because it creates an imbalance in the main tanks.

 

Huh?  I was under the impression that it's just one tank.  The second pump is just there to toss out fuel at a higher rate.

 

The one on and one off would only provide less fuel from the main tank to supplement the fuel from the wing tanks, that's all.

Huh?  I was under the impression that it's just one tank.  The second pump is just there to toss out fuel at a higher rate.

 

The one on and one off would only provide less fuel from the main tank to supplement the fuel from the wing tanks, that's all.

Well, it did so for me. One time I've used this method and I had imbalance between two main tanks. After that I've only flown with center fuel pumps 2*on/2*off. I could have done something wrong, though.

Have you tried it?

I could have done something wrong, though.

 

That's my suspicion. The center tank is one tank with two pumps. The properties of liquid dynamics would prevent an imbalance from occurring in a single tank. The use of one or two pumps on a single tank of fuel shouldn't create the imbalance you're referring to.

 

What "main" tanks are you referring to?

There's the center tank and the wing tanks. If you had an imbalance in the wing tanks, it may be because you didn't open the crossfeed for the one on / one off configuration. Take a look at the schematic below:

 

Note that the fuel pumps feed different sides from that tank. Without the crossfeed on, you'll get an imbalance in the wing tanks.

 

 

Even better. With less than 1000kg in the centre tank, you should have the pumps off for takeoff in most SOP's that I'm aware of. (so you don't empty the tank before the Climb checklist )

I presume you mean 1000lbs or 450kg? If so, that is sort of correct. More accurately, the center pumps are not to be used with less than 450kg in the center tanks on the ground. In either case (air or ground), the main tanks must be scheduled to be full (read 'planned' or 'intended') with more than 450kg in the center tank - in other words the center tank pump switches must be on with more than 450kg. This can lead to  Master Caution Fuel annunciation on takeoff when the fuel sloshes to the back of the tank, which should be briefed to prevent an unnecessary RTO! Some guys just switch the centers off for take off with less than less than 1000kgs to prevent this.

Note that the fuel pumps feed different sides from that tank. Without the crossfeed on, you'll get an imbalance in the wing tanks.

Yup, that was my mistake, I had crossfeed on "off".

And by main tanks I was referring to wing tanks.

The reason there would be a fuel inbalance with just one center pump on is not because there is an inbalance in the center tank but a fuel difference between the wing tanks. Since the aircraft draws fuel from the center tank first if a center pump is on, if the left center pump is on and the right center pump is off - the left engine will draw fuel from the center tank while the right engine will draw fuel from the right wing tank resulting in the imbalance.

Check post #18  :wink:

Spot on for John and Kyle Except on the ground, engines shutdown, APU on with with > 450kgs when using a single center tank pump (the left one) will prevent an imbalance.

 

FWIW with regard to fuel balancing in general, in reality and in flight, main tank imbalances in the NG only really happen when single engine. The Classic however was a different beast, just about every flight could utilize fuel balancing to some degree. It seems the EECs in the NG do a great job compared to the Classic's PMCs.

 

Also, checkout the QRH LOW FUEL checklist for another not too uncommon reason to use the crossfeed in flight.

This was posted here a while ago.

 

Although I really don't see a point in leaving one pump on and the other off, because it creates an imbalance in the main tanks.

 

I love this table, it helps simplify the rules I studied in the PMDG manuals.  I used to be very vigilant about the center tank switches when I flew the iFLY NG and when the PMDG NGX was first released, but since I started using FS2crew, I basically follow the SOP it uses which dictates that has the copilot leaves the center tanks on during all phases of flight until the tank is empty.  I am starting to simulate Alaskan Airlines operations, and was curious to know what their center tank policy is.  Do they use the automatic shutoff policy per FS2crew, or should I use the policy listed in this table instead?   I'd imagine it may be the latter, since they have the old classics in their fleet and I suspect crews fly both the classic and NG?   

 

I would like to get into the practice again of monitoring the center tank, but only if the airlines I simulate use it as SOP, and I have no idea which airlines do follow these rules.

I love this table, it helps simplify the rules I studied in the PMDG manuals.

Interesting. I wasn't aware how complicated this was before I started with the Boeing. I believe those ADs are now out of date. I could be wrong, but all NGs have been modified with reliable auto-shutoff center tank fuel pumps. I don't really know the history of this, but suffice to say that the PMDG manuals describe different before takeoff procedures depending levels of compliance to this AD. The up to date procedure is the one entitled "Alternative Method of Compliance" which is the simplest and makes the above table redundant.

 

The logic of the center fuel pumps with PMDG should be that they auto shutoff after a short time with the LOW PRESSURE light illuminated. If fuel is added, they should require turning OFF then ON to extinguish the light. This is unlike the behavior of the main tank pumps.

 

In the most basic form the current procedure is - Ground: Mandetory OFF if <460kg. All other phases of flight: Mandatory ON if >450kgs. Otherwise, as required.

I love this table, it helps simplify the rules I studied in the PMDG manuals. I used to be very vigilant about the center tank switches when I flew the iFLY NG and when the PMDG NGX was first released, but since I started using FS2crew, I basically follow the SOP it uses which dictates that has the copilot leaves the center tanks on during all phases of flight until the tank is empty. I am starting to simulate Alaskan Airlines operations, and was curious to know what their center tank policy is. Do they use the automatic shutoff policy per FS2crew, or should I use the policy listed in this table instead? I'd imagine it may be the latter, since they have the old classics in their fleet and I suspect crews fly both the classic and NG?

 

I would like to get into the practice again of monitoring the center tank, but only if the airlines I simulate use it as SOP, and I have no idea which airlines do follow these rules.

First, lets get the name right: Alaska Airlines.

AS operates a mix of Classics and NGs. The NGs are further broken down into planes with auto-shut off and those without. So, while Vagabondo's airline and most may have a standard fleet-wide, there are still variations.

 

What tail number are you flying?

Hi Matt, I just flew the "official" PMDG paint for the -700 (N612AS).   I will be trying out routes with the other models as well, the PMDG painted -800 (N556AS) and the -900 (no idea what tail number that one is , haven't gotten it yet)

 

It appears Alaska has a wide variety of equipment options in the NG fleet, for example, I noticed the -800 has the flight controls indicators status in the lower EICAS that I can use to test the ailerons/rudder movement before taxi, whereas the -700 does not.   Matt, aren't you a pilot for the airline, and did you contribute to livery options configuration for the PMDG NGXs?   If so, thanks!  I hope you don't mind, I may have a ton of questions down the road as I get myself acquainted with flying the line for Alaska Airlines!

I just fly with the center tank pumps on until I reach approximately 500 pounds and then the pumps get turned off. 

Hi Matt, I just flew the "official" PMDG paint for the -700 (N612AS). I will be trying out routes with the other models as well, the PMDG painted -800 (N556AS) and the -900 (no idea what tail number that one is , haven't gotten it yet)

 

It appears Alaska has a wide variety of equipment options in the NG fleet, for example, I noticed the -800 has the flight controls indicators status in the lower EICAS that I can use to test the ailerons/rudder movement before taxi, whereas the -700 does not. Matt, aren't you a pilot for the airline, and did you contribute to livery options configuration for the PMDG NGXs? If so, thanks! I hope you don't mind, I may have a ton of questions down the road as I get myself acquainted with flying the line for Alaska Airlines!

Control indicators? I helped with the config files, but they were monkeyed with once they left my desk. (Or I screwed up.) Those shouldn't be available.

 

N556AS has the auto shut-off. I think that's the plane I flew PHL-SEA last night.

Control indicators? I helped with the config files, but they were monkeyed with once they left my desk. (Or I screwed up.) Those shouldn't be available.

 

N556AS has the auto shut-off. I think that's the plane I flew PHL-SEA last night.

 

I'll be sure to remove the indicator option in the -800 next time, thanks for the heads up. 

 

Oh btw, I happen to live nearby PHL.   In fact, seeing an Alaska -800 making a sharp turn near my house on the BOJID1 arrival to runway 9R inspired me to start flying routes for this airline in the sim.  Check out my post in the screenshots forum: http://forum.avsim.net/topic/409721-alaska-airlines-737-800-ksea-to-kphl/   

 

That is so cool, to know that you may be the pilot in command of the PHL-SEA flights I see so often here!  Talk about a small world.