747-400 >FUEL TANK/ENG

[NSEU]

Folks... I just noticed that the Boeing wiring schematics show a 120 second time delay in the ">FUEL TANK/ENG" EICAS message logic.

Is it normal to get the message when the inboard wing tanks are a few hundred kilos below the outboards rather than when the tank levels are equal?

Thanks!

[JammedStab]

Probably to prevent false messages due to fuel sloshing.

[mustafagander]

After many thousands of hours B744 time I can attest that it isn't a problem for us.

[NSEU]

Thanks, gents, but I'm not trying to establish if it's a problem, just that it actually happens. i.e. when the message appears, the inboard tanks are always a few hundred kilos below the outboards.

Has anyone actually tried selecting fuel/tank prior to the message (with tanks actually equal to the last decimal place)? Perhaps then it would be a problem in choppy weather?

Just curious... not critical.

[MarkerInbound]

I'd say it's a rarity if all four tanks are equal when the message displays. But does a couple hundred kilos matter when you weigh 300+ tonnes? I will admit to being a bit slow turning off the heavy wing override pumps when there is a large difference. Old habits die hard.

[tdracer]

Probably to prevent false messages due to fuel sloshing.
Exactly - the last thing you want is an EICAS message popping up, then going away, then coming back. So most EICAS messages have a time delay to prevent nuisance messages. The exact time delay is a function of the message and it's criticality.

[Intruder]

It's easy enough to rebalance the fuel during the switchover to Tank-to-Engine operation, IF you feel the need. Main #2 is usually lower due to APU burn before takeoff.

[NSEU]

Gents, all I'm trying to do is establish whether or not this arcane logic buried deep in wiring schematics is visible in the real world. The time delay is not mentioned in my rather detailed engineering training manuals or in the maintenance manual or in the FCOMs (as far as I know). Are the inboards consistently below the outboards when the message appears or is it not noticeably visible because of randomness?

If I understand correctly, MarkerInbound is now telling me that the message can even pop up before the tanks are equal? Or is he talking about specific imbalances (left wing vs right wing, rather than inboard/outboard)? The logic specifies that the first wing to go equal will trigger the >FUEL TANK/ENG logic, so the logic is not concerned about left wing total Vs right wing total imbalances.

Thanks!

[JammedStab]

Gents, all I'm trying to do is establish whether or not this arcane logic buried deep in wiring schematics is visible in the real world. The time delay is not mentioned in my rather detailed engineering training manuals or in the maintenance manual or in the FCOMs (as far as I know). Are the inboards consistently below the outboards when the message appears or is it not noticeably visible because of randomness?

If I understand correctly, MarkerInbound is now telling me that the message can even pop up before the tanks are equal? Or is he talking about specific imbalances (left wing vs right wing, rather than inboard/outboard)? The logic specifies that the first wing to go equal will trigger the >FUEL TANK/ENG logic, so the logic is not concerned about left wing total Vs right wing total imbalances.

Thanks!

It has been a couple of years but I think there were times when tank to engine selection would be delayed on one side to ensure that it was even between inboard and outboard tanks on that side.

Subject to confirmation.

[bigduke6]

The 120 second delay seems correct. Tank 2 or 3 is normally 100-200kg lower than tank 1 or 2 when you get the message.

I believe there is also logic to give you the tank-to-engine message on the ground prior to engine start with tanks 2 or 3 containing up to 500 kg more than tanks 1 or 2.

[NSEU]

Thanks, bigduke6.

I believe there is also logic to give you the tank-to-engine message on the ground prior to engine start with tanks 2 or 3 containing up to 500 kg more than tanks 1 or 2.

Interesting. I can't find any reference to it in the wiring schematics.

The logic in the manuals does mention that the inboards have to be full to start the logic process. i.e. if you manually loaded the tanks M1=10, M2=11, M3=11 and M4=10 and configured the aircraft to burn the fuel from the inboards, in theory, you would not get the message when the tanks were equal.

[B2N2]

Interesting. I can't find any reference to it in the wiring schematics.

Why would you is my question.
Couple of hundred kilos is well within the error margin of the measuring system.

[NSEU]

It was interesting in the sense the FQIS (or EIU) would need air/ground and engine run data for the TANK/ENG message. The X-FEED CONFIG message does actually show special logic gates which include air/ground logic input.

I understand that the fuel quantity system has errors, but I see computer logic as being based on 1's and 0's, no matter how "fuzzy" the quantity measuring system is.

[JammedStab]

I believe there is also logic to give you the tank-to-engine message on the ground prior to engine start with tanks 2 or 3 containing up to 500 kg more than tanks 1 or 2.

Interesting. I can't find any reference to it in the wiring schematics.

Hmmm...maybe the pilots have more system info than the maintenance guys after all. Or maybe just a system update and you have older info.


Straight from the QRH.....

>FUEL TANK/ENG
Condition: One of these occurs with crossfeed valve 1 or 4 open:
•Main tank 2 quantity is equal to or less than main tank 1 quantity, or main tank 3 quantity is equal to or less than main tank 4 quantity
•On the ground after refueling, after initial electrical power established, or after CMC ground test; main tank 2 quantity less than or equal to main tank 1 quantity plus 500 kgs and main tank 3 quantity less than or equal to main tank 4 quantity plus 500 kgs

[tdracer]

It was interesting in the sense the FQIS (or EIU) would need air/ground and engine run data for the TANK/ENG message. The X-FEED CONFIG message does actually show special logic gates which include air/ground logic input.

All that logic would be in the EIU - which already has air/ground, engine running, and several thousand other inputs readily available.
On the 777 and later, there was an effort to move the detailed message logic out of the EIU to the system responsible for the message - basically making EIU/EICAS just a 'light bulb'. On the early 747-400, there were lots of EICAS message logic errors, because the EICAS group didn't understand the message requirements as well as the source group - moving the logic to the system helped to reduce that problem. Obviously, there are lots of messages where the source system doesn't have all the necessary data so the EIU still has do some of the logic.

[NSEU]

Thanks, gents.
After further investigation, there was actually a modification carried out to the FQPU with that on-ground stuff (I think the FQPU already had access to all the data that the new logic requires: air/ground, refuelling in progress, CMC Ground Test in progress).

A clue was found in the Fault Isolation Manual. For the boffins:

A. Fault Isolation Procedure
(1) This EICAS message alerts the flight crew to configure the fuel system to establish a tank-toengine
fuel-feed configuration.
During flight, when the fuel quantity for the inboard main tanks becomes equal to the outboard
main tanks, the EICAS advisory message FUEL TANK/ENG will show (M1 = M2 or M3 = M4).
The flight crew action is to manually close crossfeed valves 1 and 4 and put the M2 and M3
override/jettison pumps to the off position. This will initiate a tank-to-engine fuel-feed
configuration until the end of the flight.

Airline X ALL; AIRPLANES WITH FQPU BLOCK B UPGRADE (SB 28-2182)
(2) The >FUEL TANK/ENG message contains logic to prevent fuel-feed configuration messages
from showing on short flights during takeoff and initial climb. The logic allows the airplane to be
established in an early tank-to-engine configuration when the inboard main tanks are within
1000 lbs (455 kgs) of the outboard main tanks.
Additional logic will maintain the fuel-feed configuration established on the ground for 8 minutes
after the airplane transitions from ground to air. Specifically, for 8 minutes after the transition
from ground to air, the status of the tank-to-engine alert bit shall remain the same as its status
prior to the ground-to-air transition. This will prevent the >FUEL TANK/ENG and >X-FEED
CONFIG messages from showing during takeoff and initial climb.
Airline X ALL
(3) The EICAS message >FUEL TANK/ENG (ADVISORY) shows when all of these conditions are
true:
(a) The No. 1 or 4 crossfeed valve(s) are not closed.
(b) All main tanks contain more than 2000 lbs (910 kgs) of fuel.
(c) Jettison mode is not active.
(d) The TANK TO ENGINE ALERT discrete is set (2-minute time delay).
(4) The TANK TO ENGINE ALERT discrete is set when these conditions are true:
(a) The airplane is not in refuel mode (the refuel control panel door is closed and/or the
airplane is in air mode).
(b) The M2 fuel quantity is less than 32,000 lbs (14,500 kgs) or the M3 fuel quantity is less than
32,000 lbs (14,500 kgs).
NOTE: M2 fuel qty is less than M1 VTO+2000 lbs (910 kgs) or M3 fuel qty is less than M4
VTO+2000 lbs (910 kgs).
(c) One of these conditions is true:
1) The M2 fuel quantity is equal to or less than M1 or the M3 fuel quantity is equal to or
less than M4.
Airline X ALL; AIRPLANES WITH FQPU BLOCK B UPGRADE (SB 28-2182)
2) The M2 fuel quantity is not more than 1000 lbs (455 kgs) greater than the M1 fuel
quantity and the M3 fuel quantity is not more than 1000 lbs (455 kgs) greater than the M4
fuel quantity and one of these events occur:
a) The fuel quantity processor unit is powered up
b) The CMC ground test >FUEL QUANTITY IND is performed
c) The refuel panel door is cycled from open to closed and in ground mode.

The FIM doesn't mention the 120 second time delays, but I'm sure they are still in there somewhere.

Cheers

[CCA]

(d) The TANK TO ENGINE ALERT discrete is set (2-minute time delay).

You mean this don't you?

[NSEU]

You mean this don't you?

oops... yes.... disregard :O