dutch747

I posted this question on another forum and no one seems to know. I think the answer is out there somewhere....


In the 747 Classic, after Fuel tanks 2 & 3 are EQUAL to the fuel in tanks 1 & 4 PLUS reserves, we burn tank to engine leaving a crossfeed valve open.

Now, some operators leave #2 open, some leave #1 or #4 crossfeed open, and some leave both #1 & #4 crossfeed open.

Now here is the question....

What is the advantage of leaving #1 and/or #4 open vs. leaving #2 open?

The way I see it, I would rather leave #2 open. The logic being if I develop a fuel leak in the xfeed manifold. It is going to cost me fuel from an inbd tank that I can easily fix using the dump manifold or do nothing because the fuel imbalance on inbd tanks is negligible.

So what am I missing? Someone.....anyone......Bueller?

leardriver

Well here is a sinario for ya. You leave #2 open. A fuel leak starts in the manifold. With me so far? You dont notice it because you are slepping on the engineer table. The captain is viewing the over head panel while the f/o is observing the window heat system. It is the midle of the night on the other side of the world. See where I am going with this? The inner tanks hold alot of fuel. I would not want to send it to the bottom of the ocean 3-4 hours from shore.

Have fun out there!

OrionFE

I don't think it is significant which crossfeed you leave open. In our manual it says normally #1 but as long as there is one charging the manifold at all times. As far as the leek, I would close the crossfeed manifold completely and land as soon as practical.

dutch747

I don't think you read the question carefully enough.

dutch747

I would guess your right. But what advantage does leaving #1 or #4 give you?

OrionFE

Only thing I can think of, you always takeoff in a Modified configuration in a 100 and a 200 (if you are less than 17K in the center tank) to ensure the manifold is positively pressurized. If one of the outbd crossfeed valves fails in the open position you can still crossfeed to engines 1 & 4. Once you go to tank to engine you close three of them. By leaving 1 or 4 open, if you would lose power to the boost pumps, 1 & 4 sit higher and would be able to gravity feed the manifold until power is restored.

That is the best I can do. But thanks for bringing it up. I have recurrent next month. A good time to get back in the books.

leardriver

I read the question just fine. I am just having a little fun thats all. I don't miss having the sun come up in my face after being up all night and still have 9 hours to go. It dosn't make any diff which one you leave open as long as the manifold stays pressurized. Don't forget to go tank to engine before you land and have the reserve tanks emptied. But you will probably be landing with 28000 lbs or less anyway!

dutch747

Sorry for being a pain. But just would like to know. The question is clearly fuel is EQUAL in all tanks. So yes of course I realize tanks 2 & 3 or over twice the size of tanks 1 & 4. But as stated in the question, fuel is equal. So what advantage is it to have #1 and/or #4 xfeed open vs. #2? Now i can see a clear advantage to having #2 open. But don't see any advantage to having #1 or #4 open. Yet Japan Airlines, Atlas, SAI, Kallitta all prefer to keep either #1 or #4 or both open.

Why?

cac737

Now, realize of course that every operator has different philosophies with regard to aircraft ops and such.....but a reason for always keeping the #'s 1 and 4 xfeed valves open (in this case both) is because if you lose power or the valves somehow stick in the closed position, there is no back door way of moving the fuel out of those tanks. However remote the possibility, it still exists, so that is a good reason to keep both of them always open.

Now here is a follow up question to all yous guyz....what does the blue light near the xfeed valves tell ya?......yes any boeing guy can tell you that it is an in-transit light, but what specifically is it telling you that is in-transit?
is that light responding to the sensor on the valve itself?
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the answer is no. the sensor is on the drive mechanism which drives the valve. The drive mechanism is outside the fuel tank, while the valve is inside the tank, this makes replacing inop drives much easier without having to mess with fuel tanks. So this light actually tells you if the drive mechanism is actually working or not, and also indicating that the valve itself could be stuck in an intermediate position etc etc......kinda like those pesky leading edge lights.......it only takes one sensor to not make to make that pesky amber light come on.


yea yea, i know splitting hairs here with this one......but since we are hangering flying!

dutch747

Cac,

Thanks for the reply and the insight about the in-transit lights. But I still have trouble buying it. Because at the point in the flight where we leave a crossfeed open, all tanks have EQUAL fuel. So if a crossfeed valve fails in the open or closed position, who cares?

I have always been a fan of never closing #1 & #4 crossfeeds just to give me options in case of xfeed valve failure.

But still can't see the logic in leaving just one (preferably 1 or 4) that many operators use.