737NG External Power Application
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737NG External Power Application
I've been told that you don't need the Battery Switch to be ON on an NG to keep external power hooked up to the main busses on the 737NG (albeit without some of the Battery Busses being powered). Can you also apply external power without having the main battery selected ON?
If not, I'm having trouble understanding the Boeing Wiring Schematics (I suspect a diode has been missed from a diagram). Diagram 24-41-11 shows that the solenoid of the external power contactor can get power from the section of wiring between the external power socket and the contactor contacts A1, B1, C1 (The diagram shows power from this section going through a 3 phase CB "EXT PWR BPCU" to the BPCU, through a Transformer Rectifier and back out of the BPCU (pin 1 of D10898A) to pin 1 of the Ext Power Contactor and on to the solenoid, energising the contactor if the Ground Power Switch is switched on (and the external power source has been deemed to be OK)).
I see only transitting aircraft (APU kept running), so I would rarely have the opportunity to try this out for myself.
Thanks.
Regards.
Q.
If not, I'm having trouble understanding the Boeing Wiring Schematics (I suspect a diode has been missed from a diagram). Diagram 24-41-11 shows that the solenoid of the external power contactor can get power from the section of wiring between the external power socket and the contactor contacts A1, B1, C1 (The diagram shows power from this section going through a 3 phase CB "EXT PWR BPCU" to the BPCU, through a Transformer Rectifier and back out of the BPCU (pin 1 of D10898A) to pin 1 of the Ext Power Contactor and on to the solenoid, energising the contactor if the Ground Power Switch is switched on (and the external power source has been deemed to be OK)).
I see only transitting aircraft (APU kept running), so I would rarely have the opportunity to try this out for myself.
Thanks.
Regards.
Q.
Join Date: May 2002
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There are operator's options where the plane may be powered without the battery being turned on. I don't know of any US operators who have that option.
There is also an option where the APU will continue running after the generator is on line and the battery switch subsequently turned off. Once again I know of no US operators with that one either.
As to your original question, the aircraft I'm familiar with may have the battery switched off after external power is applied and many of the systems retain normal operating chacteristics. Some still require the battery switch to be turned on as you can see in the schematics.
If you turn off the battery and then remove external power it is my experience that the plane will accept external power and come to life without turning the battery on first. However things may not look quite right as some relays may change position while power is off and I promise you that you'll have faults latched in many components like the DEU's FMC's, and PSEU. Some of the displays can be very odd to say the least. One interesting fault is that you may not have any indication of right system hydraulic pressure. It's simply blank. This is because the BPCU will have a fault latched and the #2 DEU won't be operating. Until you start doing some system tests per the FIM the hydraulic indication is the only symptom. No Display Fault or anything, just a blank pressure display.
I've found that if you turn off external power with the battery switch on, then turn off the battery, our planes will not accept external power again unless the battery switch is returned on first.
Of course to apply power to the ground service bus you shouldn't need the battery on. (That doesn't seem to be set in stone either)
The correct procedure for applying external power on our aircraft is battery switch on first then external power. That way the power relays will be correctly positioned prior to AC coming on line.
If you turn on the battery first then the plane wakes up normally.
As to a diode missing in the diagram, probably not, unless your company has specified one of the options I mentioned above.
There is also an option where the APU will continue running after the generator is on line and the battery switch subsequently turned off. Once again I know of no US operators with that one either.
As to your original question, the aircraft I'm familiar with may have the battery switched off after external power is applied and many of the systems retain normal operating chacteristics. Some still require the battery switch to be turned on as you can see in the schematics.
If you turn off the battery and then remove external power it is my experience that the plane will accept external power and come to life without turning the battery on first. However things may not look quite right as some relays may change position while power is off and I promise you that you'll have faults latched in many components like the DEU's FMC's, and PSEU. Some of the displays can be very odd to say the least. One interesting fault is that you may not have any indication of right system hydraulic pressure. It's simply blank. This is because the BPCU will have a fault latched and the #2 DEU won't be operating. Until you start doing some system tests per the FIM the hydraulic indication is the only symptom. No Display Fault or anything, just a blank pressure display.
I've found that if you turn off external power with the battery switch on, then turn off the battery, our planes will not accept external power again unless the battery switch is returned on first.
Of course to apply power to the ground service bus you shouldn't need the battery on. (That doesn't seem to be set in stone either)
The correct procedure for applying external power on our aircraft is battery switch on first then external power. That way the power relays will be correctly positioned prior to AC coming on line.
If you turn on the battery first then the plane wakes up normally.
As to a diode missing in the diagram, probably not, unless your company has specified one of the options I mentioned above.
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If you notice, the power source for the External Power Contactor is Switcehd 28VDC power (CB named Bus Power Control Unit). That contactor cannot be closed until 28VDC is applied.
Avioniker was correct with everything that he said as well.
But the schematic is not wrong.
Avioniker was correct with everything that he said as well.
But the schematic is not wrong.
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"If you notice, the power source for the External Power Contactor is Switcehd 28VDC power (CB named Bus Power Control Unit). That contactor cannot be closed until 28VDC is applied."
My diagram also shows two other possible power sources for the contactor....
1. A rectified voltage from the hot side of the Ext Power Contactor (CB called EXT PWR BPCU) and...
2. The 28Vdc Bus 1 (CB called Bus Pwr Cont Unit).
The latter, of course, will not be available until power is on the main busses, but the first one seems to be available with the Ext Power plugged in (but not switched on).
The contactor control logic circuits, however, may need a particular power source(?)
Cheers.
Q.
My diagram also shows two other possible power sources for the contactor....
1. A rectified voltage from the hot side of the Ext Power Contactor (CB called EXT PWR BPCU) and...
2. The 28Vdc Bus 1 (CB called Bus Pwr Cont Unit).
The latter, of course, will not be available until power is on the main busses, but the first one seems to be available with the Ext Power plugged in (but not switched on).
The contactor control logic circuits, however, may need a particular power source(?)
Cheers.
Q.
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Be careful with the schematics.
They're operator specific.
Ground service bus doesn't require 28vdc unless the operator specified it in the purchase contract.
Normal aircraft power application does require 28vdc from the battery unless the operator specifically stated they wanted to be able to power the transfer busses without it.
They're operator specific.
Ground service bus doesn't require 28vdc unless the operator specified it in the purchase contract.
Normal aircraft power application does require 28vdc from the battery unless the operator specifically stated they wanted to be able to power the transfer busses without it.
