Crossfeed not working!

[cockpit737]

Hi,
It seems I have something strange here...
I created an imbalanced fuel situation and opened the "Crossfeed" valve on the Overhead, but nothing happens. All the 6 "Fuel pumps" are ON. The fuel does not transfer from the higher tank to the lower, and it remaines in an imbalance situation...
Any ideas?
Thank you for your help,

[tomenglish2000]

I dont fly the 737, Im a 767 guy, but I believe if you want to transfer fuel you will need to turn off some fuel pumps.

With the cross valve open:
Turn off the pumps on the side with the low tank. Turn on the pumps on the side with the high tank. This creates higher fuel pressure on the high tank side. With the fuel crossfeed open, the high tank, high pressure fuel will be used as the low pressure fuel cannot get out of the lower tank! When sufficient high tank fuel has been used to equal out the imballance, turn the low tank pumps back on and then turn off the crossfeed.

Fuel doesnt actually go into the low tank. Hope this helps.

[cockpit737]

Tom,
A big thanks to you...
I will try it tonight and will let you informed. But for sure it is logic...
Thank you

[cockpit737]

Tom,
Here are the results... Unfortunatly did not work!
First here is my 737-400 fuel cfg...

[fuel]
Center1 = 0, 0, 0, 2313, 0
LeftMain = -3, -19, 0, 1500, 0
RightMain = -3, 19, 0, 1500, 0
fuel_type = 2
number_of_tank_selectors = 2
electric_pump=0

Now what I tried...
First I put 2000 Lbs in wing 1, 4000 in wing 2 and 3000 in center tank...
I started the APU, then both engines via PMSystem, then closed the APU!
So I placed the crossfeed in open position (horizontal) using the PMSystem directly (no hardware connected)!

- When 6 fuel pumps ON, system takes fuel only from center tank, nothing from 1 or 2 tank!!!
- When center both pumps ON, wing 1 both pumps OFF, wing 2 both pump ON, takes fuel only from center tank, nothing from 1 or 2 tank and no fuel transfer from wing 2 to 1!!!
- when center pumps OFF, wing 1 pumps OFF, and wing 2 pumps ON, it take fuel from both wing 1 and 2 same fuel flow...
- When center left pump OFF, center right pump ON, wing 1 both pumps OFF, and wing 2 both pumps ON, it takes fuel from center and wing 1...
- When center left pump ON, center right pump OFF, wing 1 both pumps OFF, and wing 2 both pumps ON, it takes fuel from center and wing 2...
Strange behavior no???? Any idea?????

[tomenglish2000]

That is strange behaviour indeed.

I can explain it a bit better perhaps.
When you are loading fuel you first try and fill the wing tanks. When/if they are full then you fill the centre tank. The centre tank is ONLY used if the wing tanks are full. If the wing tanks are not full, put your fuel in them!
If not using enough fuel to fill the wing tanks you simply place enough fuel in each wing tank to fill them evenly and leave the centre tank empty.

When flying with fuel in the centre tank then all pumps should be on. If you only have fuel in the wing tanks then the centre tank pumps should be off. The centre tank pumps provide fuel at a higher pressure compared to the wing tank pumps and this ensures that the centre tank fuel is used first and that NO wing tank fuel is used until the centre tank is dry. When the centre tank goes dry its pumps cannot supply fuel (obviously) and so now the wing tank pumps start supplying fuel. At the point the centre tank exhausts its fuel, both wing tanks should be completely full (or as close as possible) which means they are ballanced.

If you follow these rules then you should never have a fuel imballance when flying with centre tank fuel loaded. (Except for what the APU has used which is nominal).

This means that for normal situations we only have to consider fuel imballance with no fuel in the centre tank. So those centre tank pumps are off. Now say the left tank is higher than the right. Turn the crossfeed valve to open. Now turn off the right hand tank pumps. Now the left tank is providing fuel at a higher pressure than the right because the right has no fuel pumps on. This means that the left tank fuel is favoured and used up. You are not actually transferring fuel from left to right tanks, just using up the extra fuel in the left tank. When sufficient extra fuel in the left tank has been used turn the right tank pumps back on. Fuel pressure is now equal and both tanks will provide fuel. Now you can turn off the crossflow.

If for any unseen reason you have an imballance and fuel in the centre tank, turn off the centre tank pumps before ballancing otherwise the higher pressure centre pumps will override the pumps on the wings, and so no wing fuel will be touched.

Hope this helps a bit better.

[cockpit737]

Tom,
Thank you so much for your explainations... I understand perfectly what you are saying...
So, this on my experiences from yesterday was not working properly...
- when center pumps OFF, wing 1 pumps OFF, and wing 2 pumps ON, it take fuel from both wing 1 and 2 same fuel flow...
On this pumps situation it should have take fuel only from wing tank 2...!
One more question... In my mind in an imbalanced fuel situaltion I tought that the higher tank should transfer fuel in the lower tank until it equals the higher tank... Am I wrong? Is it possible to transfer fuel from higher to lower?
Thank you very much Tom, your explainnation are very usefull

[tomenglish2000]

Glad that helps.

When imbalanced, the higher tank does not transfer fuel to the lower tank. Instead (because of the way you setup the fuel system) the engines take fuel only from the higher tank leaving the lower tank fuel as it is.
Example: Left tank has 5000 pounds of fuel and the Right tank has 3000 pounds of fuel. You need to take fuel from the left until it reaches 3000 pounds to match the right tank. Then you continue taking fuel from each.

What does not happen is that if Left has 5000 and Right has 3000 then the left transfers fuel to the right until both have 4000.

You are lucky that the 737 fuel system only has 3 tanks. Imagine the Concorde fuel system that CAN transfer fuel from tank to tank but to add even more complexity it has 13 tanks, and the fuel has to be balanced correctly not just left and right but also nose and tail to maintain the Centre of Gravity! http://www.airliners.net/photo/Briti...rde/0384863/L/

[cockpit737]

Tom,
Thousand of thanks for your help...
I was sure that it was possible to transfer fuel from one tank to the other... I know a little more now on that beauty of B737NG's and you'r right I would be scared if I had to manage a Concorde fuel tanksssssss Happily I did not choose to build a Concorde sim... !
All my thanks,

[cockpit737]

Hello Tom,
Here are the yesterday night results... Good and bad news...!
Whene I close both center fuel pumps it stops from taking fuel from the center tank... This is the good news...
But as I said in my previous message, when I close both left wing tanks (low fuel) with crossfeed opened and both center tanks closed the fuel is going to the engines by both wing tanks... Thats the bad news... Unhabble to stop taking fuel from the left tank... (the lower one).
Can you confirm that it works on your side????
Anyway, even if I am a little discourageous trying to make it working, it is just a little thing not working.
Is there someone else having this problem or am I the only one????
Thank you again Tom...

[Michael Carter]

The 737NG fuel distribution system sounds a lot like the 727 system.

You cannot transfer fuel from any tank to any other tank except when on the ground during a gravity fueling operation from the wing tanks to the center tank.

Normal configuration is tank-to-engine, except when the center tank has more fuel than the wing tanks can hold. Then, all three engines are fed from the center tank using the crossfeed valves and pumps until the center tank has as much fuel as the wing tanks. Then tank-to-engine feeding is done.