Center Fuel Tank Operations

[JohnSmith 0]

Fellow 777 pilots,

 

I've recently started flying longer routes since the release of the 300ER. However, I am somewhat perplexed when it comes to the operation of the center fuel tank. On some routes, there is barely any in the center tank. Maybe 4000 kilograms. In this situation, using the center pumps on the ground produces a "Fuel Low Center" EICAS warning. However, if I leave them off, a "Fuel In Center" EICAS warning is displayed once I am established in my climb.

 

Does anyone have any information on how to properly operate the center fuel tank when in this situation? I know the 737 center tanks are meant to be switched on after 10,000 feet if there is less than 5,000 pounds or something like that. Does the 777 operate using similar rules?

 

Thanks

[downscc 1,327]

This topic spans several sections in the FCOM, I recommend you use Find "center tank" in Vol 2 for an overview.  In summary, if the total fuel requires fuel in center tank then center tank fuel pumps should be on until the Low Fuel advisory message. These pumps have a higher discharge pressure than the main pumps and provide fuel as long as there is pressure, when the pressure drops it is sensed by the Electrical Load Management System and the pumps are automatically turned off so there is no concern for the operability of the pumps. Turning off the center pumps with a small amount remaining, which is what you are doing when the Low Fuel Center Tank message appears, results in remaining fuel being scavaged to the mains. Unlike the 737/747, there is no minimum fuel required in the center tank.

[JohnSmith 0]

After a bit of reading, I have made a few discoveries. Here's a section from the 777 FCOM under "Fuel Operations". I have excluded irrelevant details.

 

Before start, the main tank pump switches should all be pushed ON.

 

[Option - English Units, 777-200, with pump logic revision]

Before start, with center tank quantity more than 7,500 pounds, the center tank pump switches should be pushed ON.

 

[Option - Metric Units, 777-200, with pump logic revision]

Before start, with center tank quantity more than 3,400 kilograms, the center tank pump switches should be pushed ON.

 

[All models]

During flight, when the FUEL IN CENTER message displays the center tank pump switches should be pushed ON, when the FUEL LOW CENTER message displays the center tank pump switches should be pushed off.

 

This is section 12.20.3 in Volume 2 of the PMDG 777 FCOM if anyone wants to have a look at the entire page.

[Deaglet 0]

It happens a funny thing to me.
Consumig fuel in the central tank, It appears normally in the upper Eicas dispaly the message "Fuel Low Center" with the fuel quantity in the central tank in ambra color ...

 

obviously, 'by the book', i switch the central fuel tank pumps off, and all things seems right, with wings tank pumps activated and central fuel quantity back in white...



... but just after a little ammount of time the "FUEL IN CENTER" pops out in Upper Eicas and central fuel quantity in ambra again.



... So, I switch the pumps back on:



... and after a while, the "FUEL LOW CENTER" came back again,
 

with me switchin' the pumps off again.

 

A this time, things remain stable, and the fuel in the center tank, slowly drop due to the scanvange pumps




I don't think it's normal to switch OFF-ON-OFF twice the pumps in the central tank, but maybe there's something that I missed...

I did few test in other flights. I tryed to not switch off the pumps after the first "FUEL LOW CENTER" message, and central quantity in ambra color, letting pumps ON. (round about a 1.8 tons in the central tank) After a while the message DISAPPEARED, and the quantity turned back in white color!!! Just like the plane "realized" the was not yet the right moment. Just after few minutes, when quantity dropped below 1.2 ton, the FUEL LOW CENTER came back one more time, so I switched off the pumps, just one time, for good.

Anyone has any idea about what happened?
What I have to do? Just wait that the message disappears and reappears for the second time before switching the pumps OFF?

Thanks in advance for any help.
Regards,
Diego.

[scandinavian13 5,192]

 

 

Anyone has any idea about what happened?
What I have to do? Just wait that the message disappears and reappears for the second time before switching the pumps OFF?

 

Just run them a bit longer. The FUEL LOW CENTER is to give you a warning that you should act soon on the matter. I think the message is intended for you to act right at that moment, in a perfect situation, but flying isn't really ever perfect.

 

Fuel sloshes around in the tanks, so it may trigger the FUEL LOW CENTER warning a little earlier than it should. I usually run them down, despite the warning, until about 900 KGS/1900 LBS.

[Deaglet 0]

Thanks, mate. So I'm going to shut them OFF when the level drop below 1 t.
Thank you for your reply.

Cheers,
Diego.
 

[downscc 1,327]

Thanks, mate. So I'm going to shut them OFF when the level drop below 1 t.

Thank you for your reply.

 

 

I like to attend to things on 'schedule' so if something comes up and I am distracted the scheduled event is not missed.  When the advisory comes on, I shut down pumps. Simple.  Sure, the fuel in center advisory may come on but it will go out pretty quickly, actually quicker than letting the shut down message appear until 1.0 t.

 

This reminds me of flying in the Cessna 414 where we have to switch between AUX and MAIN before running AUX dry. The penalty is one or both engines surge then stop and it gets quiet.  With pax on board, I always switch tanks before the engine surges, discovered that surging engines and pilot exclaiming "Oh s***!" tended to rattle their nerves.

[Deaglet 0]

Thanks for your contribute, Dan. I'm going to do further tests about it.

[FEDEX061 1]

Some interesting information on the 777 fuel system.

 

http://www.infinidim.org/pt-update-eicas-fuel-scavenge-sys