PMDG777 Fuel Quantity Decrease during cruise

[pcubine 143]

Robin

 

Two things to consider. The possibility of confusion between Flight Levels in meters and feet and doing step climbs when the FMC says to do it.

[codechris 9]

Sorry, I know they are really obvious things to check, but always best to make sure you've not missed anything.

 

If it's only 10% of flights that sort of tells me that it's possibly you're hitting a strong headwind and that hasn't been calculated. In PFPX are you using ASN as the weather source? Is that setup correctly? ASN is open when you are planning in PFPX isn't it?

 

Again, it's all obvious but best to check everything

[Robinnn 13]

Robin

 

Two things to consider. The possibility of confusion between Flight Levels in meters and feet and doing step climbs when the FMC says to do it.

No sorry, no confusion about that. But thanks for thinking with me (and the rest)

 

Sorry, I know they are really obvious things to check, but always best to make sure you've not missed anything.

 

If it's only 10% of flights that sort of tells me that it's possibly you're hitting a strong headwind and that hasn't been calculated. In PFPX are you using ASN as the weather source? Is that setup correctly? ASN is open when you are planning in PFPX isn't it?

 

Again, it's all obvious but best to check everything

Yes, oboviously I have to check everything. But I do that every flight.

Very professional and I do not make any mistakes. I'm really sure about that.

 

As long this does not happen again I really can't show you (with a video) what really happens. 

The only thing I can hope, is that more people are having this issue so we can fix this.

But this is not a mayor big deal for me. But it would be nice to have this solved.

[downscc 1,327]

Very professional and I do not make any mistakes. I'm really sure about that.

 

Don't try too hard to convince us, saying things like that could be interpreted in many different ways.

 

May I suggest you save your PFPX to file and use a text editor to keep track of "actual" verses "planned" time enroute and fuel used. I've done this for many long flights and gained insights into relationships between the plan and the execution.  No need to log every fix, but keeping track of performance every 30 min will reveal pretty quickly if things are not going to plan.  It's also realistic to be looking at fuel at key waypoints such as the ETPs or re-dispatch fixes.  If you are going to trap the problem in 1 out of 10 flights this will be a good way to collect the data.

 

I have my own method that works for me, I use the blank space above the PFPX entered data and enter the actual numbers (I use Ultra Edit but any text editor should work) and I add the PERF page forecast fuel remaining in the right margin.  I found it very interesting to watch how FMS and PFPX FREM Fuel Remaining compared during flight:

                               245/98                  158.1 /  22.4       00+45  FREM 27.2
DCT      GOW       305  FL320  257/104   491    43     157.2 /  23.5   06  00:46
         GLASGOW          -45   115.40   410  3302  N5552.2 W00426.7 ...../.....

[codechris 9]

There is a section to do that in the print out from PFPX

[Mike777 163]

I do not know what causes this problem. As I just mentioned, it only happens in like 10% of my flights.

Robin,

 

I still suspect an issue with confusion about units of measure. Different

liveries - or even different individual aircraft with the same livery --

come with different units. Can you think about which liveries you use? Was the

aircraft from EHAM to HECA different from one that works?

 

This can and has happened in the real world. Do a web search for "Gimli Glider."

 

Mike

[skelsey 765]

 

 

When that is finnished I check my PROG page: At destination 8.6 (8600kg) left. Perfect!
During taxi (which is looooooooong at Amsterdam) the fuel at destination reduces a little bit, which is normal.
During climb it even decreases further, back to 5.0 at destination. At this point it got my attention. 
During cruise it goes further down, even to the point that there isn't even fuel left at destination.

 

What about the totalizer -- not the FMC calculated fuel at destination (which can for all sorts of reasons be wildly inaccurate) but the actual TOTAL FUEL display on the EICAS (I think -- sorry, familiar with the 744 rather than the T7). How does this compare with the flight plan fuel from PFPX? (My standard method is to record totalizer fuel remaining at each waypoint and subtract from the OFP fuel required (to destination from this point -- i.e. reads zero at destination) figure, which gives me a fuel remaining at destination figure which is completely independent from the FMC. Different OFP layouts will vary in the information provided, but using the totalizer value and the figure from the OFP will nonetheless give you an indication of how the actual burn is progressing compared to the OFP without involving the FMC).

 

I don't believe there's a winds issue here: the flight would have to take twice as long (i.e. nearly 8 hours) in order to burn the sort of figures we're talking about here.

 

The quantities involved do make me suspicious of a units issue: in effect, you needed roughly double the planned amount of fuel to complete the flight.

 

36252lb = 16443kg.

50000lb = 22679kg

 

36252lb + 50000lb = 86252lb = 39123kg -- suspiciously close to the original required figure of 36252kg.

[pcubine 143]

 

The quantities involved do make me suspicious of a units issue: in effect, you needed roughly double the planned amount of fuel to complete the flight.

 

36252lb = 16443kg.

50000lb = 22679kg

 

36252lb + 50000lb = 86252lb = 39123kg -- suspiciously close to the original required figure of 36252kg.

 

I agree that it appears that a units issue is involved but then I come back to what is in post #28.

"But at my FMC it looks like this: 30000 - 29995 - 29991 - 29895 - 29890 - 29885 - 29880 - 29800." These amounts must come from the FS Actions>Fuel. This is odd behavior for every third measurement. I am doing a flight right now and every decrease is by four pounds.

[downscc 1,327]

I agree that it appears that a units issue is involved but then I come back to what is in post #28.

"But at my FMC it looks like this: 30000 - 29995 - 29991 - 29895 - 29890 - 29885 - 29880 - 29800." These amounts must come from the FS Actions>Fuel. This is odd behavior for every third measurement. I am doing a flight right now and every decrease is by four pounds.

 

I agree Michael, thinking about it and first question is about an add-on interfering with the simulation clock. I don't have anything like that loaded on my rig but aren't there gadgets out there that could do this? I seem to recall someone praising the merits of a add-on to "fix" the FSX clock... just wondering.  The fuel consumption of the model is directly affected by anything that affects the FSX ticks. For example, Introduction pg 33 cautions against the use of any add-on that monitors fuel and gives you points based on a virtual airline management rule.

[kevinh 409]

It seems to me that the reduction in FMC estimated fuel remaining during a flight can only be due to one thing. Unexpectedly high headwinds. Or perhaps, perish the thought, forgetting to uplink the wind data from ASN so the FMC predicts fuel burn with still air.

[Robinnn 13]

Oke guys,

 

It is happening again so I recorded it.

 

 

It's a boring video, but it shows the fuel quantity decreasing rapidly.

 

So last week I flew with this paint (PH-BVD) from AMS to CAI and I had this problem.

The next day I flew from SIN to DPS with a different paint (PH-BVN) and I did not have this problem

And today I fly back from CAI to AMS with again the PH-BVD and I have the problem again.

 

Its to soon to tell if the paint is causing the issue because I can't remember what paint I used during the other flights.

Also, for both paints (PH-BVD and PH-BVN) I used the same panel configuration via the import function in the FMC.

[downscc 1,327]

Robin, please don't use a video to explain the problem.  I tried to watch it although I am an old fart and still use email, but the displays are blurry and I have not way to read the information I need to understand your problem and I'm not going to watch a 10 min video when you could take 5 minutes to write down a description of the problem.

 

Use your engine display on the lower DU and record your fuel flow differences between one flight and the next.  Fuel flow at a given speed and weight should be very repeatable, if you have a problem with fuel use and the fuel consumption is not indicating a big change then we know it's something outside of the PMDG simulation affecting your fuel.  As Kevin noted, winds have a major role as well as add-ons like some VA software.

[pcubine 143]

 winds have a major role

 

The winds would need to be beyond the realm of possibly to cause a flight to run out of fuel a third of the way into the flight. I looked at the PFPX OFP and I see nothing wrong with it keeping in mind that I only have the LR and this apparently is the ER. Winds aloft are being imported into the FMC. I really think the idea of a strong headwind causing this need to be discarded.

[downscc 1,327]

Agree, I am waiting for someone to provide fuel flow rates that increase unexpectedly to account for high fuel consumption.  Haven't seen that, in fact. I haven't seen much in the way of information here except unexpected results.  I am looking for the reasons why the results were obtained just like everybody else.. let's see some data.

[PinkPony 171]

See my last post: