FUEL TANK/ENG condition

[Hardy Heinlin]

Wild  guess... on ground = pitch zero, which leads to a slightly different fuel gauge indication than in cruise flight with pitch a  bit up? Enough to cause head scratching "why does the TANK/ENG indication (not) appear"?

Possibly. But why does the diagram not distinguish between air and ground? And why does that other text not mention the effects of inflight operations? Both documents only mention one value, not two values for air and ground. I think there is one approximate value somewhere between 1100 lbs and 2000 lbs, or maybe around 1100 lbs after a later update (and not referring to VTO) -- because the variable pitch attitude cannot provide a constant, precise reference level. Not only at cruise attitudes. Tank-to-engine may also occur at climb and descent attitudes.


|-|

[John H Watson]

Attitude should be compensated for by the multiple level sensors in every tank. There are 11 in each inboard tank, 6 in each outboard tank. However... 

The schematics are old and the QRH is new and we don't know what remains of the old logic. Perhaps someone can make sense of this version (even more complicated) in the Fault Isolation Manual:


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.

BAB 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.
BAB 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.
BAB 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.

[Jeroen Hoppenbrouwers]

zOMG now I opened the can of worms...

[Hardy Heinlin]

Easy. Except for the 28-2182 upgrade all these conditions are already implemented in PSX 10.32.

So the VTO+2000 stuff remains.

The 28-2182 upgrade just allows 1000 lbs more on top of the traditional "equal-at-any-level" condition for 8 minutes after lift-off. During these 8 minutes Dirk can set tank-to-engine even earlier, at 1000 lbs above equal level :-)

But the message will occur after 8 minutes. *

It seems when near tank-to-engine conditions, the tank-to-engine config is safer than letting the inboard quantities go lower by the O/J pumps, and getting X-FEED and O/J messages during initial climb.


|-|


* Edit: I was about to write that it will not occur as during these 8 minutes 1000 lbs will be burnt. But I just realized that the tanks cannot get equal when the x-feeds are closed and the outboard pumps are running.

[Hardy Heinlin]

b) The CMC ground test >FUEL QUANTITY IND is performed

Is this test manually performed before each flight, or even automatically? In other words, can we assume that it is performed before each flight in the virtual world of any simulator?


|-|

[John H Watson]

Definitely not routine. Logic only applies if Avi's CMC add-on is activated 

The FQIS may go through some kind of self test on power up, but this will be different from a CMC test.

[Hardy Heinlin]

This makes me sad. I just implemented that 8 minute stuff :-)

So do they call maintenance if they know they will be within 1000 lbs and the 8 minute thing is to be activated?

[John H Watson]

Not sure I'm with you.

The reference to the CMC ground test is in an "OR" section. It would be normal for the other two ((a) & (c)) to occur prior to a flight.

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.

EDIT: I'm not sure about (a). I don't know if they're talking about the FQPU/FQIS being powered up from a dead ship, the CB's being cycled, etc. I guess the key word is "up"... rather than just "powered".

I still don't know how to read the logic (e.g. That reference to VTO is confusing).

[John H Watson]

P.S. I guess if you were transitting an airport where there was no fuel (and had enough for the next flight), you might not get any of those three triggers. Even so, I doubt anyone would be aware of the ramifications. No one would ask maintenance to carry out a CMC test before the next flight. Having said that, a fueller might hook up the refuelling truck out of habit (before the engineers stopped him) and save the day

[Dirk Schepmann]

zOMG now I opened the can of worms...

I think it was me who opened the can of worms ... perhaps you just gave the open can a kick 

But anyway, thanks for the insight to everyone! Learning something new every day.

Cheers,
Dirk

[Hardy Heinlin]

The reference to the CMC ground test is in an "OR" section.

Ah, right. In the back of my head there was an "AND". All fine now :-)

[Hardy Heinlin]

EDIT: I'm not sure about (a). I don't know if they're talking about the FQPU/FQIS being powered up from a dead ship, the CB's being cycled, etc. I guess the key word is "up"... rather than just "powered".

In my current model you can cycle D5 and D6 on P6 to repower the FQIS in flight at any time, and this way you can restart the 8 minute timer before its time-out.



8 minute stuff is now in PSX 10.33:

http://aerowinx.com/board/index.php?topic=4191.0

[John H Watson]

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.

I don't see how cycling the CBs will recreate a ground-air transition. Won't the FQPU just see Air straight away? Is the FQPU just seeing air/ground relays or something else?

[Hardy Heinlin]

Two of the three OR conditions (power-up, CMC, refueling) include no on-ground condition in the text. Any of these three conditions can set a certain data bit which can inhibit the X FEED CONFIG message. This bit will be reset when the aircraft has been in the air for 8 minutes. The timer starts when the aircraft is airborne.

CMC implies on-ground because CMC tests usually cannot be performed in flight. And refueling implies on-ground too (it's even explicitely mentioned). But the power-up does not imply on-ground conditions. So the power-up (CB cycling) can set the bit again, even in flight. And when the bit is set and the aircraft is airborne, the 8 minute timer restarts.

That's how I understand it. The effect is not dangerous. It's just a play with the system to illustrate the principle.

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.

We could also assume this text is based on another text or diagram and perhaps has lost some logical parantheses in the final copy; maybe it originally went like this:

(
The fuel quantity processor unit is powered up
OR The CMC ground test >FUEL QUANTITY IND is performed
OR The refuel panel door is cycled from open to closed
)
AND in ground mode

It's strange anyway that it mentions the ground mode in the refueling condition; has anybody ever refueled the 747 in flight?


Cheers,

|-|ardy

[John H Watson]

It's strange anyway that it mentions the ground mode in the refueling condition; has anybody ever refueled the 747 in flight?

I've seen the refuelling panel under the wing left open due to miscommunication between engineers and refuellers (but noticed just prior to pushback). The panel is hinged on the forward edge. When fully open, the door is latched with a mechanically resistive device (or some kind of spring clip, I can't remember). Also at the fully open position, a magnetic switch is activated (which signals "fuelling in progress"). The door may be blown away from the fully open position (depending on the strength of the wind and the condition of the resistive device/clip). This should end the "fuelling in progress" logic. Perhaps, if the resistive device/clip was faulty (too tight), the fuel door may not move away from the fully open position until the aircraft lifted off. i.e. it would satisfy the open to close cycling logic in flight.

(Aside) Sometimes if the magnet part of the magnetic sensor falls off, we have to fit a temporary magnet to fool the magnetic switch during refuelling. If the magnet is left on, it can create all kinds of chaos in flight.

Any of these three conditions can set a certain data bit which can inhibit the X FEED CONFIG message.

Sorry, I'm still trying to wrap my head around this stuff. The new logic is designed to prevent (inhibit) both messages (>FUEL TANK ENG and X FEED CONFIG) appearing during takeoff and initial climb, yet all the logic seems focussed on enabling the messages   

(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.

You obviously want to take off with no messages and have no new messages appear for the first 8 minutes after the ground to air transition (if at all). Does the new inhibit logic just apply to immediately after liftoff until 8 minutes has elapsed? (or is there a ground element?). e.g. if the tanks start out exactly equal (and the tanks are in a TANK to ENG configuration... and the engines are individually started 1~4, but #2 has start problems and takes a while to start (Meanwhile #1 is burning fuel and is now below #2). Do we now need to start talking about inhibiting the X-FEED CONFIG message or should that remain active? Maybe I should post the logic for that (from the FIM)?

[Hardy Heinlin]

The new logic is designed to prevent (inhibit) both messages (>FUEL TANK ENG and X FEED CONFIG) appearing during takeoff and initial climb, ...

Both messages cannot appear at the same time anyway -- regardless of the upgrade.

There is a main-tanks-equal point on the timeline.

Before this point, only the X FEED CONFIG message can appear (if fuel feed is closed to early).

After this point, only the TANK TO ENG message can appear (if fuel feed is still open).

When we have 1000 lbs more on the inboard, we are before the point. Before the point, we only need to inhibit the X FEED CONFIG message. Before the point, the TANK TO ENG message will not appear anyway.

When we have 1000 lbs more on the inboard, and we close the fuel feed, we get the X FEED CONFIG message. We need to inhibit that. Just that.

[John H Watson]

Thanks... getting clearer now.

Just an excerpt from the FIM to remind me....

(2) The TANK TO ENGINE ALERT discrete is not set when any of these conditions are true:
(a) The airplane is in refuel mode (in ground mode and the refuel control panel door is open).
(b) The M2 fuel quantity is more than 32,000 lbs (14,500 kgs) or the M3 fuel quantity is more
than 32,000 lbs (14,500 kgs).
NOTE: M2 fuel qty is more than M1 VTO+2000 lbs (910 kgs) and M3 fuel qty is more than
M4 VTO+2000 lbs (910 kgs).
(c) The M2 fuel quantity is more than M1 and the M3 fuel quantity is more than M4.
(d) The M2 fuel quantity is more than 1000 lbs (455 kgs) greater than M1 and the M3 fuel
quantity is more than 1000 lbs (455 kgs) greater than M4.

I see now at (a), that the message can appear in the air with the fuelling door fully open (and other inhibits are not applicable). This is not a likely scenario, though.

[Hardy Heinlin]

(c) The M2 fuel quantity is more than M1 and the M3 fuel quantity is more than M4.

(c) is only valid when the discrete is not set yet. Once it is set, (c) cannot reset it. Only the VTO+2000 condition can reset it. (c) is part of the hysterisis.

I see now at (a), that the message can appear in the air with the fuelling door open (and other inhibits are not applicable)

Understood. But why should the on-ground condition only be valid in connection with the refueling action? Aside from the fact that an inflight timer reset triggered by the refueling system is neither dangerous nor probable. Why should the on-ground condition not be connected with the power-up action as well? That action is even more likely to happen in flight (momentary DC bus failure).

I bet the on-ground thing is a "text layout" mistake and should actually refer to all three OR-conditions, not just to the refueling action.


By the way, if the fueling door closure is the trigger point in time where the 1000 lbs excess is measured, the final difference may be higher than 1000 lbs because, as we know, there is a lag in the sensed quantity indication. When the discrete has been set while the difference was 900 lbs, and the sensed difference now increases to 1100 lbs, you can now reset the discrete by repowering the FQIS. The repowering event measures the difference again, and it now sees it's more than 1000 lbs.

[John H Watson]

I'm wondering exactly what happens as a result of these three things (especially with the old software):

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.

With a), does powering up the FQPU on the ground (only) make the cards go through a self test and stop the message appearing/reappearing? (even if the logic says it should)
With b), on the ground (Ground Tests need the airplane to be on the ground), does it clear messages and stop them reappearing?
With c), the message is inhibited with the fuelling panel open. What happens when the door is re-closed? Does it re-arm the message or does the message normally stay away?

Is the new logic piggybacked on the old logic and designed to inhibit the normal(old) behaviour of the FQIS logic in response to these events. 

Re a) Note that some aircraft computers behave differently on power up. e.g. when the Yaw Damper is powered up on the ground it runs through a test cycle. When the Yaw Damper is powered up in the air, it doesn't go through a test cycle (it gets straight to work).

Hope this makes sense 

(EDIT Disregard... If the airplane behaved this way with the old logic, it would be dangerous... The pilots wouldn't get the message during their preflights.

[Hardy Heinlin]

I'm wondering exactly what happens as a result of these three things ...

It sets a discrete in the memory of the EIU (or elsewhere). This discrete inhibits the X FEED CONFIG message. Upon lift-off an 8 minute countdown starts. When the countdown is done, the discrete is reset, and the X FEED CONFIG message is no longer inhibited.

That's how I understand it.

This upgrade is just designed for the range between 0 and 1000 lbs. Outside this range it has no effect, and all will work as usual.