Fuel Tank balancing (in-flight)

[softreset]

Reference material - https://www.smartcockpit.com/docs/CL605-FUEL_SYSTEM.pdf

Page: 16

Fuel tanks are expected to be within 400 lbs / 181 kgs of each other on taxi / takeoff / landing OR 800 lbs / 362 kgs during flight, to avoid Fuel Imbalance EICAS message.

• First, orient yourself on the overhead panel, we'll be working in the 2nd row (or middle row) of the FUEL panel area. (Fuel.PNG)
• All three of these buttons should be extinguished under normal operations.

Gravity XFLOW

Press to begin the process, there's a 1 second delay after pushing the button for it to illuminate OPEN. This will allow fuel to flow freely between both main (wing-based) tanks. There is no EICAS notification that this is open/occurring. (gravity.png)

To observe what's happening in the sim. You can go to the Challenger 650 menu -> Study -> Engines -> Fuel System...

You'll notice that whatever Main Tank (either Left or Right) has more fuel that this will continue to consume, whereas the side with less fuel will not. Thus balancing the tanks.

L (R) to AUX XFLOW

Press the appropriate button to begin the process, there's a 1 second delay after pushing the button for it to illuminate ON. This will remove fuel from whatever side is select and transfer it to the auxiliary tank.

• Determine which tank has more weight in fuel
• Left side? Choose L to AUX (L to AUX.png)
• Right side? Choose R to AUX (R to AUX.png)

There is a lock out mechanism that does not allow you to press the opposite button when one is already selected. Pressing L to AUX, for example, then pressing R to AUX, will no longer illuminate the L to AUX button. Pressing the R to AUX button again will resume the L to AUX transfer and the button will illuminate ON again.

Once you're done balancing the tank, make sure to deselect the previously selected L (R) to AUX button. The aircraft will now suspend fuel consumption from the two main tanks and pull from the AUX tank until depleted.

*Edit* Thanks! @airforce2
As long as a main tank is below 93% full, the aux tank feeds the main tank to maintain the level at 93%.  Once a main drops to 93%, the main quantity will remain constant and the aux level will drop.  It will appear that the engines are burning from the aux tank, but in reality the engines are burning from the mains and that fuel is being simultaneously replaced in the main tank from the aux tank ejector xfer pumps.

To observe what's happening in the sim. You can go to the Challenger 650 menu -> Study -> Engines -> Fuel System...

Note: The plane will display an EICAS warning message: FUEL XFLOW SOV OPEN if either button is ON for more than 50 seconds and the tanks are balanced within 100 lbs.

 

Edited April 1, 2022 by softreset

[xplana]

this was helpful!

[GouliAndPlanes]

Defo helful ! 

[Ed Moore]

Just to add to this I highly recommend setting a timer on your phone or something if you start a transfer to aux - multiple times I have ended up with an imbalance the other way because I forgot it was running.

[JT8DNoise]

On 1/13/2022 at 1:55 AM, Ed Moore said:

Just to add to this I highly recommend setting a timer on your phone or something if you start a transfer to aux - multiple times I have ended up with an imbalance the other way because I forgot it was running.

We use a flip down FLOW sign to remind us, a audio warning as you suggested is a great idea 

[BlackBird_GR]

On 1/13/2022 at 10:55 AM, Ed Moore said:

Just to add to this I highly recommend setting a timer on your phone or something if you start a transfer to aux - multiple times I have ended up with an imbalance the other way because I forgot it was running.

As @softreset already point out with his last note as long the tanks are almost balanced you get on EICAS a warning (with audio alert) FUEL XFLOW SOV OPEN to remind you to close L(R) 2AUX FLOW.

[softreset]

Thanks @BlackBird_GR. You beat me to it. There is a 50-second delay.

Personally, anything <250 pounds I don't bother correcting. At anything over 250 pounds, the transfer pump is on long enough to always trigger the EICAS / and audio alert to where setting an independent monitor doesn't seem necessary.

[Ed Moore]

9 hours ago, BlackBird_GR said:

As @softreset already point out with his last note as long the tanks are almost balanced you get on EICAS a warning (with audio alert) FUEL XFLOW SOV OPEN to remind you to close L(R) 2AUX FLOW.

You do, but it's far from impossible to miss!

[airforce2]

On 1/8/2022 at 9:48 PM, softreset said:

Once you're done balancing the tank, make sure to deselect the previously selected L (R) to AUX button. The aircraft will now suspend fuel consumption from the two main tanks and pull from the AUX tank until depleted.

As I understand it, the engines never suspend consumption from the mains--they feed *only* from the main tanks, never directly from the aux tanks.  Though the collector tanks are physically located in the aux tank, they remain functional components of the main tank system, in that they only receive fuel from the mains. 

As long as a main tank is below 93% full, the aux tank feeds the main tank to maintain the level at 93%.  Once a main drops to 93%, the main quantity will remain constant and the aux level will drop.  It will appear that the engines are burning from the aux tank, but in reality the engines are burning from the mains and that fuel is being simultaneously replaced in the main tank from the aux tank ejector xfer pumps.

Regards

Bob

Edited March 30, 2022 by airforce2

[ois650]

2 hours ago, airforce2 said:

As I understand it, the engines never suspend consumption from the mains--they feed *only* from the main tanks, never directly from the aux tanks.  Though the collector tanks are physically located in the aux tank, they remain functional components of the main tank system, in that they only receive fuel from the mains. 

Correct

[softreset]

On 3/30/2022 at 10:53 AM, airforce2 said:

As I understand it, the engines never suspend consumption from the mains--they feed *only* from the main tanks, never directly from the aux tanks.  Though the collector tanks are physically located in the aux tank, they remain functional components of the main tank system, in that they only receive fuel from the mains. 

Thanks Bob, you're absolutely right. I've edited my original post to reflect that.

[KirkR]

The above reference helped me understand why I develop an imbalance on the ground.  The APU's fuel source is from the right main tank, and that is indeed the one that goes lower when I am sitting in the plane continuing to learn its systems after I have started the engines.  If I had to pay for the gas and maintenance then this would be self correcting!

 

[dbw11]

On 1/13/2022 at 2:55 AM, Ed Moore said:

Just to add to this I highly recommend setting a timer on your phone or something if you start a transfer to aux - multiple times I have ended up with an imbalance the other way because I forgot it was running.

Used to put a checklist between the throttles so as to not forget.

[HussamAli]

Thanks for the procedure clarification to rebalance the tanks. Today I had an imbalance of 800 lbs , and it took me over 25 mins to balance them. Is this normal ? 
If on the ground and the APU is on, can we preemptively prevent an imbalance by turning on the R to Aux button ?

[Pils]

17 minutes ago, HussamAli said:

Thanks for the procedure clarification to rebalance the tanks. Today I had an imbalance of 800 lbs , and it took me over 25 mins to balance them. Is this normal ? 
If on the ground and the APU is on, can we preemptively prevent an imbalance by turning on the R to Aux button ?

800 lbs is a significant imbalance so I’m not surprised it took a while. “To Aux” isn’t a pump-driven transfer, purely gravity.

[FYG001]

In a long conversation with BBA yesterday we learned that BBA advises to equalize left and right mains BEFORE departure using the L (usually) to AUX pumps, even if the difference is lower that the limit of 400 lbs. This in order to avoid CAS fuel imbalance messages after departure which could be triggered by banking and 'sloshing' in the tanks

[simking1]

twice now I was on final and suffered flame out on 1 engine left because the fuel was not equal 20 in left 50 I followed the above procedures cross valve opened  so it should have had continuous fuel for both engines or balanced it equal  but nothing moved into that left tank..i don't understand according to the flight plan i should have had enough.

[Pils]

4 hours ago, simking1 said:

twice now I was on final and suffered flame out on 1 engine left because the fuel was not equal 20 in left 50 I followed the above procedures cross valve opened  so it should have had continuous fuel for both engines or balanced it equal  but nothing moved into that left tank..i don't understand according to the flight plan i should have had enough.

Once you’re down to those levels the sensors are unreliable and not all fuel in the tanks is even usable. You should be planning to land with 4,000 lbs, obviously something went rather wrong in your fuel planning.

[simking1]

yep lol I think the whole galleons from the fuel truck vs lbs screws me up I suck at math  good thing for apps 

 

Edited February 24 by simking1

[Prefect99]

On 2/11/2024 at 11:24 PM, simking1 said:

yep lol I think the whole galleons from the fuel truck vs lbs screws me up I suck at math  good thing for apps 

 

Your OFP will give you fuel weight, so only need to divide by density to get volume. No need for a conversion.

Even then, you should get a warning when doing PERF INIT before departure. And even then be aware of your current fuel burn in flight and time to destination - less than an hour spare and you should be asking questions. Divert to nearest suitable is always an option, don't press on to destination with low fuel.

Maybe the most useful point for you to consider is, unlike a car, aircraft fuel should always be thought of in terms of time.

[Lauramks2]

On 1/13/2022 at 12:55 AM, Ed Moore said:

Just to add to this I highly recommend setting a timer on your phone or something if you start a transfer to aux - multiple times I have ended up with an imbalance the other way because I forgot it was running.

It is easy to forget and a timer is great. If it is only a few hundred pounds, I point my left finger to the ceiling, resting on my elbow until I put the button back.