View Full Version : B737 Transfer Bus Breaker

27th Sep 2005, 18:16
A colleague has been demonstrated this trick in the sim:

The Transfer Bus Breaker pops, and of course you are left with no Transfer Bus (1 or 2, doesn't matter) without any direct indication of the failure.

BUT if you deselect the related Generator from the Generator Bus, you recover the Transfer Bus!

I knew this trick could be used in case of a Transfer Bus failure, to force the transfer relay to switch to the opposite side, but the behaviour of all this with the breaker popped is quite amazing...

So what does the breaker control exactly?

29th Sep 2005, 15:20
That's a great question, especially because during my last sim I had a Xfr bus breaker popup. We just caught it because we saw what we had lost on the overhead and then we just fixed it by reset and it did not bother us anymore (at least the TRE behind did not) :)
I'm having a look at the Elec schematic right now... if you loose Xfr bus 2 for example, and you deselect GEN 2, the relay goes on alternate... right... therefore the Xfr bus Cb should control the "normal" position relay ? I'm just wondering... waiting for explanations !:confused: :ok:

18th Feb 2006, 10:58
Hi guys,

I came across this thread and I was wondering what exactly we win by de-selecting the Generator from its bus to recover the TFR Bus.

In fact, you find yourself with the TFR Bus recoverd, but with the BUS OFF light illuminated (and in case of GEN2, TR3 INOP). So do we really win a lot ?

Things can get worse ;
- if you loose the associated Hydraulic EDP, you loose the complete hydraulic system (this would not happen if you keep GEN connected)
- if you have an generator or engine failure opposite side, you find yourself on battery (this would not happen if you kept GEN connected)

To answer your question : The TFR BUS IND CB powers the Bus indication (amber light + EPAP light)

Any similar trick on NG ?

23rd Feb 2006, 08:38
You are right Pat, this decision has to be carefully assessed...

Just a few considerations:
If you deselect the generator, you lose the hyd system or all generators, as you stated, but this if a case if the other generator fails.

It would take a subsequent failure, as you call them in your excellent book.

And in that case you can easily reconnect the first gen on line.

By losing a Transfer bus, you lose, depending on the wirings of the various airplanes which are unfortunately quite different, the CM1 or CM2 flight instruments; with a loss of Transfer bus 2 you will lose the right IRS after 5 minutes (!); you have to avoid icing conditions; you lose the various indications, trims and so on...

I don't know, I've always considered the Transfer bus more important than the Generator bus, but of course it depends on the day...

Let me know what you think.
Cheers, LEM


27th Feb 2006, 03:37

I agree that a BUS OFF failure is less harmful than a TFR BUS OFF. I just wanted to draw some attention to the subsequent failure, in which case you end up on the Battery.

I have never done this trick in the simulator, I will certainly try it soon.



28th Feb 2006, 03:12
We once had a captain who would write up strange defects in the Tech Log. When we cleared them as 'normal operation', he would argue that it "doesn't work like that in the simulator". We told him that its ok to tell the simulator people their simulator doesn't work like the real aeroplane, but not the other way round.

Some of the above may indeed be the way it works on the real aircraft, but a gentle reminder to beware of assuming that the simulator correctly simulates everything is never amiss... :)