Airliners.net

I am looking at the 737-400, but I guess that 737NG are the same in this respect (?)

I'd firstly like to ask what the functions of the various positions of the DC knob on the O/H panel actually are (STBY PWR; BAT BUS; BAT; TR1; TR2; TR3 & TEST). When would each of those positions be selected by the pilots?

What are 'normal operations' for DC power? Is the knob left on Standby, or is it switched to BAT when the a/c is powered (that is, with APU running? Or only with engines running and generators on?)? And when engines are running, to TR1, 2 or 3?

Finally, is there a different procedure for DC power if power for the aircraft is being obtained from a GPU, and if the APU is running? I tend (I am not a real-world pilot I should add! Probably very obvious!) to run my initial cockpit setup powered from the GPU and start the APU a little later (saves a bit of virtual fuel!)...

Hope the questions make sense..

Thanks.

Martin

... I didn't phrase some of the last part quite correctly. Given that Standby DC power comes via either the GPU or the APU, under what circumstances would the selector be moved to other positions (like BAT)?

My best recollection is is that the switch positions select what the meter above those 2 switches displays. DC power selections on the left switch and AC power selections on the right switch. Kinda hard to follow, but should help answer your questions. https://www.youtube.com/watch?v=En87CjqiXzk

Found this overview that looks to be even better.
https://www.youtube.com/watch?v=n7lB_QryM74

Martinlest wrote:

I am looking at the 737-400, but I guess that 737NG are the same in this respect (?)

I'd firstly like to ask what the functions of the various positions of the DC knob on the O/H panel actually are (STBY PWR; BAT BUS; BAT; TR1; TR2; TR3 & TEST). When would each of those positions be selected by the pilots?

What are 'normal operations' for DC power? Is the knob left on Standby, or is it switched to BAT when the a/c is powered (that is, with APU running? Or only with engines running and generators on?)? And when engines are running, to TR1, 2 or 3?

Finally, is there a different procedure for DC power if power for the aircraft is being obtained from a GPU, and if the APU is running? I tend (I am not a real-world pilot I should add! Probably very obvious!) to run my initial cockpit setup powered from the GPU and start the APU a little later (saves a bit of virtual fuel!)...

Hope the questions make sense..

Thanks.

Martin

It has been many years since I last stepped into a B737. If I understand you correctly, the DC knob mentioned above is the voltage selector knob to indicate the DC voltages of the different DC buses -- Standby Power Bus, Battery Bus and so on. Moving the knob to different positions does not alter anything on the airplane itself, the selector just picks up the DC voltage from the selected bus and indicates the voltage on the display above the knob.
The same goes for the AC selector knob next to the DC selector knob -- it is the selector to check the voltages on the different AC buses.
Generally speaking, there is not much necessity to monitor DC & AC voltages, unless some DC or AC buses drop off, due to failures or other reasons. Monitoring the DC voltage is very important, however, when starting the APU using the airplane battery alone (without support from a GPU or engine generators). The 28V DC from the battery could drop to pretty low values when the APU starter is cranking up.
I will try to hop into a B737 in the next day or two to take a closer look at the overhead panel.

Moving the knob to different positions does not alter anything on the airplane itself, the selector just picks up the DC voltage from the selected bus and indicates the voltage on the display above the knob.

That rings a very loud bell in my head (and I feel a bit daft for having asked the question now!!). Somehow, over the past couple of years I have forgotten this and started assuming that the knobs actually control the source: but no, I am sure you are correct, they just indicate voltages in the different systems.

I am still unsure what the systems actually are however (apart from obvious ones like BAT, GRD PWR or APU of course). TR1, 2 and 3 are...? And BAT BUS? (I remember the Bat Car from my childhood TV days, but the Bat Bus??)...

Thanks for the video link - I have been ploughing through pdf manuals on this same subject since I posted my question.

Martin

TR usually means Transformer Rectifier which converts ac power to dc power.

Everything that is electrical is connected to a bus. It’s similar to fluids (liquids) being connected to a header or (gases) being connected to a manifold.

OK... I'll watch that video more fully when I have a moment (half an hour or whatever it is)... Maybe the other questions I had will be answered there. Thanks.

Martinlest wrote:

I am still unsure what the systems actually are however (apart from obvious ones like BAT, GRD PWR or APU of course). TR1, 2 and 3 are...? And BAT BUS? (I remember the Bat Car from my childhood TV days, but the Bat Bus??)...

The way the electrical buses are named can be pretty confusing. Different manufacturers have different ways of naming the electrical buses, just like for other aircraft systems.

My memory is not too great nowadays, here is a description from the manual:

Battery Buses
The hot battery bus usually receives power from the main battery or main battery charger. The auxiliary battery and auxiliary battery charger connects in parallel with the main battery during non-normal conditions to help supply power.
The battery bus normally receives power from TRU 3. The battery bus receives power from the battery if TRU 3 has no output.
The switched hot battery bus receives power from the hot battery bus when the battery switch (P5 panel) is in the ON position.

Standby Buses
The DC standby bus usually receives power from DC bus 1. The hot battery bus may also supply power to the DC standby bus.

DC Buses
DC bus 1 usually receives power from TRU 1. However, the bus can receive power from TRU 2 or TRU 3 through the bus tie relay. This relay is usually energized.
DC bus 2 usually receive power from TRU 2. TRU 3 supplies power if TRU 2 fails. DC bus 2 may also receive power from TRU 1 through the bus tie relay.

As you can see, to understand just the DC buses alone will take quite a bit of time