FUEL TANK/ENG condition

[Hardy Heinlin]

What do you think about my new theory that the 120 sec timers might be started by something not related to the FQIS output?

[John H Watson]

The hysterisis is provided by the reset logic of the "equal+2000 lbs" stuff.

Sorry.. I'm still trying to catch up here. I'm not 100% sure how that reset/set gate works... or if it is a true reset/set gate.

http://www.members.iinet.net.au/~b744er@ozemail.com.au/PSX/FUELTANKENGlogic.gif

This is how I see it:
The "VTO" reset is a fixed fuel quantity value. i.e. inboard qty = outboard VTO qty plus 2000lbs (not just equal qty plus 2000lbs). When any inboard tank fuel qty goes below (or is equal to) outboard VTO plus 2000, zero logic outputs from the "VTO" gate to the inverter on the next AND gate and (assuming there are no huge 2000lbs variances) that next AND gate will always have an OK signal on the bottom leg. This leaves the M2<=M1 or M3<=M4 logic to output a tanks equal signal. I can see that minor variations in quantity due to turbulence can produce a varying TANKS EQUAL output to the EIU (where the mysterious 120 second timer exists).  That's why I think the timer is required (at least for the TANK/ENG message).

The VTO gate is an AND gate. I don't know if that adds an extra layer of complexity(?).

[Jeroen Hoppenbrouwers]

"One to two hundred kg" may very well mean "one hundred to two hundred".

[John H Watson]

"One to two hundred kg" may very well mean "one hundred to two hundred".

Hoppie...  1kg is not visible ... nor 10kg

[Jeroen Hoppenbrouwers]

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.

Just making sure.

[Hardy Heinlin]

That's a good point, Hoppie. I thought he meant to refer his "1", which cannot be seen on the EICAS, to the theory in the manuals that say "anything beyond equal". Sort of a poetic "1". But I think he's no poet and actually meant to say "100" to 200 because that can be seen on the EICAS indeed.
So I agree. I bet he said (and meant to write) "one hundred to two hundred".

The hysterisis is provided by the reset logic of the "equal+2000 lbs" stuff.

Sorry.. I'm still trying to catch up here. I'm not 100% sure how that reset/set gate works... or if it is a true reset/set gate.

I think it's a reset/set gate. Otherwise the VTO+2000 stuff wouldn't make sense. It's the only gate on the diagram that has SET/RESET labels. So they must have a special meaning. Those +2000 lbs provide a safe hysterisis zone.

Another theory: As this diagram is a compilation of two documents -- FQIS and EIU -- maybe they refer to two different versions. We have two hysterises here, one using 2000 pounds, the other using 120 seconds. Perhaps there was a version where just pounds or just seconds were used. So now in this compiled diagram one of the two methods may be a relict from an older version and should be removed.

[John H Watson]

Sorry, I thought this kind of expression was universal. e.g. 2~3000 usually means 2000~3000. Just a shorthand way of writing it.

The fault isolation manual* text is a written expression of all the logic and contains both the VTO and the 120 seconds. I don't think one set of logic is obsolete. Perhaps there is some situation which requires both. Might the VTO logic stop the messages appearing and disappearing all the time during refuelling? In the real world, during refuelling, the tanks are not refilled at the same rate.... Fuelling favours the tanks closest to the tanker (but imbalances are from time to time corrected).

The 120 second logic may be just to help with quantity fluctuations around the time the tanks reach the same value in flight.

*Note that the FIM is up to date as it contains the additional new logic.

[Hardy Heinlin]

Might the VTO logic stop the messages appearing and disappearing all the time during refuelling? In the real world, during refuelling, the tanks are not refilled at the same rate.... Fuelling favours the tanks closest to the tanker (but imbalances are from time to time corrected).

So it could be that the 120 sec delay for the X FEED CONFIG is in fact designed for the refueling process, not for the fuel burn process(?)

[John H Watson]

Every time I look at the diagrams I seem to read them differently, but no, I don't see how 120 seconds would be long enough to take care of inboard/outboard imbalances during refuelling. I thought the VTO might offer longer term message inhibits (in combination with other parts of the logic). Anyway, looking at the FIM again, I see that the logic inhibits both messages during refuelling anyway.

In a YouTube video, a pilot said selecting the crossfeeds on during preflight "arms" the logic, but I don't know if this is part of the old or new logic or even if this is correct. The pilots do seem to turn on the crossfeeds irrespective of load. I don't know if this is simply a procedure to help the pilots remember to turn on the crossfeeds or if it does arm something.

[simonijs]

Most likely I will be talking nonsense here (I am not very technically minded...), but could it be possible that these 120 seconds in the drawings only apply to aircraft that use main tank 2 for APU fuel?
If the APU is used for 20-30 minutes before Take-Off, main tank 2 will contain between 100 - 150 kgs less fuel than main tank 3. In flight, main tank 2 will reach "equal to or less than (main tank 1)", when main tank 3 quantity is still above the fuel quantity in main tank 4. Depending on duration of APU usage on the ground and fuel flow in flight, it will take a minute or more before main tank 3 also conforms to "equal to or less than ...", thus satisfying   > FUEL TANK/ENG.

Regards,
Simple Simon

[John H Watson]

but could it be possible that these 120 seconds in the drawings only apply to aircraft that use main tank 2 for APU fuel?

Nice theory. Note, however, that our fleet has both types, but they seem to have the same logic in the FCOM and Maintenance Manuals. Also, 150kg is not a lot in the real world. Engines burn fuel at different rates. I may even have seen 100kg difference between random tanks directly after refuelling.

[John H Watson]

As an afterthought... I seem to recall (vaguely) that the APUs which use both inboard tanks still tend to favour the left inboard tank (slightly). Perhaps it's because the fuel has to travel a greater distance from the right wing. The fuel line to the APU still goes via the #2 tank.

Independent verification required