View Full Version : boeing 747 fuel system


flybik
24th Nov 2009, 23:29
Short from PMDG maual:

Any engine can draw fuel from any fuel tank, fuel can only be suction fed from the main wings tanks.
Each main tank and stab tank has AC boost pump + inboard main tanks and center wing tank have override/jettison AC pump.
Center tank has a scavenge pump that will automaticly transfer fuel to main tank 2 if center tank fuel pumps detect low pressure.
A vented surge tank is located near the tip of each wing and provide positive pressure on fuel tanks during cruise.
Each reserve tank has two automatic transfer valves that alow gravity transfering of fuel to inboard tank when inboard tank fuel quantity decreases below 40,000 lbs.
Outboard main tanks have each one manual transfer valve that allow fuel (7,000 lbs remain in each outboard tank) to be gravity transfered to inboard tank. Valves open automaticaly durring jettison if inboard tank fuel quantity decreases below 20,000 lbs.
When center wing fuel tank quantity decreases below 80,000 lbs fuel will be transfered from stab tank.
APU drives fuel from main tank 2 that also has DC boost pump.

Questions:
1. Does each engine has accessory drive attached fuel suction pump?
2. If so from where does this pump suction fuel from - where is the pump's inlet connected? Can engine one suction fuel from any tank?
3. Does stab tank really has boost pump, since the symbol for pump (on secondary EICAS under FUEL) is the same as for overide/jettison pumps and fuel is also fed from this tank in fuel jettison operation?
4. Where does stab tank fuel outlet goes (they are only pictured when fuel jettison is armer)? Can stab tank pumps provide fuel to the crossfeed manifold as other boost pumps?
5. What is a vented surge tank?
6. Can fuel from reserve and outboard tanks only be gravity transfered to inboard tanks?
7. What is the scavenge pump flow and is it AC or DC powered?



Wirelock
25th Nov 2009, 00:06
it's like you want us to do your homework.

SMOC
25th Nov 2009, 00:16
Questions:

1. Does each engine has accessory drive attached fuel suction pump?

Yes a HP and LP pump or First and second stage.

2. If so from where does this pump suction fuel from - where is the pump's inlet connected?

The pumps inlet is connected to the same pipe from the tank where the boost pumps are connected there is just an additional pipe that bypasses the boost pumps.

Can engine one suction fuel from any tank?

Technically yes but realistically no, Boeing doesn't say yes as it probably won't work as it would have to go via the crossfeed system.

3. Does stab tank really has boost pump, since the symbol for pump (on secondary EICAS under FUEL) is the same as for overide/jettison pumps and fuel is also fed from this tank in fuel jettison operation?

They are Transfer/Jettison pumps same as the Override/Jettison pumps.
Will have to double check the MM though.

4. Where does stab tank fuel outlet goes (they are only pictured when fuel jettison is armer)?

To the center tank.

edit: Except during jettison then fuel goes to the jettison manifold.

Can stab tank pumps provide fuel to the crossfeed manifold as other boost pumps?

No it just fills the CWT therefore you have to wait till the center tank is below approx 36.5T (edit: as above).

5. What is a vented surge tank?

Should a tank become full due to over fueling or expansion of the fuel it will be forced into the surge tank, should this tank fill, it will come out the vent, normal ops means the vent a NACA duct provides a positive head of pressure to all the tanks. Air pressure has to be able equalize inside the tanks ie as fuel is used air needs to replace it in the tank this is done by the NACA duct and via the surge tanks as they are connected to all the tanks.

6. Can fuel from reserve and outboard tanks only be gravity transfered to inboard tanks?

Yes from the reserve tank as there are no pumps. The Outboard to inboard transfer is by gravity and only during jettison the outboard tank may still be providing fuel to the engine and crossfeed manifold via it's boost pumps.

7. What is the scavenge pump flow and is it AC or DC powered?

I'll need to get that from the Maintenance manual.

sb_sfo
25th Nov 2009, 00:54
Last time I scavenged the last 100 pounds out of a center tank it took a little north of 10 minutes. Believe it's an AC pump, but not sure.

NSEU
25th Nov 2009, 03:14
The electric scavenge pump is AC with DC control. It's only a small pump and can take hours to shift fuel. Under automatic operation it stops after 2 hours.

During jettison, according to the Boeing schematics, the HST plumbing has a path directly to the Jettison manifold and to the Jettison Nozzles.

Rgds
NSEU

P.S. Again, I'm surprised you didn't ask these questions on the PMDG AVSIM forums. The people who created the simulator had input from professional pilots and engineers.

NSEU
25th Nov 2009, 08:12
My manuals give a nominal value of 771Kg/hour for CWT scavenge pump flow. However, this may vary.

In theory, fuel should not get into the venting system because of float valves fitted to the vents. However, improper seating of valves, fuel sloshing arount, etc, may allow fuel to get into the venting plumbing.

There are flame arresters and pressure relief valves for added safety.

I just checked our manuals and the HST pumps have the same part number/s as the Override Jettison pumps. If you look at the Fuel synoptic, the pump symbols should be the same size (at least on the real aircraft).

Can stab tank pumps provide fuel to the crossfeed manifold as other boost pumps?

If you mean directly, then only by engineer intervention on the ground.

Rgds.
NSEU

bigduke6
25th Nov 2009, 11:28
I believe center wing tank (CWT) scavenge pump type is a customer option. The (single) AC electric pump is very slow, as noted in prior replies. Dual??? jet pumps are much quicker.

bigduke6
25th Nov 2009, 11:51
To correct myself, after pulling the book out............


"CWT fuel is scavenged by four jet pumps, two pumping into each main tank 2 and
3. Scavenge begins when main tank 2 or 3 fuel quantity decreases to
approximately 27,200 kilograms............."

Tinwacker
25th Nov 2009, 12:08
Earlier B747 series aircraft had the single AC powered scavenge pump feeding No.2 tank. This was a problem if pump failed or filters blocked with approx 1.5 ton of unuseable fuel remaining.
Later production aircraft were fitted with hydro mech jet pumps feeding both the No.2 and 3 tanks which also reduced the possiblity of fuel imbalance and any unuseable fuel remaining, compared to the single scav pump system.

TW

NSEU
25th Nov 2009, 23:07
I assume the CWT fuel was scavenged into the #2 tank to offset APU fuel burn. Those aircraft which have jet scavenge pumps, Also seem to have two boost pumps (one in #2 tank and one in #3 tank) to provide fuel to the APU to balance the levels in the tanks.

Well, that's my theory :}