B737 classic Fuel Pump

[mmrassi]

Dear

I got a question about fuel pumps in 737-400. I saw that when all pumps are one and we disconnect one Gen. and put the Bus Transfer switch to off position all the pumps get off except the one. (the Fw pump on the live side).

This makes me confuse because according to the system plan only those pumps feeding from the dead Gen. should turn off.

Thanks for your attention.

[snydersnapshots]

Dear

I got a question about fuel pumps in 737-400. I saw that when all pumps are one and we disconnect one Gen. and put the STBY Power switch to off position all the pumps get off except the one. (the Fw pump on the live side).

This makes me confuse because according to the system plan only those pumps feeding from the dead Gen. should turn off.

Thanks for your attention.

I'm going back in time here--it's been 7 years since I was on the 737 (but I'll be going back to it soon, so this is good review). I'll try to answer your question.

First of all, you mention the STBY Power switch. Did you mean that? Or the Bus Transfer Switch?

The standby power switch should have nothing to do with the fuel pumps. Basically turning that switch off de-energizes the standby AC and DC busses and the standby inverter. The standby buses power the minimum items necessary for safe flight. The captain's flight instruments, the number 1 nav and number 1 comm radios, and some basic lighting. On standby power the busses are powered by the battery, so there are NO high load items on the standby busses.

OK. I found a good schematic that shows power sources. Here's the rundown:

The forward boost pumps are powered by the respective main bus, i.e. the pump on the left side is powered by the left generator and is inop if the left generator is lost. Ditto for the right.

The aft boost pumps are powered by the transfer busses. If a generator fails, then the items on that generator's transfer bus will transfer automatically to the opposite generator. So:

If the left generator fails, then the bus-tie breakers will close, allowing the left aft fuel pump to be powered by the right generator through the bus transfer system. The right side operates the same, of course.

The center tank fuel pumps operate off the MAIN busses on the opposite side. So the LEFT center pump is powered by the RIGHT (#2) main bus and the Right center pump is powered by the Left (#1) main bus. The theory here is that if you lose the left generator, the center pump on that left side is still powered.

SO...Back to the original question. All pumps on and losing one generator takes you down to one pump (fwd pump in the live side)

Both generators on, all pumps on.

Remove the #1 (left) generator with the Bus transfer switch open (so the transfer busses will NOT transfer to the opposite generator)
You lose:
Main bus 1 (Left FWD and Right Center)
Transfer bus 1 (Left Aft)

You still have:
Main Bus 2 (Right FWD and Left Center)
Transfer bus 2 (Right Aft)

So you should NOT lose the aft pump for the engine with the operating generator. You've got me puzzled...

[mmrassi]

Thanks for your answer,

I apologize that I mistakenly wrote STBY switch that the correct word was Bus Transfer Switch. Sorry for that mistake although you guess what I mean!

I asked other captain and he told me this is something made by Boeing and he said he doesn't know the reason but he tested that condition and he got the same output! (Aft pump gone!)

now I think we should take a look at maintenance manuals to findout the answer.