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CaptainLars

Fuel Disagree Message, Diversion, Fuel Jettison...

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Hi,

something cool happened to me, but I don't know whether it's a simulated failure or rather a simulation failure. I was going on a trip from RJAA to SBGR, so I selected the JAL 77L with 3 auxiliary tanks. I know that in reality, there are no 77Ls with 3 aux tanks, but there's also no service from Tokyo to Sao Paulo...

Anyways, I took off from Narita and about 2 hours of flight time, well shortly before I would arrive at the outermost of the Aleutian islands, I got an EICAS message saying "Fuel Disagree". I noticed that I really had considerably less fuel than expected (the aux tanks were already completely drained). I was quite thrilled and brought up the ECL -> Fuel Disagree Checklist -> Fuel Leak Checklist. There was no fuel leak that I could detect, and so the plane wanted me to "use the more accurate value" of Totalizer vs. Calculated fuel quantity. Now I wondered which to choose. Judging from the hours flown so far, it had to be the calculated value, judging from the content of the tanks as displayed on EICAS, it had to be the totalizer value. I figured out I'd go with what the aircraft showed me, and I "cheated" a bit and took a look at the fuel quantity under "FS Actions" on the center FMC, and also in the W&B menu of the sim, and they all agreed with the totalizer value, which was about 30,000 lbs less than the calculated fuel.

After selecting the totalizer value, a quick look at the Progress page revealed that I would have no more than 1900 lbs of fuel left in Sao Paulo. So I turned the aircraft around and flew back to Japan, a 21-minute fuel jettison included, and safely landed in Narita.

Of course I researched a bit, and I found out that a) Boeing warns about this Fuel Disagree issue on the real plane (FCOM 1), and b) there have been simulation related issues of this kind back in 2013 and 2015 (Avsim). I wonder what was the cause of my failure. It never happened before and I did pretty much everything as I always do, the only difference being the fact that I entered the fuel value selecting "100%" instead of entering a number, since I had to use every bit of fuel possible. Upon taxi, I remember that everything seemed normal to me, but shortly after takeoff, I recall switching off the Aux Fuel Pump comparably early, i. e., compared to the one other flight where I needed it.

Could I this be a simulation issue, or is it a PMDG feature? If it's a PMDG feature, what was the simulated effect of having suddenly lost 30,000 lbs of fuel? I couldn't detect a fuel leak monitoring the wing tanks for over half an hour, and there seems to have been no drain afterwards.

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Interesting, I am focused on your entry of fuel using the 100% load button.  This is just the kind of thing that we might run into during beta testing, the bugs usually just appear rather than being found with any planned methodology.  It's always, "uh I wonder why it did that" moments that lead to the odd little bug that might not be found in years of use by thousands. It could be that the combination of 100% load and auxiliary tanks, or something else.  If you can repeat it would you please document it and submit to PMDG Product Support.

RJAA-SBGR got my attention.10,000 nm! Three auxiliary tanks? I've never tried that, only one auxiliary in my fleet and I don't recall who has it.  The great circle route heads South East of the US, interesting.

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On 22.9.2017 at 10:15 PM, downscc said:

Interesting, I am focused on your entry of fuel using the 100% load button.  This is just the kind of thing that we might run into during beta testing, the bugs usually just appear rather than being found with any planned methodology.  It's always, "uh I wonder why it did that" moments that lead to the odd little bug that might not be found in years of use by thousands. It could be that the combination of 100% load and auxiliary tanks, or something else.  If you can repeat it would you please document it and submit to PMDG Product Support.

RJAA-SBGR got my attention.10,000 nm! Three auxiliary tanks? I've never tried that, only one auxiliary in my fleet and I don't recall who has it.  The great circle route heads South East of the US, interesting.

Thanks for the input, I will try to recreate the situation and see whether I can get the same result.

The good thing about RJAA-SBGR is that it's actually doable with full pax + 5 tons of cargo. Trying LFPG-NTAA eastbound, flying north of Siberia (GC distance is "just" 8,500nm going via L.A.), is much more difficult, because of the necessity of ETOPS segments at the end of the route, and a lack of suitable redispatch airports.

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On 22.9.2017 at 10:15 PM, downscc said:

Interesting, I am focused on your entry of fuel using the 100% load button.  This is just the kind of thing that we might run into during beta testing, the bugs usually just appear rather than being found with any planned methodology.  It's always, "uh I wonder why it did that" moments that lead to the odd little bug that might not be found in years of use by thousands. It could be that the combination of 100% load and auxiliary tanks, or something else.  If you can repeat it would you please document it and submit to PMDG Product Support.

I tested today, using the percent method and setting pretty much everything like I did on that occasion, and wasn't able to recreate it.

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17 hours ago, CaptainLars said:

I tested today, using the percent method and setting pretty much everything like I did on that occasion, and wasn't able to recreate it.

That would be too easy.  Maybe it will happen again and if you compare the two scenarios something might appear salient.

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3 hours ago, downscc said:

That would be too easy.  Maybe it will happen again and if you compare the two scenarios something might appear salient.

I still want to get to Sao Paulo, I hope this time without any Disagree.

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I added Tropicalsim SBGR 2017 recently and flew AAL B77W AA950 KJFK-SBGR without anything unexpected happening. I never use 100% to fill the tanks so I am still wondering about that.

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21 hours ago, downscc said:

I added Tropicalsim SBGR 2017 recently and flew AAL B77W AA950 KJFK-SBGR without anything unexpected happening. I never use 100% to fill the tanks so I am still wondering about that.

Yesterday I made a new attempt to fly to Sao Paulo, saved and quit the sim. Today I resumed the flight and the plane greeted me with a "Fuel Low Aux" message, because "overnight" I had lost 5900 lbs of fuel... I entered the Calculated value into the center CDU, the Totalizer value jumped up, and so did the Calculated value. Now I have the correct amount of fuel in my aeroplane, but the wrong calculation.

So, I guess the issue is "Fuel in Aux" + "Resumed flight". It doesn't seem to be able to memorize the fuel in the aux tanks. Now if I like, I could go and find out what happens if you switch the aux fuel pump to off before saving, but I guess just the same. Or I could go and find out what happens when I save with P3D instead of my ATC program's save function.

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Meanwhile I arrived at SBGR and now I can state the following:

  1. When resuming a flight without fuel in Aux, the fuel quantity remains unaffected.
  2. The FMC calculates the remaining fuel on board at landing using the Calculated value, ignoring the Totalizer value, i. e., if you have, say, 29k lbs FOB (Totalizer), the FMC predicts the remaining FOB would be 27k lbs (assuming 35k lbs Calculated value FOB minus 8k lbs to be used). In reality (i. e., Totalizer value) the FOB to remain is going to be 21k lbs.
  3. After landing and parked at the gate, the Totalizer and the Calculated fuel values reconcile and do not diverge any more.

I'll test different conditions in the upcoming days.

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Submitting a ticket, since there seems to be no workaround around this problem; the only possible workaround being to fly at least until your auxiliary fuel tank is drained and only then save and resume.

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