What sequence should the fuel tanks be used in the B744 and B748? Assuming there is fuel in the centre tanks, I tend to use it first and then use fuel from the outer tanks first, in this sequence:

• At takeoff all the pumps are on and cross feeds open.
• After takeoff, close (turn off) the cross feeds between engines 1&2 and 3&4 and let the centre tanks feed fuel.
• Once the centre tanks are almost empty, turn off the centre pumps.
• Once tanks 1 and 4 are almost empty, turn those pumps off and open (turn on) the cross feed valves.

I've tried understanding Chapter 12 of the PMDG FCOM manual, but it's not clicking, so if anybody can point me to an easier explanation, that would be great.

Thanks,

Pete

 

1 hour ago, retepnodrog said:

What sequence should the fuel tanks be used in the B744 and B748? Assuming there is fuel in the centre tanks, I tend to use it first and then use fuel from the outer tanks first, in this sequence:

• At takeoff all the pumps are on and cross feeds open.
• After takeoff, close (turn off) the cross feeds between engines 1&2 and 3&4 and let the centre tanks feed fuel.
• Once the centre tanks are almost empty, turn off the centre pumps.
• Once tanks 1 and 4 are almost empty, turn those pumps off and open (turn on) the cross feed valves.

I've tried understanding Chapter 12 of the PMDG FCOM manual, but it's not clicking, so if anybody can point me to an easier explanation, that would be great.

Thanks,

Pete

 

If your flight is long enough to require fuel in the center tank, the center tank fuel is always used first until empty. While the center tank is in use, it is feeding all 4 engines.

Once the center tank is empty, tank 2 is used to feed both engines 1 and 2 (left wing) and tank 3 is used to feed both engines 3 and 4 (right wing). This requires that the outer crossfeeds remain open, and the tank 2 and 3 demand pumps remain “on”.

Tanks 2 and 3 are much larger than tanks 1 and 4 and will normally have more fuel to start. Since the two inner wing tanks initially feed both engines on that wing, (once the center tank is empty), their quantities will decrease, while the quantities in the outer wing tanks remains unchanged. On a long flight, the outer wing tank fuel is typically not used until several hours have passed.

When the amount of fuel in tank 2 has dropped enough to equal the amount in tank 1, then the cross feeds will be turned off, along with the demand pumps. The same is done with the cross feeds and demand pumps for tanks 3 and 4 on the right wing.

This is known as “tank to engine” configuration, in which each tank feeds only its own engine - tank 1 feeds engine 1, tank 2 feeds engine 2 etc.

Once tank-to-engine mode begins, (and continuing until the end of the flight), the quantities in tanks 1, 2, 3 and 4 will all remain close to equal, and will all decrease together at the same rate.

There are relatively small reserve tanks (one in each wing) that will empty their contents into tanks 2 and 3 when there is sufficient room. This happens by a simple gravity transfer and requires no action by the pilot.

At takeoff all pumps are on, and outer crossfeeds open. When the center tank is empty, the two center tank pumps are turned off. When the quantity in tank 1 = tank 2 and tank 3 = tank 4, then the tank 2 and 3 demand pumps are turned off (but the main pumps stay on) and the outer cross feeds are closed. The inner crossfeeds are automatic (they have covers over the switch) and are normally not operated by the pilot.

Edited April 17, 20215 yr by JRBarrett

Thank you very much @JRBarrett for taking the time to explain this so well. I've never got this sequence right, so I'm looking for to following these steps in my next flight on the 747!

Excellent exploration there from Jim.

Now you’ve got the hang of it we’ll move on to advanced real world operations 😀

Now, I’m not sure if this is simulated by PMDG but knowing them it probably is, but I’ll just go over this for anyone who is interested.

You may remember the loss of a 747 off Long Island in the 90’s the investigation put this down to an explosion in the centre wing tank caused by an exposed fuel pump which was damaged and caused a spark and igniting vapours.

To mitigate against having running fuel pumps sucking on dry air a new procedure and software was introduced in the 747.

Basically.. in the climb , with a sensed pitch attitude greater than 5 degrees,with centre wing fuel you’ll get an EICAS message FUEL LOW CTR L/R once the centre fuel reaches approximately 3200kgs. This prompts you to turn the centre tank pumps off, as the fuel level is getting low and settling in the rear of the tank during the climb this minimum figure ensures the pumps remain covered.

Then, once you level off in the cruise with the pitch below 5 degrees again you’ll get a FUEL OVD CTR EICAS message this prompts you to turn the centre pumps back on to start drawing fuel again.

Finally once the centre fuel reaches 1300KG you’ll get FUEL LOW CTR EICAS again , at which point you turn the centre pumps off for good to stop them becoming uncovered . 
The remaining 1300kg is then drawn into The main tanks using motive flow scavenge pumps.

If this is simulated, that’s why you’ll see these messages with centre tank operations.

It’s nothing to get bogged down with remembering figures, just turn the centres off in the climb when you get the message , then on again when you level off and get the next message. Then it’ll tell you to turn them off for good when you get the final message.

The 747 fuel system although fairly easy to operate , is quite a complicated system to understand .

17 minutes ago, jon b said:

To mitigate against having running fuel pumps sucking on dry air a new procedure and software was introduced in the 747.

Thanks a lot for this excellent information! I haven´t flown the PMDG 747 for some months now. But I´m almost sure this is simulated. I recall I found this always a bit strange....now I know it isn´t....it´s not a bug but an important feature! 😉

Thanks Jon!

Marcus

Edited April 17, 20215 yr by mpo910

10 hours ago, retepnodrog said:

Thank you very much @JRBarrett for taking the time to explain this so well. I've never got this sequence right, so I'm looking for to following these steps in my next flight on the 747!

Also, when the quantities in tanks 1.2,3 and 4 all match, you should get a CAS message indicating that it is time to switch to tank-to-engine mode.

Thank you both Jim and Jon for the additional information. It gives me a better appreciation of the great job PMDG have gone to of modelling these systems. One of the things I love about flight simulation is all these additional layers I learn about, and the willingness of people like yourselves to share your knowledge

one short note, there is a difference for reserve tank feeding between B747-400 and 747-8, it works as follows:

B747-400: reserve tanks 2 and 3 gravitiy feed to main tanks 2 and 3 if qty there is less than 18000 kg / 6000 gal
B747-8: reserve 1 and 4 tanks gravity feed to tanks 1 and 4 if qty there is down to 6,080 kg / 2000 gal

Best regards Ralf

Edited April 20, 20215 yr by cseu