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Author Topic: FUEL TANK/ENG condition  (Read 2098 times)

Dirk Schepmann

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FUEL TANK/ENG condition
« on: Tue, 17 Apr 2018 09:07 »
Hi all,

I have a question regarding the system Logic of the Tank-to-Engine condition.
As we all know, when the amount of fuel in tanks 2 and or 3 is equal or less than the fuel in tanks 1 and 4, the related EICAS message appears to remind the crew to re-configure the tanks.

But what happens when a pilot is faster (or smarter ;-)) than the system logic? I tried it out with PSX and monitored the amount of fuel in the tanks. As soon as the amount in tank 2 and 3 were equal to 1 and 4 (13.2 tons), I quickly configured the tank to engine config and the EICAS message never appeared. This is what I expected.

Interestingly, I couldn’t reproduce this in the PMDG product. Instead, I got a FUEL X-FEED message, although the amount of fuel was equal in all tanks. PMDG claims that the fuel indicated on the synoptic page is not accurate and that the O/J pumps have been switched off too early. They also state that the system logic requires pilots to wait for the FUEL TANK/ENG message to show up before they actually turn off the O/J pumps and crossfeed valves.

I couldn‘t find detailed information about this in my documents.
In reality, pilots will probably never spend unnecessary time to cheat with the system like I did ;.) - but what is the actual logic behind it? Some tolerance with the amounts (like it obviously is in PSX) or a strict equal tank condition?

Best regards,
Dirk 

United744

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Re: FUEL TANK/ENG condition
« Reply #1 on: Tue, 17 Apr 2018 11:08 »
IIRC there is a bracket where no messages will be generated. It is to prevent spurious alerts.

e.g. Below 10T you get "TANK TO ENG", and above 12T you get "OVRD PUMP X" and "FUEL XFEED"

So if you switch when between these two levels, you will not get an alert.

However, in PSX, the logic seems to work that if it is NOT "TANK TO ENG" then it will alert after a few seconds for the other option.

I have raised this before, as sometimes PSX would request the OVRD PUMPS ON again even when the aircraft should be TANK TO ENG.

Note this was a while ago, so I'm not sure if this has been changed recently.

Hardy Heinlin

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Re: FUEL TANK/ENG condition
« Reply #2 on: Tue, 17 Apr 2018 11:54 »
Hi Dirk,

in PSX (and certainly in the real system) there is a hysterisis of some kilograms to avoid nuisance messages. Remember, the message logic is sharp and the physical fuel volume-and-density is a big dynamic object; when the sensed volumes and sensed densities are dancing around a trigger value by just the width of a molecule you would get blinking nuisance messages.


Regards,

|-|ardy

John H Watson

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Re: FUEL TANK/ENG condition
« Reply #3 on: Tue, 17 Apr 2018 12:03 »
In the Boeing Wiring Schematics, there is mention of a 120 second time delay for the FUEL TANK/ENG after the fuel has reached an equal quantity. Add this to pilot reaction time and I can see how the fuel quantities in the tanks will be well and truly balanced (i.e. minor fuel quantity measurement variations will be resolved). I guess it's more "hit and miss" if you reconfigure prior to the message appearing.

Hardy Heinlin

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Re: FUEL TANK/ENG condition
« Reply #4 on: Tue, 17 Apr 2018 12:17 »
This 120 sec delay is the time required to burn approx. 300 kg in total. It makes sense as it works like a quantity based hysterisis. The gap of 120 sec on the time line is also a gap of 300 kg on the quantity line. Divided by the number of tanks, each tank will refer to a fraction of 300.

Hardy Heinlin

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Re: FUEL TANK/ENG condition
« Reply #5 on: Wed, 18 Apr 2018 05:03 »
In typical cruise flight, if the message is delayed by 120 seconds, the message will appear when the inboard main tanks each contain 200 kg less than their respective onside outboard main tank.

So readers of the manuals shouldn't be confused if the text reads "equal or less". It's always way less than "equal". Or the message timer is started when the inboard tanks are 200 kg above, and at the timeout the tanks are equal.

John H Watson

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Re: FUEL TANK/ENG condition
« Reply #6 on: Wed, 18 Apr 2018 06:09 »
Quote
In typical cruise flight, if the message is delayed by 120 seconds, the message will appear when the inboard main tanks each contain 200 kg less than their respective onside outboard main tank.

That was my thought, too, and this prevented the flip/flop between messages. Pilot input required? When I checked fuel levels on the ground after a flight, I couldn't really say if this was so because of (normal) uneven engine fuel burn and APU usage.

For engineers... the logic is shown on SSM 28-41-02 Page 101 sheet 3 (although for some strange reason, the alert is called a memo message).


John H Watson

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Re: FUEL TANK/ENG condition
« Reply #7 on: Sat, 21 Apr 2018 02:58 »
I just noticed in PSX that I'm getting the >FUEL TANK/ENG message with an inboard tank consistently above the adjacent outboard tank (by 0.1 tonnes). The F/O, if activated, also responds prematurely.

I can't remember if we discussed this during beta testing, but the logic is:

If the fuel in the inboard and outboard tank is equal (or less than) on any wing, the message will appear. You can reset the message by reconfiguring the fuel panel or by:

(during refuelling) increasing the level of the fuel in the inboard tank by 2000lbs more than the outboard...  but it has to be greater on both wings.




« Last edit: Sat, 21 Apr 2018 03:12 by John H Watson »

Hardy Heinlin

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Re: FUEL TANK/ENG condition
« Reply #8 on: Sat, 21 Apr 2018 04:00 »
This 0.1 on the EICAS is due to display rounding. The true sensed (and lagging) difference in the background is not higher than exactly zero.

Anyway, in the next update that 120 sec delay will rigorously take effect, and thus, in flight, the message will only appear when the difference exceeds ca. 200 kg.


Quote
If the fuel in the inboard and outboard tank is equal (or less than) on any wing, ...

Is this pair-wise just left wing and just right wing? Or does "any wing" mean "anywhere cross-wise", e.g. M2 <= M4?

In PSX since version 10.0.0, tank-to-engine begins if ...

Code: [Select]
if M2 <= M1
or M2 <= M4
or M3 <= M1
or M3 <= M4
« Last edit: Sat, 21 Apr 2018 05:12 by Hardy Heinlin »

John H Watson

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Re: FUEL TANK/ENG condition
« Reply #9 on: Sat, 21 Apr 2018 10:20 »
The first inboard tank to go equal with its adjacent (same wing outboard tank) tank will trigger the 120 second timer.

[(M2<=M1) or (M3<=M4)] plus 120seconds = message.

The reset is a bit more complicated than I thought and involves M2 and M3 reaching the VTO  (max volume) of the inboard tanks plus 2000lbs

FUEL TANK/ENG Logic
« Last edit: Wed, 25 Apr 2018 07:52 by John H Watson »

Dirk Schepmann

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Re: FUEL TANK/ENG condition
« Reply #10 on: Sat, 21 Apr 2018 10:45 »
Thanks for the replies.

But I think it‘s safe to assume that if the pilot doesn‘t necessarily has to wait for the TANK/ENG message to appear, correct?

If the pilot selects the tank-to-engine configuration at the appropriate time, he‘ll neither get a crossfeed advisory nor a TANK/ENG message.

After seeing so many discussions about alert inhibits, timers and other system logic stuff I really start to admire the engineers who implemented all this stuff roughly 30 years ago.

John H Watson

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Re: FUEL TANK/ENG condition
« Reply #11 on: Sat, 21 Apr 2018 13:36 »
Quote
If the pilot selects the tank-to-engine configuration at the appropriate time, he‘ll neither get a crossfeed advisory nor a TANK/ENG message.

In a perfect world, yes. However, the fuel quantity system is not entirely perfect. Although there are multiple sensors in each tank at different points in the tank, if the aircraft attitude/speed changes quickly or the aircraft hits turbulence, the fuel quantity processor may register a false level during this X-FEED CONFIG ~ FUEL TANK/ENG transition.

There is also a delay on the X-FEED CONFIG message, so it may also help reduce message flip-flop.

Hardy Heinlin

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Re: FUEL TANK/ENG condition
« Reply #12 on: Sat, 21 Apr 2018 13:41 »
On the Set/Reset AND-gate, shouldn't there be a feedback loop from the gate output back to the "SET" input, so that the SET input remains set when M2>M1 and M3>M4?

M2>M1 and M3>M4 must not reset the SET state. Only the VTO stuff may reset it.

Or is it sufficient if the symbology includes the word "SET"? Is this a standard word indicating that this input is self-holding?

Jeroen Hoppenbrouwers

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Re: FUEL TANK/ENG condition
« Reply #13 on: Sat, 21 Apr 2018 13:45 »
And to add engineering hell, all wiring associated with the FQIS needs to be treated as if it was the fuse wire to the bomb. When installing SATCOM, for example, you need to keep your wires (audio, 429, power) 2 inch or so away from any FQIS bundle to avoid sparking between wires and potential induction of high voltages to the fuel quantity sensors and setting off an explosion.

Modern systems use fibre optics for the FQIS, and only for the FQIS... talk about precautions.


Hoppie

John H Watson

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Re: FUEL TANK/ENG condition
« Reply #14 on: Sat, 21 Apr 2018 13:51 »
Quote
Or is it sufficient if the symbology includes the word "SET"? Is this a standard word indicating that this input is self-holding?

Most likely.

That VTO logic is interesting. I always assumed that you would get the TANK/ENG message if you loaded the tanks:

M1  10.0, M2 11.0, M3 11.0, M4 10.0...

 and configured the aircraft to burn fuel from the inboards.

Of course, you would have to manually control the refuelling valves to get this configuration prior to flight. Normally, however, you would have the refuelling system carry out everything automatically and so you wouldn't get that configuration.

(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)
« Last edit: Sat, 21 Apr 2018 14:06 by John H Watson »

John H Watson

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Re: FUEL TANK/ENG condition
« Reply #15 on: Sun, 22 Apr 2018 01:12 »
Ugh... more complications...

Just reported on PPRuNe...

In someone's QRH and I now see it in ours...

>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

I didn't think of looking in the QRH for such detailed information.



Hardy Heinlin

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Re: FUEL TANK/ENG condition
« Reply #16 on: Sun, 22 Apr 2018 01:29 »
So in these cases the reset occurs at plus 1100 LBS (500 kg) instead of plus 2000 LBS?

Perhaps these two different values are not really different; maybe they refer to the same hysterisis function as shown in that logic gates diagram. Taking all that sensor lag and sensor tolerances into account, those 450 kg (900 lbs) are just peanuts, aren't they? Subtract a few pounds from the 2000 value and add a few to the 1100, and consider the rounded indications on the EICAS, there won't be any dramatic difference. Aren't these values, 2000 and 1100, just approximations anyway?

John H Watson

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Re: FUEL TANK/ENG condition
« Reply #17 on: Sun, 22 Apr 2018 04:44 »
My first thought was that the reset logic was unrelated. I thought this new logic might apply to the situation where the inboards were a few hundred kilos (but less than 500) above the outboards with the outboard tanks not necessarily full. A sort of a preflight reminder. This tolerance probably allows for APU fuel usage (and any messages related to the inboards being below the outboards as a result of APU fuel usage).

e.g..... M1 10.0, M2  10.5, M3 M 10.5, M4 10.0 ...  TANK/ENG message appears, but not at M1 10.0, M2 10.6, M3 10.6, M4 10.0

Hardy Heinlin

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Re: FUEL TANK/ENG condition
« Reply #18 on: Sun, 22 Apr 2018 11:57 »
Is that logic diagram very old? Perhaps in a later version the memo message was turned into an advisory and the hysterisis of 2000 lbs changed to approx. 1100 lbs -- and they forgot to update the diagram?

Edit:
That diagram doesn't specify whether those 2000 lbs refer to inflight or to on-ground resets. -- The other text "On the ground after refueling, ..." etc. seems to refer to on-ground operations only. However, on a normal flight the tank quantities will not increase; so after all this other text too actually refers to all conditions, including inflight, I think. It probably mentions refueling etc. only because such actions can increase the quantities, and the actual purpose of this text is probably just to tell the values. What I'm trying to say is that both the diagram and the other text probably refer to the same hysterisis function, and that the hysterisis probably makes no difference between inflight and on-ground conditions (see diagram), and that this "VTO plus 2000 lbs" thing is, perhaps, now to be replaced by "inboard tank (not VTO) plus 500 kg".
« Last edit: Sun, 22 Apr 2018 13:03 by Hardy Heinlin »

Jeroen Hoppenbrouwers

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Re: FUEL TANK/ENG condition
« Reply #19 on: Sun, 22 Apr 2018 14:07 »
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"?


Hoppie