747 Question for Flight Engineers

[Dutch74]

I've asked this before, but no answer so am going to ask again. Hope fully someone will answer...

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?:ugh:

[Rainboe]

Good question! You know, I flew the Classic for 10 years and I never knew why. Never understood it. I await the answer with interest!

[28L]

And there I was thinking it was going to be one of the ATQ's :ok:

[Spanner Turner]

mmmmm, not a definitive answer but here are some thoughts nonetheless.

I CAN see a disadvantage in having 1 AND 4 crossfeeds open (or for that matter 2 AND 3, or basically one crossfeed on each side of the ship).

Located in the centre of the crossfeed manifold is the "flow equalisation valve" or "flip/flop" valve as we call it. It's function is to reduce the chances of fuel imbalance due to the normal variations in boost pump output press by only allowing crossflow left to right when the diff pressure across the valve is above approx 2-3psi.

Example:
Tank to Engine feed.
Crossfeed valves 1 AND 4 open.
Combined output pressure of #1 boost pumps 15psi(old worn out units)
Combined output pressure of #4 boost pumps 20psi(new units fitted)

Result:
In addition to feeding #4 engine, the output of #4 boost pumps will overcome the "flow equalisation valve" due to the lower pressure supplied to the left side by #1 pumps and will supplement the fuel feed to #1 engine. It has a clear path as 1 and 4 crossfeeds are open.
#4 tank quantity will decrease faster than #1 = imbalance.

With only ONE crossfeed valve open across the ship, there is nowhere for the fuel to go other than keeping the crossfeed manifold pressurised up to the other three x-feed valves.



mmmmmm, Why is it number #2 crossfeed that most operators use to keep the crossfeed manifold pressurised?

As I mentioned, not definitive, but it could just be a case of physical location. The output line(fuel feed line) from #2 boost pumps is closest(in a physical sense) to the crossfeed manifold pressure switch so there's less chance of the switch going low pressure.
(For ease of maintenance,i.e quick under wing access, the switch itself is located next to one of the #1 boost pumps but it's sensing port is right next to the #2 boost pumps outlet) Hope that didn't confuse.


:ok:

[Dutch74]

Nice try Spanner. I knew about the flow equalization valve but it still doesn't explain advantages/disadvantages of keeping #2 xfeed vs. #1 and/or #4 xfeed after fuel is balanced between all tanks.

Now I know the reason we keep the manifold pressurized is to prevent pressure "bumps" in the manifold. But still the question lingers....

[Jumbo Driver]

Good question! You know, I flew the Classic for 10 years and I never knew why. Never understood it. I await the answer with interest!

I flew the Classic for over 20 years - and I don't know either... !

I do recall that we left one Crossfeed Valve open (#1 or #4) when "Tank to Engine" in order to pressurise the Crossfeed Manifold.

Which side of the Flow Equalisation Valve does the CWT feed into the Crossfeed Manifold - or do the OVRD/JETT pumps feed into the manifold individually either side of the valve (in which case the panel schematic is misleading)?

Is this relevant to the question, I wonder?


JD
:)

[Spanner Turner]

Which side of the Flow Equalisation Valve does the CWT feed into the Crossfeed Manifold - or do the OVRD/JETT pumps feed into the manifold individually either side of the valve (in which case the panel schematic is misleading)?



Correct, Left Ovrd/Jett pump supplies to left side of flow equalisation valve and the R/H Ovrd/Jett pump discharges to the right side.

Not sure how this makes the panel schematic misleading, as on all the 747's i've worked on, the flow equalisation valve isn't even depicted on the P4 panel.


Dutch74 said;
I knew about the flow equalization valve but it still doesn't explain advantages/disadvantages of keeping #2 xfeed vs. #1 and/or #4 xfeed after fuel is balanced between all tanks.

Well, it does actually explain a potential disadvantage of having more than one crossfeed open.

Jumbo Driver's procedures would back this up.

I do recall that we left one Crossfeed Valve open (#1 or #4) when "Tank to Engine" in order to pressurise the Crossfeed Manifold.

[Jumbo Driver]

Not sure how this makes the panel schematic misleading, as on all the 747's i've worked on, the flow equalisation valve isn't even depicted on the P4 panel.

Spanner Turner, I agree the flow equalisation valve is not shown on the P4 (lower) panel.

However, if you recall, the panel schematic combines the two lines from each of the OVRD/JETT pumps into just one line, before it then feeds vertically down into the manifold. Space saving on the panel, I guess - but that is what I meant by "misleading".



JD
:)

[Spanner Turner]

However, if you recall, the panel schematic combines the two lines from each of the OVRD/JETT pumps into just one line, before it then feeds vertically down into the manifold

Oh, I can see how your aircraft panels may be confusing. My company aircraft (most of them retired now) P4 panels depicted two individual vertical lines, one from each centre pump down to the horizontal cross manifold. (no 'Y' junction) I've dug out my old training notes and they are the same. The Boeing panels book that I have though shows the 'Y' junction that you are familiar with.

We had following combinations 747-200 P & W, 747-200 RB211, 747SP RB211 and 747-300 RB211(still a few of these kicking around)

It's been a while since I've seen one of the -200 panels so maybe our old girls did have the 'Y' depiction!!!

[Jumbo Driver]

As I recall, all ours had the "Y" junction just below the OVD/JETT PUMP switches - maybe it was a delivery option to standardise between different tank configurations - I really can't remember now.

We had -100 P&W/JT9D-7 and -200/RB211-524 - sadly, all are long gone ...


JD
:)

[Dutch74]

Damn... I've been without internet for a week and wow! you guys really took a stab at this. I appreciate your help in solving this mystery.

I don't quite get where the ovdr/jtsn pumps/plumbing fit into this. Because there is no fuel in the ctr tank when the mystery starts.

I've posted this question on other boards without joy. Isn't it cool how we are all using the net to work together to solve a mystery?

[Dutch74]

Spanner Turner said:
"Why is it number #2 crossfeed that most operators use to keep the crossfeed manifold pressurised?"

This is simple. the philosophy here is to protect if the manifold breaks. By leaving #2 open, if a fuel leak develops in the manifold, and you discover it, close the manifold off completely. At least you can transfer fuel using the dump valves back into #2 (or #3) so you can balance it back out or save yourself from starving the engine.