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News: The latest PSX update (version 10.30 from 10 April 2018) is available at: http://aerowinx.com/board/index.php?topic=4191.0

Author Topic: FUEL TANK/ENG condition  (Read 190 times)

Dirk Schepmann

  • Join date: Aug 2014
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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

  • Join date: Oct 2014
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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).