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rennman

CDU fuel info very wrong

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On the ground,  the CDU progress page indicated 30 as fuel remaining at end of route.

 

During cruise it went down to 10.

 

At destination is was at 3.

 

Any ideas?

 

 

 

 

Mike Ryder.

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A few possible causes:

(1) Did you encounter headwind during cruise? If there was strong headwinds (50+ kts) you will burn much more fuel than FMC predicted.

(2) Were you flying at optimal cruising altitude? If not the fuel burn can be quite different.

(3) Maybe the ZFW (zero fuel weight) entered into the CDU is incorrect. For example if ZFW is entered into the gross weight field the FMC will calculate predict incorrectly.

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I had a 140 kt headwind the whole cruise (I dont know if that is possible?)...But...I cleared the weather and fuel remaining did not increase...

 

I think I put in the wrong fuel weight...

 

So...2 questions..How do I find the right ZFW...and how do I see what the winds will be like 2000 miles away before I take off?

 

Thanks...

 

Mike Ryder.

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2000 nm trip into a 140 knot headwind, makes a journey equivalent to roughly 2,600 nm. Those extra 600nm will easily eat up 27,000 lbs of fuel.

 

Right click the ZFW LSK and it will populate it with the correct number, or you can note it down when you load the aircraft.

 

Program like Active Sky can tell you destination weather, and if you load a flightplan into it will tell you your head/tail wind component for the trip.

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That will sort me out completely!...Thank you so much for taking the time to show me these simple, but essential steps...

 

 

But I'm still wondering why the remaining fuel calc did not change when I cleared the headwind?

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But I'm still wondering why the remaining fuel calc did not change when I cleared the headwind?

How did you clear the headwind and at what point of the flight did you clear the headwind?

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I deactivated AS and cleared weather in flightsim...The wind indicator went to 0 (off)...and I was only halfway through a 4000 mile flight.

 

I was expecting a big gain but fuel remaining stayed at 10...

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I think you're catching yourself up in the logic here.

 

Earlier, you were surprised that the FMC value was changing over time. This was due to environmental variables. The FMC is expecting to move at X TAS (assuming zero wind, X GS as well). The wind is delaying your arrivals to the waypoints versus the no-wind prediction, so the fuel amounts are decreasing.

 

When you then remove the environmental variable, you remove the excess depletion of the fuel versus the no-wind prediction; you're not going to suddenly see a jump.

 

 

 

 

I'll explain in alternate terms:

  • From A to B is 1000 nm, and you're planning on traveling at 200 knots. You will arrive at your destination in 5 hours. If fuel burn is planned to be 200 pounds per hour, you will burn 1000 pounds of fuel. If you're carrying 1500 pounds of fuel, you will arrive with 500 pounds.
  • For that same trip (of 1000 nm), you're planning on travelling at 200 knots, but you encounter wind of 50 knots. You do not tell the computer about this, so it still assumes you will arrive in 5 hours, with 500 pounds of fuel. It recomputes over each waypoint. Assuming you only have one waypoint at the halfway point, it sees 500 nm left, which should only take 2.5 more hours (still assuming no wind), but you've also burnt 667 pounds of fuel getting to that point, so the fuel prediction is now 333 (1500 initial fuel, minus 667 burned = 833 remaining; 500 pounds of fuel is required to go the rest of the way, so 833-500 = 333). If the wind suddenly died off at the mid point, the prediction would still be 333. In this scenario, the computer is not at all curious about how or why there's "extra" fuel missing. More fuel was burned and it accepts this as a fact. Since no wind was provided, it assumes you will continue and encounter no wind (and it also assumes that you encountered no wind).
  • For that same trip (of 1000), with the same speed and wind: you do tell the computer about the wind at the beginning of the flight. It assumes you will arrive in 6:40, with 167 pounds of fuel remaining. Provided the wind predictions are accurate, you should land at or near this amount (and should probably consider adding on more fuel).

This isn't an issue with the systems as much as a misunderstanding of how the system works regarding planned v actual conditions.

 

 

Also: please remember to include your name in your posts - first and last.

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  •  It recomputes over each waypoint. 

 

That explains it...I didnt wait for the next waypoint before I killed the flight...

 

Mike Ryder

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 It recomputes over each waypoint. 

 

There are a bunch of conditions that it'll recalculate. I was trying to keep it as simple as possible for the purposes of the illustration.

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