FUEL TANK/ENG condition

[John H Watson]

I was just trying to find a reason for why the text only said "on the ground" for the refuel panel door. I'm not entirely convinced that it's a bracketing problem.

Anyway, here's the FIM tests for the >X FEED CONFIG

841. EICAS Message >X FEED CONFIG (ADVISORY) - Fault Isolation
A. Fault Isolation Procedure
(1) The EICAS message > X FEED CONFIG (ADVISORY) will show when one or more of these
conditions are true:
(a) The No. 2 and/or 3 crossfeed valve(s) are closed with the flaps in the retracted position (10-
second time delay).
(b) The No. 2 and 3 crossfeed valves are not automatically commanded to the closed position
(takeoff command) with the flaps set for takeoff and the airplane in ground mode (one
minute time delay).
(c) The No. 1 and/or 4 crossfeed valve(s) are closed and the TANK TO ENGINE ALERT discrete
is not set (two minute time delay).
(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.

(3) Corrective Action
747-400
  (a) If the EICAS message >FUEL X FEED CONFIG (ADVISORY) shows, then do these steps to
clear the message:
1) Close the refuel control panel, P42 (if open).
2) Put the applicable crossfeed valve(s) in the correct position:
a) Open the No. 2 and 3 crossfeed valves if the flaps are retracted (not in the takeoff
position).
b) Make sure the No. 2 and 3 crossfeed valves are closed (automatic operation) when
the flaps are extended for takeoff and the airplane is in ground mode.
c) Open the No. 1 or 4 crossfeed valve(s) if the inboard main tank(s) contain more fuel
than the outboard main tank(s).
BAB ALL; AIRPLANES WITH FQPU BLOCK B UPGRADE (SB 28-2182)
d) Close the No. 1 and 4 crossfeed valves if: The M2 fuel quantity is not more than 1000
lbs (455 kgs) greater than M1 and the M3 fuel quantity is not more than 1000 lbs (455
kgs) greater than M4 and one or more of these events have been accomplished:
<1> The fuel quantity processor unit is powered up
<2> The CMC ground test >FUEL QUANTITY IND is performed
<3> The refuel panel door is cycled from open to closed and in ground mode.

(b) If the airplane is in the correct system configuration and the EICAS message >FUEL X
FEED CONFIG (ADVISORY) continues to show, then do these steps:
1) Confirm the correct airplane configuration per the above logic.
2) Make sure a magnet is not installed on the fueling power control switch (S681) (P42
control panel).
NOTE: The magnet may have been left on the S681 control switch after a refuel
operation. On the ground, with the magnet attached to the S681 control switch,
the FQPU will be in refuel mode, regardless of refuel door open/closed
position. With this condition, the >FUEL X FEED CONFIG (ADVISORY) will
continue to show until the No. 1 or 4 crossfeed valves are opened. This
condition can cause an incorrect preflight fuel-feed configuration (No. 1 and 4
crossfeed valves open and main tanks even).
3) Make sure the refuel panel door is closed and properly latched.
4) Use the input monitoring system to check the REFUEL DOOR OPEN discrete: E/15/355/
11.
a) Bit 15 = one (DOOR OPEN)
b) Bit 15 = zero (DOOR CLOSED)
5) If bit 15 is incorrect for the state of the door (open/closed), then do a check of the wiring
from the S681 control switch to the FQPU, M7860 (SSM 28-21-08) (WDM 28-21-23).
6) If the door open discrete is OK, then coninue.
(c) If the >FUEL X FEED CONFIG (ADVISORY) continues to show, do these steps:
1) If the EIU DISAGREE (STATUS) also shows, first do the corrective action for EIU
DISAGREE (PAGEBLOCK 31-61-00/101 Config 1).
2) Look for one or more of these CMCS messages:
a) 27600, 27602, 27627, 27628, 27679, 27687, 28676, 28677, 28774

[Hardy Heinlin]

(c) The No. 1 and/or 4 crossfeed valve(s) are closed and the TANK TO ENGINE ALERT discrete
is not set (two minute time delay).

Does that mean that the X FEED CONFIG message is still enabled until the 2 minute countdown is over?

I thought there are three phases:

Phase 1: Tanks not equal, X FEED CONFIG msg enabled, FUEL TANK ENG msg disabled.

Phase 2: Tanks equal, X FEED CONFIG msg no longer enabled, 2 minute timer running, FUEL TANK ENG msg still disabled.

Phase 3: 2 minute timer passed, FUEL TANK ENG msg enabled.


In phase 2 both messages are disabled, I thought. The above quote suggests that the crew will always see a message, either X FEED CONFIG if they act too early, or FUEL TANK ENG if they don't act:

Phase 1: Tanks not equal, X FEED CONFIG msg enabled, FUEL TANK ENG msg disabled.

Phase 2: Tanks equal, X FEED CONFIG msg still enabled, 2 minute timer running, FUEL TANK ENG msg still disabled.

Phase 3: 2 minute timer passed, X FEED CONFIG msg disabled, FUEL TANK ENG msg enabled.

[Phil Bunch]

Don't forget that the US President's personal 747 can be refueled in the air.  Since it is fairly old, I wonder if much common software or hardware is shared by commercial 747s and "Air Force One"?  Air Force One is based on a 747-200B. 

Interestingly, the proposed replacement Air Force One (also a 747), may not have in-air refueling capability:

https://www.military.com/dodbuzz/2017/09/26/congress-questions-air-force-ones-lack-of-refueling-capability

[Hardy Heinlin]

I forgot "Air Force One" indeed :-)

[John H Watson]

Any clues here?

(Old software/hardware)

X FEED CONFIG logic diagram

[Hardy Heinlin]

These logic lines that start with the word "120 SEC" come from nowhere. The diagram doesn't indicate what condition will start the 120 SEC timer.

For the FUEL TANK ENG message we know that the timer is started when the "MAIN TANKS EQUAL" signal is set (from the SET/RESET OR-gate output).

For the X FEED CONFIG message, which has its own 120 SEC input to another AND-gate, I don't know what condition will start that timer. That AND-gate has an inverted input from the "MAIN TANKS EQUAL" signal. Now is the timer starter linked with a point before the invertion or after the invertion? If the starter sits before the invertion, it starts running when the tanks become equal (inflight scenario); if it sits after the invertion, it starts when the inboard quantities become higher (refueling scenario).

If we assume that the designers wanted to have a delay for inflight scenarios (not refueling scenarios), the timer starter probably sits before the invertion, i.e. at the same spot where the other 120 SEC timer is started.

[John H Watson]

These logic lines that start with the word "120 SEC" come from nowhere. The diagram doesn't indicate what condition will start the 120 SEC timer.

You have to wonder why they simply didn't put a timer in the circuit instead of an AND gate. Timer symbols can have a "0" or "1" symbol indicating a delay on a particular input. What would a delay on all inputs do to the logic? (Give the pilots a reprieve from any messages for a few minutes?)

[Hardy Heinlin]

Is there any 747 pilot on the forum who has ever set the tank-to-engine configuration at a time when the inboard quantities were within 0.0 to 0.2 tons below the outboards, i.e. before the FUEL TANK ENG message appeared? (It only appears when more than 0.2 tons below, not when equal.) And when you did so, was the X FEED CONFIG message shown until the differences were greater than ca. 0.2 tons?

I conclude from the complete diagrams that there is no point in time where neither the X FEED CONFIG nor the FUEL TANK ENG message is shown. Either one will always appear. The crew can't hit the right moment. They can only act too early or too late. No neutral phase in between. And the change-over point is never at the tanks-equal condition as most manuals suggest, but 2 minutes later.


|-|ardy

[mark744]

This will probably be no help whatsoever, as no rigorous investigation. but I have occasionally set tank-to-engine before the EICAS message appears, (when the tanks were clearly balanced and I was impatient). Usually then get no EICAS message. Occasionally I have subsequently got Crossfeed Config message, and had to briefly go back to the previous pump config. But I didn't notice exact tank quantities. I'll try to be more methodical, maybe balance 2 to 3 and 1 to 4 before tank-to-engine, to get ideal conditions, then investigate.

[Hardy Heinlin]

Thank you. I think the balance is not required for the message logic. Just one inboard needs to be 0.1 tons lower than its onside outboard. At that point the timer has been running for about 1 minute, and thus the X FEED CONFIG message should still appear when setting tank-to-engine.

[mark744]

maybe by the time I had completed moving the switches, the conditions were then correct to not then give the X-FEED CONFIG message ?

[Hardy Heinlin]

Yes, maybe there's another, undocumented delay on top of the 2 minute delay, just before the final message trigger. Perhaps 10 or so seconds ...

[Jeroen Hoppenbrouwers]

I've seen EICAS itself doing delays of 7-10 seconds after I flip the bit going to it. So unless EICAS has the 120 second timer, I would not be surprised to see an additional bit of delay, to know for sure that the 429 word on the bus is stable.

SELCAL though needs about 1 second only, if at all.


Hoppie

[torrence]

 Philosophical Comment: This thread makes me wonder if some of the "message-timing logic" that isn't absolutely critical for safety wasn't just thrown together to get the job done on the real aircrafts and software. 

Cheers,
Torrence

[Hardy Heinlin]

I've seen EICAS itself doing delays ...

You mean the symbol generator of the EICAS screen? Or the EIU box?

If I read the diagram correctly, the first three logic gates to the left are in the fuel quantity system, and the rest to the right sits in the EIU, including the 120 sec delays.


|-|ardy

[John H Watson]

If I read the diagram correctly, the first three logic gates to the left are in the fuel quantity system, and the rest to the right sits in the EIU, including the 120 sec delays.

Correct. I joined two diagrams together, one of the FQIS, one of the EIUs

[Jeroen Hoppenbrouwers]

You mean the symbol generator of the EICAS screen? Or the EIU box?

The EIU. I never talk directly to the display units. I think the symbol generator is just dumb and paints immediately whatever is sent to it.

Does anybody know whether the symbol generator contains the message texts, or the EIU? In other words, whether the whole message is a single symbol?


Hoppie

[Hardy Heinlin]

Will anyone disagree if I add those 2 minutes to the X FEED CONFIG message logic as well, as indicated in the diagram? I'd like to get this part finished.

This will not only inhibit the FUEL TANK ENG for 2 minutes after the tanks became equal (implemented in PSX 10.33); it will also show the X FEED CONFIG message when tank-to-engine is configured during that same 2 minute phase.


|-|ardy

[Hardy Heinlin]

These logic lines that start with the word "120 SEC" come from nowhere. The diagram doesn't indicate what condition will start the 120 SEC timer.

You have to wonder why they simply didn't put a timer in the circuit instead of an AND gate. Timer symbols can have a "0" or "1" symbol indicating a delay on a particular input. What would a delay on all inputs do to the logic? (Give the pilots a reprieve from any messages for a few minutes?)

Reconsidering these "open-end" signal paths of both 120 sec delays ...

It could be that these delay paths are started by an undocumented "extra feature" (instead of being directly started by the output of the fuel quantity system). This "extra feature" might be something completely different; e.g. related to power-up, air-ground, refueling, jettison, whatever. If one of these systems can start the delays, the 120 seconds may already be passed when the fuel quantity system sends the main-tanks-equal signal to the EIUs.

This may also explain why they didn't put the timer in the circuit but installed an extra path into an AND gate.


|-|ardy

(EDIT: A PPRuNer has finally given us the answer we were looking for. He says it's normal for the inboards to be 1~200kg below the outboards when the message appears)

If it's the whole range from 1 to 200 kg, there must be something special that does not always delay the signal by 120 seconds.

I think I'll remove that delay from PSX until we know for sure what condition starts the delay. It has no hysterisis function anyway. The hysterisis is provided by the reset logic of the "equal+2000 lbs" stuff.

[John H Watson]

A PPRuNer has finally given us the answer we were looking for. He says it's normal for the inboards to be 1~200kg below the outboards when the message appears)

As we know, cockpit observations are often generalisations until we start focussing on the subject. Was the pilot really looking at same wing differences or opposite wing differences.

Does the text fully agree with the logic diagram or is it too vague? (excluding the service bulletin)