.

From the B767 Maintenance Training ManualClose and Trip CommandsWhen the BUS TIE switches are in the latched (AUTO) position, the BTBs operate automatically to maintainpower to the main buses from any operatinggenerator, with this power source priority: - Asscociated IDG - APU Generator - Opposite IDGNote that external power (EP) has priority over all other power sources when EP is manually selected on the ground. The associated IDG has priority during engine start-up.

"The associated IDG has priority during engine start-up."Sorry, I don't understand what you're trying to say here. Associated with what?"From the B767 Maintenance Training ManualClose and Trip CommandsWhen the BUS TIE switches are in the latched (AUTO)position, the BTBs operate automatically to maintainpower to the main buses from any operatinggenerator, with this power source priority:- Asscociated IDG- APU Generator- Opposite IDGNote that external power (EP) has priority over all other power sources when EP is manually selected on the ground."I understand all of this, thanks (Our manuals are identical). They simply don't tell me (unequivocally) that a single engine will take over _both_ sides of the bus system where the previous power source for both sides of the bus was Ext Power.The statement about Associated IDG, APU Gen and Opposite Gen priority, refers, I believe, to one side of the bus only (You can have the different sides of the bus powered by different power sources). All I really need to know is can you have this arrangement with External Power and engines? (With the Bus Ties in Auto). If not, would it be possible to have split sources with a Bus Tie open? (e.g. Power the whole aircraft with Ext Power, then, say, open the Right Bus Tie (killing power to the Right Main Bus), then start the Right engine. Would external power go to the Left Main Bus and Right Engine Gen Power go to the Right Main Bus?). If you reset the Bus Tie to AUTO in this example would the single engine gen take over the Left and Right Main Busses?Thanks.Cheers.Ian.

Let me just answer this before it all turns to mush after being 737NG brainwashed :)Just speaking from experience here (and memory). When starting the engines on ground power, ground power will be disconnected when the first IDG comes online automatically. It is NEVER possible for the GPU to power one side of the electrical system while ANY other A/C generator powers the other side. The APU gen CAN be used in conjunction with an IDG.So when starting with APU electrical power, the APU gen powers the entire system. The first IDG that comes online during start, will only take over its own side of the electrical system. That should clear things up I think!Iz