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737 Electrical System Questions

Posted: Fri Jul 24, 2020 8:05 pm
by DanielBednar
Hey guys, a couple questions about 73 electrics:

1) If you look at the schematic, TR 3 has a diode located between it and its DC buses that are operated in parallel with the other TR’s. What’s the point of the diode there, and why do TR 1 & 2 not have one?

2) When operating under a single generator, the load shed begins with the galley and main buses on XFER bus 2, then galley and main buses on XFER bus 1, and if there’s still too much load, all IFE buses are shed. I’m wondering, why are the in-flight entertainment buses given higher priority than the main & galley buses? Why would Boeing rather keep IFE buses until the last possible moment?

3) Looking at some loads connected to the CAB/UTIL switch, there’s mention of FWD & AFT door-area heaters. What the heck are those, and why do the doors need to be heated?

4) Regrading the TR UNIT light: why does this light illuminate (in-flight) if TR 1 or TR 2 & TR 3 fail? Why not just have the light illuminate (in-flight) if any of them fail? Why is that specific arrangement required for the illumination of that light?

5) The DC system is 28v but the Ni-Cad batteries are 24v. Why the inconsistency between the voltage of the battery and the voltage of the overall DC system?

Thanks to all who answer.

Re: 737 Electrical System Questions

Posted: Sat Jul 25, 2020 2:17 am
by DL_Mech
DanielBednar wrote:
Hey guys, a couple questions about 73 electrics:

1) If you look at the schematic, TR 3 has a diode located between it and its DC buses that are operated in parallel with the other TR’s. What’s the point of the diode there, and why do TR 1 & 2 not have one?


Normal operation of the 737NG is with DC1 and DC2 busses in parallel through the bus tie relay R9. R9 is opened during autoland so that DC sources are separate and independent. DC1 is powered by TR1 normally and the main battery as the backup. DC2 is powered by TR2 normally and by TR3 through that diode as the backup. I’m not 100% sure of this as my manual doesn’t say squat about that diode.


DanielBednar wrote:
2) When operating under a single generator, the load shed begins with the galley and main buses on XFER bus 2, then galley and main buses on XFER bus 1, and if there’s still too much load, all IFE buses are shed. I’m wondering, why are the in-flight entertainment buses given higher priority than the main & galley buses? Why would Boeing rather keep IFE buses until the last possible moment?


Sorry, my manual is old and the IFE bus came much later.

DanielBednar wrote:

3) Looking at some loads connected to the CAB/UTIL switch, there’s mention of FWD & AFT door-area heaters. What the heck are those, and why do the doors need to be heated?


Door heaters are small electrical elements that provide warm air around the bottom of the doors (similar to a hair dryer without the fan). The areas around the doors (and overwing exits) can get very cold.

DanielBednar wrote:

4) Regrading the TR UNIT light: why does this light illuminate (in-flight) if TR 1 or TR 2 & TR 3 fail? Why not just have the light illuminate (in-flight) if any of them fail? Why is that specific arrangement required for the illumination of that light?


TR3 picks up when TR2 fails. Why bother the pilot if you don’t have to? Maintenance will know about it as a chapter 24 fail in the MCDU.

DanielBednar wrote:

5) The DC system is 28v but the Ni-Cad batteries are 24v. Why the inconsistency between the voltage of the battery and the voltage of the overall DC system?


I thought the batteries were 28v?

Re: 737 Electrical System Questions

Posted: Sat Jul 25, 2020 11:13 am
by Legs
Very basically, system voltage and therefore the charger voltage has to be at a higher potential than the batteries in order to get some charge into them. 24V battery on a 28V system is standard.

Re: 737 Electrical System Questions

Posted: Sat Jul 25, 2020 2:30 pm
by Dalmd88
DanielBednar wrote:

4) Regrading the TR UNIT light: why does this light illuminate (in-flight) if TR 1 or TR 2 & TR 3 fail? Why not just have the light illuminate (in-flight) if any of them fail? Why is that specific arrangement required for the illumination of that light?



TR3 picks up when TR2 fails. Why bother the pilot if you don’t have to? Maintenance will know about it as a chapter 24 fail in the MCDU.

It's been while since I did airframe work, but this rang a little bell in my memory. If TR2 or TR3 fail there is no light in flight, but doesn't the TRU Fail light on the ground for a failure of any of the three units. Don't bother the pilots in flight of the single 2 or 3 failure, but indicate there is an issue on the ground. I could be wrong, but it just seems like one of those things I recall from NG school.

Re: 737 Electrical System Questions

Posted: Sat Jul 25, 2020 6:50 pm
by DL_Mech
Dalmd88 wrote:

It's been while since I did airframe work, but this rang a little bell in my memory. If TR2 or TR3 fail there is no light in flight, but doesn't the TRU Fail light on the ground for a failure of any of the three units. Don't bother the pilots in flight of the single 2 or 3 failure, but indicate there is an issue on the ground. I could be wrong, but it just seems like one of those things I recall from NG school.


If you press and hold the six pack annunciator on the ground (six pack recall), it will illuminate the “elec” light on the P5-5 Generator Drive and Standby Power panel indicating a fault in the electrical system. You would get more information by interrogating chapter 24 in the MCDU.

Re: 737 Electrical System Questions

Posted: Sat Jul 25, 2020 8:49 pm
by Lpbri
TR3 normally powers the battery bus. Having a diode prevents the battery bus from charging the battery and allows only the battery charger to charge the battery

Re: 737 Electrical System Questions

Posted: Sun Jul 26, 2020 1:38 pm
by Dalmd88
DL_Mech wrote:
Dalmd88 wrote:

It's been while since I did airframe work, but this rang a little bell in my memory. If TR2 or TR3 fail there is no light in flight, but doesn't the TRU Fail light on the ground for a failure of any of the three units. Don't bother the pilots in flight of the single 2 or 3 failure, but indicate there is an issue on the ground. I could be wrong, but it just seems like one of those things I recall from NG school.


If you press and hold the six pack annunciator on the ground (six pack recall), it will illuminate the “elec” light on the P5-5 Generator Drive and Standby Power panel indicating a fault in the electrical system. You would get more information by interrogating chapter 24 in the MCDU.

Thanks for the memory jog. I knew there was some sort of on ground only indication of a fault. Like I said it's been ten years since I worked the flight deck on one of these. The memory tends to fade.