Electrics: question for the experts

[henkie 0]

Hi, I have two questions regarding the CAB/UTIL switch on the overhead and the logic behind it. 1. When ground power is connected you can see the amperage difference on the AC/DC metering panel when switching the CAB/UTIL switch. In my case it draws 39A extra when the switch is on.However, when I am on APU-power there is no difference at all. The amperage stays on 28A regardless of the switch position. Is this suppose to work like this? 2. Regarding the FCOM, one of the components that are powered via the CAB/UTIL switch are the Logo Lights. However the logo lights stays on even with the switch to off. Again, I'm not sure about this. Maybe they are talking about other kind of logo lights? Thank you for reading! Daniel Verhaal by the way: Thank you PMDG for this great product! It is stunning!

[VIPERGTSR01 22]

Yes I have been wondering the same thing and posted about the logo lights (and other items) not being isolated when the CAB/UTIL switch is manually tripped off. I have no answer yet. *EDIT* Further checking: The in my current setup the APU has a draw of 50A (400hz, 115V) while for the exact same situation switched over the the Ground power the total draw is 89A (400hz 115V). However If I shut off the CAB/UTIL it makes the ground power draw of.... exactly 50A. Seems to me (without thorough proof) the CAB/UTIL isn't being drawn on the APU at all, although it still functions as it should (logo lights etc). I haven't checked it on the IDG's yet. It's something very minor but should be easy to fix up assuming we haven't got it wrong ourselves.

[henkie 0]

I just checked it with the IDG's. The CAB/UTIL bus draws 20A on both sides when the aircraft is powered by the IDG's. So that is about the same as the 39A when using ground power in my case. I'm very interested why there is no extra draw when the systems are powered by the APU. Are we overlooking something here?

[g_precentralis 191]

Maybe, just maybe Automatic Load Shedding cut it off due overload? But, there is no reason for overload because APU can deliver enough power. Lj. Prodanovic

[VIPERGTSR01 22]

I can't think of anything we overlooked, it seems to all add up now that the ground and IDG have been checked in comparison to the APU. The external AC powers the same busses (AC transfer) as the APU and even the IDG's so I can't see anything we missed. I also noticed something else: When the AC ground power is available but not switched on there is no battery draw (no other AC source either), but when I remove the cart using the CDU (remember I never actually switched on the cart) I get a 79A current draw from the DC battery in my current config. It seems just having the cart connected to the receptacle but not actually switched on still stops the battery from drawing power but nothing is functioning as if the cart was on until you do actually switch it on. However the APU doesn't act in this way.

Maybe, just maybe Automatic Load Shedding cut it off due overload? But, there is no reason for overload because APU can deliver enough power. Lj. Prodanovic

Thats a good point, like you say though it should be able to handle it anyway. Especially on the ground. Plus you can still see the logo lights on with APU.

[ajacied551 1]

The Logo light is controlled by the CAB/UTIL switch on the real aircraft, also it controls the electrical system's in the cabin such as the galley's and lavatory water heaters except for the Inflight Entertainment System and the cabin light's. So the logo light is not correctly simulated. There are more thing's that are not correctly simulated as it should work on the real Aircraft. As for the battery discharging with external power AVAIL you are right, But the Aircraft is simulated on the GROUND service bus. I think.

[henkie 0]

I also noticed something else: When the AC ground power is available but not switched on there is no battery draw (no other AC source either), but when I remove the cart using the CDU (remember I never actually switched on the cart) I get a 79A current draw from the DC battery in my current config. It seems just having the cart connected to the receptacle but not actually switched on still stops the battery from drawing power but nothing is functioning as if the cart was on until you do actually switch it on. However the APU doesn't act in this way.

Good spot Jay. I've got the same results. Regarding the FCOM also the recirculation fans should be connected via the CAB/UTIL bus which is not the case right now. I hope this thread will be spotted by one of the developers!

[lonewulf 1]

Just another thing with the Electrics that came to my attention:When the APU is available the blue "Generator Offline" (or whatever it reads) light is on, indicating that the generator would be available. However I can't see any voltage/frequency indication with APU selected on the rotary switch.. It only appears when the APU generator is engaged online. Compared to the engine generators this indication is different. They show the voltage/frequency even when still idling. Is that correct on the APU? Oskar

[ajacied551 1]

Just another thing with the Electrics that came to my attention:When the APU is available the blue "Generator Offline" (or whatever it reads) light is on, indicating that the generator would be available. However I can't see any voltage/frequency indication with APU selected on the rotary switch.. It only appears when the APU generator is engaged online. Compared to the engine generators this indication is different. They show the voltage/frequency even when still idling. Is that correct on the APU? Oskar

Hi Oskar The Blue APU GEN OFF BUS Light illuminates to show that the APU is running and is ready to load but the APU Generator is not in use as a power source. This light goes OFF when APU generator is the electrical power source or when the APU is shutdown. so Yes it's normal that the voltage / Frequency is not visible on the AC meter indicator, only when the APU generator is the power source feeding the AC busses it will show you the AC Voltage and Frequency. The Blue GEN OFF BUS light is always ON except when the Engine Generator is the power source feeding the AC Busses The blue light is also ON when the engine is OFF . After engine start you want to now if the quality is good of Engine Generator so after engine start, The AC meter show's the AC voltage and frequency when the engine generator is running.

[alvin 0]

Hey guys! I already reported this one with several other things I suspect might be things that are not perfectly simulated at this moment. Concerning GRD PWR connected, but not turned on, I studied FCOM quite a lot about this, and I explained the situation to myself this way: GRD PWR switch in OFF position cuts ground power from the main AC buses, but does not cut ground power from the charger unit (FCOM is ambiguous as it goes for that). The charger in turn powers Battery Buses, and Standy Buses - this is why the battery doesn't get drained, while you get some power. The GND PWR meter in turn measures only AC input, so this is why you see nothing there. One thing that kept me puzzled though was why the battery doesn't get charged when GND PWR is connected (and turned off). This is what RSR wrote me in response to my ticket In scenario #2 you have opened the battery bus tie breaker, thus removing it from the system. It will not accept a charge even thought the chargers are available.This logic is designed to protect the airplane from a battery overheat condition. Of other funny things I found: * TRs never show any DC drainage * Dome light DIM uses more DC power than Dome light OFF * It uses this power even when it doesn't give any light (battery power only) * wheel well light uses more energy in OFF position By the way, how great a simulator must be when people focus on such small things.. "hey! this amperage is 2amps too low!" :) cheers!Jakub

[Ty53650 1]

I'm curious on this as well... was under the impression that the electrics were modeled 100%

[lonewulf 1]

...The Blue APU GEN OFF BUS Light illuminates to show that the APU is running and is ready to load but the APU Generator is not in use as a power source. This light goes OFF when APU generator is the electrical power source or when the APU is shutdown. so Yes it's normal that the voltage / Frequency is not visible on the AC meter indicator, only when the APU generator is the power source feeding the AC busses it will show you the AC Voltage and Frequency...

Thank you Mark, for your explanation. However I will never be able to understand this kind of logic I can check the power output on the engine's generators before connecting but I can't do so on the APU. If I can't I don't need the indication when it is feedig already. i would rather check BEFORE connecting. Oh well ... every A/C has its' own logic and we have to admire it nevertheless Oskar

[Houghton11 12]

Ref: AMM 24-22-00 Task 24-22-00-862-002

8) Turn the AC meter selector switch on the panel P5 to APU GEN.(a) Make sure that the voltmeter shows 115 ± 5 volts ac.( Make sure the CPS FREQ meter shows: 1) UNLOADED 405-410 Hz, LOADED 397-417 Hz.(other types)2) UNLOADED 395-405 Hz, LOADED 395-405 Hz. SUBTASK 24-22-00-862-044(9) Put the APU GEN No. 1 switch on the panel P5 to ON.NOTE: This will supply power to the ac generator bus 1

Reading from this you should be able to see the APU voltage and cycles before you apply it to the AC BUS.

Input power to the battery charger is from 115-volt ac ground service bus or' date=' if this bus loses power,from the 115-volt ac main bus 2.[/quote'] The ground service bus is only energized I believe when the Ground Service switch on the F/A attendants panel is set to on, and is not functional when the normal Ground Power switch on the overhead is set to on...in which all the typical things the Ground Service bus runs (interior cabin lights...cleaning stuff) are now powered through their respective Main Bus'

[lonewulf 1]

... Reading from this you should be able to see the APU voltage and cycles before you apply it to the AC BUS...

That would make more sense to me. I take it that you are citing from an original maintenance manual. Oskar

[ajacied551 1]

Ref: AMM 24-22-00 Task 24-22-00-862-002 Reading from this you should be able to see the APU voltage and cycles before you apply it to the AC BUS.

Hi Patrick You are Looking in the AMM of B737-300/400/500 That is completely different Aircraft also The APU power plant is different, so don't compare the NG with the PG. The 737-600/700/800/900 AMM TASK 24-22-00-860-815. (supply APU generator Power).