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Guest bruchpilot

Electrical system question

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Guest bruchpilot

I don't see why it should be difficult to model load shed. Apart from the sensor issue you described, it happens any time less than two IDGs power the busses.Yesterday I posted that with PMDG loadshedding does not occur when only one IDG is powering the busses.Well, forget my post: I was testing it while still on the ground. It should happen only while in "AIR MODE". I don't have the time right now to check it out and see, if AIR MODE is modelled.Seems like I have to do a test flight today. (Or is anybody here who can do it in the meantime? Start with APU running and powering the busses. If it changes to the IDGs when you push the throttles: Lucky us: AIR MODE modelled :-))GreetingsEd

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Not something you see everyday but I doubt the light coming on is a sim fault. Did some of the recirc fans stop? Try resetting the R utility bus and see what happens.CheersSteve


Cheers

Steve Hall

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Guest bruchpilot

Resetting the R util bus didn't do anything. I also read that the recirc fans are powered by the util bus. Where would I see they fail? No EICAS indications. But I think that's how it is in reality as well.

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"I don't see why it should be difficult to model load shed."So you happen to know what each airplane system draws in terms of current?... I don't. It's not in our manuals.Consider the number of systems on the 744, each with dozens of individual components... pumps, fans, lights, heaters, motors, computers.... Are these components active? Have they been automatically or manually switched off? Would, say a pump be under load (driving flaps) or just idling?You would have to figure out how much you would gain by the progressive depowering of Untility and Galley loads (the load sheds systematically .. i.e. in a specific order).If a main bus is isolated, its utility and galley busses would have to be factored out of the loadshed sequence.To make things more difficult, loadshed current values are not proportional to the number of generators operating."Yesterday I posted that with PMDG loadshedding does not occur when only one IDG is powering the busses."Well there should be loadshedding.. in the air and on the ground with one generator operating (and lots of systems drawing current). In the air, I read in my training notes, loadshedding is instantaneous. On the ground, there are varying time delays for specific load values.On the 767, there are loadsheds for particular configurations.. E.g. If four hydraulic demand pumps are turned on, C2 pumps will be switched off (irrespective of generator load). It also has current-sensing type loadshed. As far as I can remember, the 744 does not have specific configuration loadsheds. It is all current-based. This is why the DDG says, for operating the aircraft with limited power available, you _may_ see such and such messages on your EICAS screens.Hope this makes sense.Cheers.Q>

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Guest bruchpilot

Q,didn't mean that it is possible to simulate it in every way... but the basic model of load shed if only one IDG is operating? Why not?Ed

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The same current analyses would still have to be made for each item in each airplane system. On one gen, you may not get loadshed (other than one specific item I can think of) if you have all your high current devices switched off (demand pumps, fans, fuel pumps, etc). Would you get loadshed if you intentionally switched off power to one or two busses? Probably not.On one Ext power source or APU gen, we can power an aircraft without loadshed if we switch off fuel pumps and hydraulic pumps (although it is risky putting so much current through one plug). Will switching on 1, 2, 3, 4.... fuel pumps trigger loadshed in this config? Will that pax in seat 52C switching on his reading light trigger loadshed? Who knows.Cheers.Ian R>

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The recirc fans would appear as a status message. Have you cleared all the faults in your failures settings?Steve


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Steve Hall

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That link you gave us certainly provided some interesting reading, Ed (not sure what happened to the link, however). As with all information on the internet, it may not be up to date, accurate, etc (unfortunately, the same applies to some of my manuals :( ) The link talked about "-6/" and "-7" BCU's... and our aircraft are now at "-8". However, at least one of the things that was supposedly fixed with -6/-7 is still evident on the real aircraft! (so I'm not sure if the author left Boeing before the -6's and -7's were released).Some of my manuals don't mention single gen loadshedding at all, but I started reading one airline's maintenance manual, and it does mention it. AssUuming this type of loadshedding is on all 744's, we may have something to work with here (although the Freighter may be a little different). Note, however, I'm trying to figure out if my understanding of what happens when the throttles are advanced fits in with what is said on your link (it's still a little confusing).Anyway, the article is interesting reading (even though some of it is giving me a major headache :( ).Thanks.Cheers.Q>

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Guest D17S

Pulled up a relatively unambiguous 767 description/operation today. The BPCU (buss power control units) monitors the load supplied by any source (yes, source) and kicks off the utility buss relays (deenergizes the UBRs and any slaved galley buss relays). This shedding event will occur if a load of 90Kva occurs for 4.5 minutes or 112.5 Kva occurs for 4 seconds. There is no shedding of individual components. Anything that is unlucky enough to be powered by the utility or galley power buss for the affected main AC buss will shut down . . . like some fuel pumps. The utility buss switch light illuminates. The buss can be reset by cycling the switch. The GCU (Generator control unit) also monitors source loads will trip the associated BTB (buss tie breaker) under this overload circumstance. This turns on the buss tie switch light and can be reset by cycling the switch. Shedding based on throttle position is only related to single generator operation (in the 767). If only one generator is operating, all utility and galley busses are shed if: 1) That single IDG is used to power all the main AC busses or, 2) Thrust levers are advanced on the ground or3) The airplane is in flight. If a second source comes back, all busses are reset. Again, this is a 2 engine 767. But the basic philosophy has got to be about the same.

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Sam, the 767 has lots of different types of loadshedding. Generator Loss Loadshedding, pre-engine start loadshedding (ground only), Engine Start (with APU as source) loadshedding... and Fuel Jettison loadshedding. Some loadsheds are current AND aircraft configuration-based. Others are not. It might not be helpful comparing the two aircraft.Cheers.Q>

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Guest D17S

For a 744 with 1 or 2 remaining sources (possibly caused by the loss of 2 or 3 generators or a pre-engine start / engine start condition where only 1 APU generator is available), the utility/galley busses will not auto-shed? On a 744, one APU generator can power everything, including a pre-flight dinner special for 500 hungry (and cramped) pax? Not with out smoke, I think. Sounds like these 767 functions are providing a very helpful comparator to, actually, any modern aircraft electrical system . . . including a 744. It makes perfect sense that an electrical system would dump loads based on available sources. The 767 does this and so will any modern aircraft electrical system. My observation is describing a 767, but providing basic descriptions of any modern aircraft electrical system will provide some understanding about how a 744 works. It's a good start toward recognizing basic functions . . . and could certainly highlight similar philosophies between a 2 v 4 engine Boeing airplane. Load shed design functions are common and relatively simple. However on the non-simple side, there's some serious teamwork going on between the BPCU and the GCU as they work toward isolation of power QUALITY problems. Certainly power quality can cause overloads to occur. However in these cases, the load shed event is responding to the overload's effect by tripping UBRs and GCRs, not the overload's cause. If the load shedding the utility and galley buss does not resolve the power QUALITY problem, the BPCU and the GCU will continue an attempt to isolate the problem, but this is not load shedding. It is isolation. For the over load "symptom" of this electrical drama, the utility and galley busses trip. That's it. This is really just describing a basic for design. At that basic level, I strongly suspect the 767's philosophy carried thru for the 744. Why wouldn

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Guest bruchpilot

Speculation, Speculation. What would really help now is a developer of the Sim who could give us the answer to all our questions...Hello? Are you out there?Ed

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