Auto fuel/tank config is not always working for me (748)

[Andreas Stangenes 1,628]

I have the fuel/tank config set to auto because I mostly fly overnight longhaul flights when I sleep. Sometimes everything is fine when I return, but often it's not. Ie. today I was flying from Buenos Aires to Frankfurt (13,5 hours), and after several hours into the flight, the stab L R warning showed up (they were off when I left the cockpit, and should have been turned on (when ctr was empty?). Also the CTR L and R were not configured correctly (they were on when I left the plane, but should have been turned off). 

I guess this automatic setting of the fuel and tank system is supposed to be for people like me who leave their flights, but it seems it's not working a 100% of the time. At least in the sense that the 744 could be left unattended because of the pause on TOD + auto tank/fuel setting. Is this no longer the logic behind this setting?

[Captain Kevin 586]

Auto tank/config will ONLY reconfigure your fuel tanks during cruise when tanks 2 and 3 match tanks 1 and 4, and in the case of the 747-8, reserve tanks 1 and 4. It will not do anything with either the stabilizer tanks or the center tanks, nor was it ever designed to do so.

[Andreas Stangenes 1,628]

1 hour ago, Captain Kevin said:

Auto tank/config will ONLY reconfigure your fuel tanks during cruise when tanks 2 and 3 match tanks 1 and 4, and in the case of the 747-8, reserve tanks 1 and 4. It will not do anything with either the stabilizer tanks or the center tanks, nor was it ever designed to do so.

Ok, thanks for your reply, Kevin! 

This issue, I guess, mostly comes into play on longer flights when more of the tanks are filled. This was my first flight that ever utilized the stab tanks. I start off with the overrides on so that all four engines draw fuel from the center tanks. With auto tank/fuel set to on, I am expecting the logic to automatically turn off the overrides. In this particular flight I also had fuel in the stab tanks, and I am assuming that the center tank, when empty, is to be turned off while the stab tank is transferred to center tanks. Then the stab tanks are to be turned off while you resume drawing from the center tanks. 

It's a pity that the auto fuel/tank feature doesn't handle this complexity because then I cannot leave the flight on it's on for long hauls the way I could on the 744. What is the point of the auto feature on the 748?

[Captain Kevin 586]

The auto tank/eng feature will only, by definition, reconfigure your tanks when you get the TANK/ENG message on the EICAS. The only time you will get that message is as I described, when tanks 2 and 3 match tanks 1 and 4.

[downscc 1,327]

6 hours ago, Andreas Stangenes said:

What is the point of the auto feature on the 748?

The TANK/ENG condition is a case where mismanagement can lead to problems with fuel imbalance.  The other cases are generally leaving a pump running without any fuel.  This will not lead to the same problems as a fuel imbalance in the simulation.

[Eric_Parker 4]

Hate to disagree but it doesn't always work. It used to work on the -400 but if it fails in mid-flight on the -8 and you are away from the flight deck for an extended period it could result in a flame out. I tried it for the first time today from KJFK to EGLL and over the Atlantic Ocean I had a serious fuel supply problem. None of the fuel from the reserve tanks were feeding and main 2 and 3 were empty. I quit the flight and returned to KJK to troubleshoot and learned that auto-switching was a fair luck situation at best. I decided pausing the sim was a better choice.

 

Eric Parker

KSMF

[kevinh 409]

 

8 hours ago, Eric_Parker said:

Hate to disagree but it doesn't always work. It used to work on the -400 but if it fails in mid-flight on the -8 and you are away from the flight deck for an extended period it could result in a flame out. I tried it for the first time today from KJFK to EGLL and over the Atlantic Ocean I had a serious fuel supply problem. None of the fuel from the reserve tanks were feeding and main 2 and 3 were empty. I quit the flight and returned to KJK to troubleshoot and learned that auto-switching was a fair luck situation at best. I decided pausing the sim was a better choice.

 

Eric Parker

KSMF

The only time I used it in the 747-8 it worked flawlessly. Yet on the basis of your one failure you say it’s a “fair luck situation at best”? Note, in the -8 the reserve tanks feed main tanks 1 and 4, not 2 and 3 as in the -400. 

It sounds like tanks 1 and 4 were supplying engines 2 and 3 through the open crossfeeds, as they should. Rather than quit your flight, why not continue like that? 

Edited December 17, 2018 by kevinh

[downscc 1,327]

13 hours ago, Eric_Parker said:

auto-switching was a fair luck situation at best

A little push back here.  Without the details I don't think you can support this proposition.  The auto tank/eng reconfiguration works, but it only works in specific conditions such as level cruise flight.  If you are climbing when it's time to switch then it's not going to cover that.  It is an option only for cruise flight.

[Jetlinker 13]

Admittedly I don’t have much experience with the pax birds as I primarily fly freighters, but if there was fuel in the STAB tanks shouldn’t those pumps have been left on? The logic for transfer to the CWT is automatic according to the FCOM, and this occurs when CWT fuel reaches 36470kg or less. From what I recall if a tank has fuel in it the pumps should be on until the low pressure message alerts you to turn them off... I think you may be making it more difficult than it needs to be. The fuel cards on the 400 and -8 for the most part will take care of everything for you aside from just turning pumps off and the FUEL TANK/ENG configuration.

As others have said though the auto configuration sim feature will only handle the fuel tank to engine. When you return to your sim you should have tanks drained of fuel and pumps to turn off but nothing more. Also, as a side note, in the real aircraft even if the pumps are left on they will auto shutoff after a period of time to prevent overheating... so no harm no foul if the switches aren't turned off promptly.

Edited December 17, 2018 by Jetlinker

[downscc 1,327]

56 minutes ago, Jetlinker said:

From what I recall if a tank has fuel in it the pumps should be on until the low pressure message alerts you to turn them off..

That is my understanding, the message should come up with the stab level is below 2500 lb (1100 kg) so it's a fuel remaining rather than pressure thing.  I had a little glitch with these just now during a step climb, the stab lo message comes on and I turn off the pumps but the message stayed on.  So I pulled up the non normal list and couldn't find anything there.  Turned pumps on then off and no more message.  I don't think the climb should have had anything to do with this but I've got nothing.

[Jetlinker 13]

1 minute ago, downscc said:

That is my understanding, the message should come up with the stab level is below 2500 lb (1100 kg) so it's a fuel remaining rather than pressure thing.  I had a little glitch with these just now during a step climb, the stab lo message comes on and I turn off the pumps but the message stayed on.  So I pulled up the non normal list and couldn't find anything there.  Turned pumps on then off and no more message.  I don't think the climb should have had anything to do with this but I've got nothing.

I’ve had that happen as well with the center tanks. I just figured it was fuel quantity fluctuations due to shifting in the tanks caused by the change in pitch... but I’m not sure. 

[Jetlinker 13]

And I stand corrected, it is the low quantity message that will appear first, followed eventually by the low pressure if the pumps are not turned off. 

[Eric_Parker 4]

In my case, there was no fuel in the stab tanks and, yes the reserves feed #1 & #4 which is not how it works in the -400. I was expecting the override switches to automatically deselect as it had in the -400. It didn't. So the override pumps depleted the #2 & #3 tanks and since they never turned off, they kept sucking fuel until it was gone. If the fuel from the reserves weren't going to feed, there wasn't going to be enough to make it London. What I didn't know was the fuel from the reserves would not feed until the #1 & #4 levels were below approx. 13,000 lbs (6.1 kg). I also was not aware there was an auto/on switch for the reserve tanks. I have since run some tests to learn what went wrong and my conclusion is to turn the override pumps off after takeoff and not use them again unless there is a fuel balance problem. And most likely the override pumps are not needed for takeoff either although I like to use them.

New to the -8 are electronic checklists which told me there was a fuel imbalance and to turn the crossfeeds off. D'oh!

 

Always something new to learn.

Eric Parker

KSMF

[kevinh 409]

Having the override pumps on is normal until you reach the tank/eng switchover point. They aren’t there just for takeoff, they provide pressure to supply engines 1 and 4, overriding boost pressure from tank 1 and 4 boost pumps.  

There was no reason to think that the reserve tanks would not to supply tank 1 and 4. Though if you were expecting them to have supplied 2 and 3 then you might have concluded the transfer valves were stuck closed. Even without the reserve tanks there should have been enough fuel to make it, assuming you had planned the fuel load with appropriate reserves.

[Jetlinker 13]

If you start manually configuring the fuel system it overrides the logic provided by the fuel management card, which isn’t how the airplane is intended to be operated. Generally unless there is a need to balance tanks you want to let the system work according to logic which is to have override pumps on until FUEL TANK/ENG